Combating climate change is the race of our lifetime. That much, at least, is clear. And it’s not just any race. It’s a race of uncertain length, and uncertain stakes. None of that means it isn’t one worth tackling. Indeed, while the danger is grave, climate change also provides opportunities to act, and, yes, to profit. The Chinese word for “crisis” is famously made up of two characters: that for danger (危), and that for opportunity (机).
Gernot Wagner & Martin L. Weitzman.
Yes, China is adding many tons of pollution to the atmosphere through its rapid building of fossil-fueled energy. But China is also at the forefront of the search for solutions. Following the Chinese tradition of “shidian” (试点), wherein prior to launching a large government program it first ought to be tested in multiple regions through a series of pilots, China is now experimenting with several regional cap-and-trade systems. These pilots may well be a metaphor for tackling climate change more broadly. No single piece of legislation, no single technology will solve it all. Climate change most likely won’t create one trillionaire. Rather, it will turn many, many tinkerers and inventors in their proverbial garages into multi-millionaires, each of whom will solve a piece of the overall puzzle.
These innovative solutions are cropping up as we speak, often aided by the right kinds of policies. The rapid price decline of solar photovoltaic technologies is but one example. It is precisely this interplay of smart policy and smart technology that will lead to the kinds of clean energy breakthroughs necessary to match the magnitude of the problem. Indeed, it’s a virtuous circle of clean technology making the right kinds of policies more likely, and policy returning the favor. China’s commitment to begin capping greenhouse gas emissions from industry and energy is a significant step in the right direction. So is the U.S. Clean Power Plan, for the first time limiting emissions from power plants.
The Paris Climate Agreement builds an important foundation for much more action to come. As it must. Little that has happened over the past year—policies in Beijing, Washington, and Paris included—has changed the basic climate calculus. The forces pointing toward the necessity for much more significant action on the mitigation front are as strong as ever. So are the forces pointing toward solar geoengineering.
The “shock” in Climate Shock is real. So is the opportunity. All that begins with taking the economics seriously.
Pop Quiz TWO QUICK QUESTIONS:
– Do you think climate change is an urgent problem?
– Do you think getting the world off fossil fuels is difficult?
If you answered “Yes” to both of these questions, welcome. You’ll nod along, on occasion even cheer, while reading this book. You’ll feel reaffirmed. You are also in the minority. The vast majority of people answer “Yes” to one or the other question, but not both. If you answered “Yes” only to the first question, you probably think of yourself as a committed environmentalist.
You may think climate change is the issue facing society. It’s bad. It’s worse than most of us think. It’s hitting home already, and it will strike us with full force. We should be pulling out all the stops: solar panels, bike lanes, the whole lot. You’re right, in part. Climate change is an urgent problem. But you’re fooling yourself if you think getting off fossil fuels will be simple. It will be one of the most difficult challenges modern civilization has ever faced, and it will require the most sustained, well-managed, globally cooperative effort the human species has ever mounted.
If you answered “Yes” only to the second question, chances are you don’t think climate change is the defining problem of our generation. That doesn’t necessarily mean you’re a “skeptic” or “denier” of the underlying scientific evidence; you may still think global warming is worthy of our attention.
But realism dictates that we can’t stop life as we know it to mitigate a problem that’ll take decades or centuries to show its full force. Look, some people are suffering right now because of lack of energy. And whatever the United States, Europe, or other high emitters do to rein in their energy consumption will be nullified by China, India, and the rest catching up with the rich world’s standard of living. You know there are trade-offs.
You also know that solar panels and bike lanes alone won’t do. You, too, are right, but none of that makes climate change any less of a problem. The long lead time for solutions and the complex global web of players are precisely why we must act decisively, today. If you are an economist, chances are you answered “Yes” to the second question. Standard economic treatments all but prescribe the stance of the “realist.” After all, economists live and breathe trade-offs. Your love for your children may go beyond anything in this world, but as economists we are obligated to say that, strictly speaking, it’s not infinite. As a parent, you may invest enormous sums of money and time into your children, but you, too, face trade-offs: between doing your day job and reading bedtime stories, between indulging now and teaching for later.
Trade-offs are particularly relevant on an average, national, or global level. And they are perhaps nowhere more apparent on the planetary scale than in the case of climate change. It’s the ultimate battle of growth versus the environment.
Stronger climate policy now implies higher, immediate economic costs. Coal-fired power plants will become obsolete sooner or won’t be built in the first place. That comes with costs, for coal plant owners and electricity consumers alike. The big trade-off question then is how these costs compare to the benefits of action, both because of lower carbon pollution and because of economic returns from investing in cleaner, leaner technologies today. Economists often cast themselves as the rational arbiters in the middle of the debate. Our air is worse now than it was during the Stone Age, but life expectancy is a lot higher, too. Sea levels are rising, threatening hundreds of millions of lives and livelihoods, but societies have moved cities before. Getting off fossil fuels will be tough, but human ingenuity—technological change—will surely save the day once again. Life will be different, but who’s to say it will be worse.
Markets have given us longer lives and untold riches. Let properly guided market forces do their magic. There’s a lot to be said for that logic. But the operative words are “properly guided.” What precisely are the costs of unabated climate change? What’s known, what’s unknown, what’s unknowable? And where does what we don’t know lead us?
That last question is the key one: Most everything we know tells us climate change is bad. Most everything we don’t know tells us it’s probably much worse. “Bad” or “worse” doesn’t mean hopeless. In fact, almost every prediction in this book is prefaced by a version of the words unless we act. We don’t venture predictions only to see them become true. We talk about where unfettered economic forces may lead in order to guide them in a more productive, better direction.
And guide we can. In many ways, putting a proper price on carbon isn’t a question of if, it’s a question of when.
911 THANK RUSSIAN POLICE CORRUPTION for footage that eluded NASA and every other space agency. On February 15, 2013, an asteroid as wide as 20 meters (66 feet) exploded in the sky above the Russian city of Chelyabinsk during the morning commute hours, causing a blast brighter than the sun. It didn’t take long for some spectacular videos to appear online, mostly from dashboard cameras many Russian drivers have to protect themselves against the whims of traffic cops. The blast injured 1,500, most because of glass shattered by the explosion.
It was a sobering wakeup call for space agencies to ramp up their asteroid detection and defense capabilities. The money for such efforts is perennially in short supply. But the technical means are there, or at least they could be. A U.S. National Academy study estimates it would take ten years and around $2 or 3 billion to launch a test to deflect an asteroid bound to hit Earth.
It may not be as glamorous as sending a man to the moon within the decade, but it may be at least as important. While the Chelyabinsk asteroid would have been too small to deflect, it would have still been nice to know about it in advance. The chance of a larger asteroid hitting us is small, but it’s there. Educated guesses put it as a 1-in-1,000-year event. That’s a 10 percent chance each century. We haven’t yet spent the money to know for sure.
The fact, though, is that a few billion dollars would allow NASA and others both to catalogue the hazards and to defend against them. That’s a small amount when measured against the costs of a potentially civilization-destroying threat.
Around 65 million years ago it was a giant asteroid that caused the globe’s fifth major extinction event, the dinosaurs. Climate change isn’t exactly hurtling toward us through outer space. It’s entirely homegrown. But the potential devastation is just as real. Elizabeth Kolbert argues convincingly based on her book The Sixth Extinction how this time around: “We are the asteroid.” In fact, by one recent scientific assessment, we are slated to experience global changes at rates that are at least ten times faster than at any point in the past 65 million years.
As Hurricane Sandy was whipping the Eastern Seaboard, leaving Manhattan below the Empire State Building partially flooded and almost entirely without power, New York governor Andrew Cuomo wryly told President Barack Obama that: “We have a 100-year flood every two years now.” Hurricane Irene in August 2011 caused the first-ever preemptive weather-related shutdown of the entire, century-old New York City subway and bus system. It took only fourteen months for the second shutdown. Sandy hit in October 2012. All told, Irene killed 49 and displaced over 2.3 million. Sandy killed 147 and displaced 375,000. New York, of course, is far from unique here.
Typhoon Haiyan slammed the Philippines in November 2013, killing at least 6,000 people and displacing four million. Not even a year earlier, Typhoon Bopha struck the country, killing over a thousand and displacing 1.8 million.
The European summer heat wave in 2003 killed 15,000 in France alone, over 70,000 in Europe.
The list goes on, spanning both poor and rich countries and continents. Society as a whole—especially in rich places like the United States and Europe—has never been as well equipped to cope with these catastrophes as it is today. As is so often the case, the poor suffer the most. That makes these recent deaths and displacements in places like New York all the more remarkable.
What likens these storms and other extreme climatic events to asteroids is that they both can be costly, in dollars and in deaths. The important and clear differences show that the climate problem is costlier still.
First the obvious: Major storms have hit long before humans started adding carbon dioxide to the atmosphere.
However, warmer average temperatures imply more energy in the atmosphere implies more extreme storms, floods, and droughts. The waters off the coast of New York were 3°C (5.4°F) warmer than average during the days before Sandy. The waters off the coast of the Philippines were 3°C (5.4°F) warmer than average just as Haiyan was intensifying on its path to make landfall. Coincidence? Perhaps.
The increase off New York happened at the surface. The increase off the Philippines happened 100 meters (330 feet) below. But the burden of proof seems to rest on those questioning the link from higher temperatures to more intense storms. That’s particularly true, since the best research goes much beyond drawing circumstantial links. The science isn’t settled yet, but the latest research suggests that climate change will lead both to more and bigger storms. Though hurricanes are among the toughest climatic events to link directly to climate change, mainly because of how rare they are. It’s easier to draw the direct link from climate change to more common events like extreme temperatures, floods, and droughts.
Think of it like drunk driving: Drinking increases the chance of a car crash, but plenty of crashes happen without elevated blood alcohol levels. Or liken it to doping in sports: No single Barry Bonds home run or Lance Armstrong Tour de France stage win can be attributed to doping, nor did doping act alone. Bonds still had to hit the ball, and Armstrong still had to pedal. But doping surely helped them hit farther and bike faster.
Major storms, like home run records and multiple Le Tour wins, have happened before. None of that means steroids or elevated levels of red blood cells in an athlete’s blood had no effect.
Something similar holds for elevated levels of carbon dioxide in the atmosphere. Researchers are getting increasingly better at using “attribution science” to identify the human footprint even in single events. The UK’s National Weather Service, more commonly known as the Met Office, has a Climate Monitoring and Attribution team churning out studies that do just that.
One such study found with 90 percent confidence that “human influence has at least doubled the risk of a heatwave exceeding [a] threshold magnitude” of mean summer temperature that was met in Europe in 2003, and in no other year since 1851. Links will only become clearer in the future, both because the science is getting better and because extreme weather events are becoming ever more extreme.
Governor Cuomo’s “100-year flood every two years” comment may have been a throw-away line, but he was on to something. By the end of the century, we can expect today’s 100-year flood to hit as frequently as once every three to twenty years.
That’s a century out, long after our lifetimes, but we know that we can’t wait that long to act. Already, the annual chance of storm waters breaching Manhattan seawalls has increased from around 1 percent in the 19th century to 20 to 25 percent today. That means lower Manhattan can expect some amount of flooding every four to five years. Unlike with asteroids, there’s no $2-to-3-billion, ten-year NASA program to avoid the impact of storms and other extreme climatic events like floods and droughts. Nor is there a quick fix for less dramatic events like the ever faster rising seas. As a first line of defense, higher seawalls would surely help.
But they can go only so far for so long. Higher seas make storm surges all the more powerful, and higher seas themselves come with plenty of costs of their own. Imagine standing in the harbor of your favorite coastal city.
Then imagine standing there at the end of the century with sea levels having risen by 0.3 to 1 meters (1 to 3 feet). It will only be a matter of time before higher seawalls won’t do, when the only option will be retreat. By then, it will be too late to act. We can’t re-create glaciers and polar ice caps, at least not in human timescales. The severity of the problems will have been locked in by past action, or lack thereof. Future generations will be largely powerless against their own fate.
One possible response that attempts to provide a quick fix is large-scale geoengineering: shooting small reflective particles into the stratosphere in an attempt to cool the planet. Geoengineering is far from perfect. It comes with lots of potential side effects, and it’s no replacement for decreasing emissions in the first place. Still, it may be a useful, temporary complement to more fundamental measures. (We will start exploring the full implications of geoengineering in chapter 5.)
None of what we’ve talked about thus far even deals with the true worst-case scenarios. Having the climatic equivalent of ever more Chelyabinsk-like asteroids hit us is bad, but there are ways to cope. For relatively small asteroids, it’s seeking shelter and moving away from windows. For relatively small climatic changes, it’s moving to slightly cooler climates and higher shores. That’s often easier said than done, but at least it’s doable. For much more dramatic climatic consequences—such as a crippling of the world’s productive agricultural lands—it’s tough to imagine how we’d cope in a way that wouldn’t cause serious disruptions.
Meanwhile, standard economic models don’t include much of this thinking. Many observers regard average global warming of greater than 2°C (3.6°F) above preindustrial levels as having the potential to trigger events deserving of various shades of the label “catastrophe.” Economists typically have a hard time making sense of that term. They need dollar figures. Does a catastrophe then cost 10 percent of global economic output? 50 percent? More?
While it’s indeed necessary to translate impacts into dollars and cents, such benefit-cost analyses can act as only one guide for how society ought to respond. We should also take into account the potential for planet-as-we-know-it-altering changes in the first place. First and foremost, climate change is a risk management problem—a catastrophic risk management problem on a planetary scale, to be more precise.
CAMELS IN CANADA
If one wanted to imagine an all but intractable public policy problem, climate change would be pretty close to the ideal. Today’s storms, floods, and wildfires notwithstanding, the worst effects of global warming will be felt long after our lifetimes, likely in the most unpredictable of ways. Climate change is unlike any other environmental problem, really unlike any other public policy problem. It’s almost uniquely global, uniquely long-term, uniquely irreversible, and uniquely uncertain—certainly unique in the combination of all four.
These four factors, call them the Big Four, are what make climate change so difficult to solve. So difficult that—short of a major jolt of the global, collective conscience—it may well prove too difficult to tackle climate change just by decreasing emissions and adjusting to some of the already unavoidable consequences. At the very least we’ll need to add suffering to the list. The rich will adapt. The poor will suffer.
Then there’s the almost inevitable-sounding geoengineering, attempting a global-scale techno fix for a seemingly intractable problem. The most prominent geoengineering idea would have us deliver tiny sulfur-based particles into the stratosphere in an attempt to engineer an artificial sun shield of sorts to help cool the planet. Everything we know about the economics of climate change seems to point us in that direction.
Geoengineering is so cheap to do crudely, and it has such high leverage, that it almost has the exact opposite properties of carbon pollution. It’s the “free-rider” effect of carbon pollution that has caused the problem: it’s in no one’s narrow self-interest to do enough. It’s the “free-driver” effect that may push us to geoengineer our way out of it: it’s so cheap that someone will surely do it based on their own self-interest, broader consequences be damned. But let’s not go there quite yet.
Let’s first tackle The Big Four in turn, beginning with why climate change is the ultimate “free-rider” problem: Climate change is uniquely global. Beijing’s smog is bad. So bad, that it comes with real and dramatic health effects that have prompted city officials to close schools and take other drastic actions.
But Beijing’s smog—or that in Mexico City or Los Angeles, for that matter—is mostly confined to the city. Chinese soot may register at measuring stations on the U.S. West Coast, much like Saharan dust may on occasion blow to central Europe. But all these effects are still regional.
That’s not true for carbon dioxide. It doesn’t matter where on the planet a ton is being emitted. Impacts may be regional, but the phenomenon is global and—among environmental problems—almost uniquely so. The ozone hole over the Antarctic is bad, but even at its height it has never reached the level of engulfing the globe. The same goes, say, for biodiversity loss or deforestation. These are regional problems. It’s climate change that ties them together into phenomena with global implications.
The global nature of global warming is also Strike One against enacting sensible climate policy. It’s tough enough to get voters to enact pollution limits on themselves, when those limits benefit them and only them, and when the benefits of action outweigh the costs. It’s a whole lot tougher to get voters to enact pollution limits on themselves if the costs are felt domestically but the benefits are global: a planetary “free-rider” problem.
Climate change is uniquely long-term. The past decade was the warmest in human history. The one before was the second-warmest. The one before that was the third-warmest. “Americans are noticing changes all around them,” as the 2014 U.S. National Climate Assessment puts it.
Changes are nowhere as evident as above the Arctic Circle: Arctic sea ice has lost half of its area and three-quarters of its volume in only the past thirty years. The Foreign Policy article describing “The Coming Arctic Boom” takes all of this as given. Then there are the visible changes all around. Again, from the National Climate Assessment: “Residents of some coastal cities see their streets flood more regularly during storms and high tides. Inland cities near large rivers also experience more flooding, especially in the Midwest and Northeast. Insurance rates are rising in some vulnerable locations, and insurance is no longer available in others. Hotter and drier weather and earlier snowmelt mean that wildfires in the West start earlier in the spring, last later into the fall, and burn more acreage.”
Climate change is here, and it’s here to stay. None of that should mask the fact that most of the worst consequences of climate change are still remote, often caged in global, long-term averages: global average surface temperature projections for 2100, or global average sea level projections for decades and centuries out.
Strike Two against sensible climate policy: the worst effects are far off—never mind that avoiding these predictions would entail acting now.
Climate change is uniquely irreversible. Even if we stopped emitting carbon tomorrow, we would have decades of warming and centuries of sea-level rise locked in. The eventual, full melting of large West Antarctic ice sheets may already be unstoppable. More extreme weather events are already here and will be with us for some time to come.
Strike Three. Over two-thirds of the excess carbon dioxide in the atmosphere that wasn’t there when humans started burning coal will still be present a hundred years from now. Well over one-third will still be there in 1,000 years. These changes are long-term, and—at least in human timescales—virtually irreversible.
Strike Four. As if three strikes weren’t enough, there’s another unique characteristic of climate change to round out the Big Four, and it may be the biggest one of them all: uncertainty—everything we know that we don’t know, and perhaps more importantly, what we don’t yet know we don’t know.
Last time concentrations of carbon dioxide were as high as they are today, at 400 parts per million (ppm), the geological clock read “Pliocene.” That was over three million years ago, when natural variations, not cars and factories, were responsible for the extra carbon in the air. Global average temperatures were around 1–2.5°C (1.8 to 4.5°F) warmer than today, sea levels were up to 20 meters (66 feet) higher, and camels lived in Canada. We wouldn’t expect any of these dramatic changes today.
The greenhouse effect needs decades to centuries to come into full force. Despite the recent changes in the Arctic, ice sheets need decades to centuries to melt. Global sea levels take decades to centuries to adjust accordingly. Carbon dioxide concentrations may have been at 400 ppm three million years ago, whereas rising sea levels lagged decades or centuries behind. That time difference is important and points to the long-term nature and irreversibility of it all. See strikes two and three.
But all that’s small consolation, and there’s an important twist to strike four.
The best available climate models come close in their temperature projections to what the world experienced during the Pliocene, but they aren’t predicting sea levels of 20 meters (66 feet) higher. Nor do they predict camels wandering around Canada. Not now. Not hundreds of years from now. That’s true for two important reasons.
First, most climate models are unduly skewed toward the known, sometimes making them much too conservative. Until recently, most climate models predicted rising sea levels only based on thermal expansion of the oceans (and the melting of mountain glaciers), but they did not include the effects of melting ice sheets.
Warmer waters take up more space, leading to higher sea levels. That mechanism alone has indeed contributed to over a third of sea-level rise in the past two decades. It’s also clear that melting glaciers in Greenland and Antarctica raise sea levels, but by how much is highly uncertain. Call it a “known unknown.” Until recently, scientific understanding of melting polar ice caps had been so poor that most models simply left it out.
Second, even though climate models do get a lot of things right, there are fundamental things that we don’t understand about the way the climate works.
The averages are bad enough. While 0.1°C (0.2°F) of average global surface warming per decade sounds rather manageable and perhaps even pleasant, few dispute that a century or more of warming at this pace would lead to serious costs. But these averages hide two distinct sets of uncertainties that could pose the real problems.
The first set of uncertainties is inherent in any kind of global, long-term estimate. Presenting just the global average numbers masks at least four important facts: First, temperatures in the past century have been increasing at an increasing rate. Second, despite that generally increasing trend, temperatures fluctuate across years and decades. (Hence the infamous “decade without warming.”) Third, air over the oceans is usually cooler than over land. Since two-thirds of the world is ocean, a global average increase of 0.07°C (0.13°F) per decade translated to about a 0.11°C (0.20°F) increase over land.
Finally, temperatures over the poles have warmed more than elsewhere. Arctic temperatures are expected to increase at a rate more than twice the global average. That’s particularly bad, since the poles are also where most of the world’s remaining ice is. Melting ice on land above sea level means higher seas, as the latest sea-level projections now officially acknowledge. Then there are the real, deep-seated uncertainties. To arrive at any of these projections—average or otherwise—requires taking several steps, each with its own set of known and, most vexingly, unknown unknowns.
Uncertainties exist around the amounts of global warming pollutants we emit, the link between emissions and atmospheric concentrations, the link between concentrations and temperatures, the link between temperatures and physical climate damages, the link between physical damages and their consequences, and, at least as important, how society will respond: what coping measures will be undertaken, and how effective they will prove to be.
Nailing down one of these steps—the link between concentrations and eventual temperature increases—has proven particularly elusive. The past three decades of amazing advances in climate science have gotten us no closer to pinpointing the true answer. Double the carbon dioxide concentrations in the atmosphere—something that will surely happen, unless we enact ambitious climate policies now—and eventually global average temperatures are likely to go up by between 1.5 and 4.5°C (2.7 and 8°F). Our confidence in that range has increased, but what’s now called the “likely” range hasn’t changed since the late 1970s, a fact we will revisit in chapter 3, “Fat Tails.”
The very term “fat tails” also points to another problem: 1.5 to 4.5°C (2.7 to 8°F) is “likely” in the best sense of that word. The chance is good that we will indeed find ourselves somewhere in that range for how temperatures react when concentrations double, what’s known as “climate sensitivity.” But there’s also a chance we won’t.
The Inter-governmental Panel on Climate Change (IPCC) describes anything below 1°C (1.8°F) as “extremely unlikely.” That assessment is pretty believable, given that the world has already warmed by 0.8°C (1.4°F), and we haven’t even yet doubled carbon dioxide concentrations from preindustrial levels. (The 400 ppm that the world just passed is a 40 percent increase over preindustrial levels of 280 ppm.) There’s also a chance that final temperatures from a doubling of carbon dioxide concentrations will end up above 4.5°C (8°F). It’s “unlikely,” but we can’t discount the possibility.
Meanwhile, global average warming of 4.5°C (8°F) is beyond the pale of most imagination. Recall the camels in Canada, or at least a planet that none of us would recognize. But that 4.5°C (8°F) doesn’t yet tell the full story. Climate sensitivity describes what happens when concentrations of carbon dioxide in the atmosphere double.
What if carbon dioxide concentrations more than double?
The International Energy Agency (IEA) predicts levels of 700 ppm, or two-and-a-half times preindustrial levels. Now we are looking at a “likely” range of temperatures between 2 and 6°C (3.6 and 11°F). Climate science warns that average global warming above 2°C (3.6°F) could trigger potentially devastating events.
It’s unclear what label to use for global average warming of 6°C (11°F): “catastrophic” no longer seems to do it justice. Mark Lynas, who has painstakingly detailed climate impacts degree by frightening degree, ends his book Six Degrees just there. The introduction to the final chapter on 6°C (11°F) begins with a reference to Dante’s Sixth Circle of Hell.
HELIX, a recently started project funded by the European Union, aims to determine global and regional impacts of specific levels of temperature rise. It, too, ends at 6°C (11°F). And per our own calculations in chapter 3, we are looking at an eventual chance of around 10 percent of exceeding that mark.
Whenever science points to the very real potential of these types of catastrophic outcomes, cognitive dissonance kicks in. Facts might be facts, the reasoning goes, but throwing too many of them at you at once will all but guarantee that you will dismiss them out of hand. It just feels like it can’t or shouldn’t be true.
That fickleness of human nature and the limits of our understanding are at the core of the climate policy dilemma. Smarts alone don’t seem to make much of a difference here. Solving the dilemma will take a completely different way of thinking.
THE BATHTUB PROBLEM
Think of the atmosphere as a giant bathtub. There’s a faucet—emissions from human activity—and a drain—the planet’s ability to absorb that pollution. For most of human civilization and hundreds of thousands of years before, the inflow and the outflow were in relative balance.
Then humans started burning coal and turned on the faucet far beyond what the drain could handle. The levels of carbon in the atmosphere began to rise to levels last seen in the Pliocene , over three million years ago.
What to do?
That’s the question John Sterman, an MIT professor, asked two hundred graduate students. More specifically , he asked what to do to stabilize concentrations of carbon dioxide in the atmosphere close to present levels .
How far do we need to go in turning off the faucet in order to stabilize concentrations? Here’s what not to do: stabilizing the flow of carbon into the atmosphere today won’t stabilize the carbon already there at close to present levels. You’re still adding carbon. Just because the inflow remains steady year after year, doesn’t mean the amount already in the tub doesn’t go up. Inflow and outflow need to be in balance , and that won’t happen at current levels of carbon dioxide in the tub (currently at 400 ppm) unless the inflow goes down by a lot.
That seems like an obvious point. It also seems to get lost on the average MIT graduate student, and these students aren’t exactly “average’. Still, over 80 percent of them in Sterman’s study seem to confuse the faucet with the tub. They confuse stabilizing the inflow with stabilizing the level.
CLIMATE SHOCK. The Economic Consequences of a Hotter Planet
by Gernot Wagner and Martin L. Weitzman.
get it at Amazon.com
THE SIXTH EXTINCTION
An Unnatural History.
If there is danger in the human trajectory, it is not so much in the survival of our own species as in the fulfillment of the ultimate irony of organic evolution: that in the instant of achieving self-understanding through the mind of man, life has doomed its most beautiful creations. — E. O. Wilson
Centuries of centuries and only in the present do things happen. — Jorge Luis Borges
Beginnings, it’s said, are apt to be shadowy. So it is with this story, which starts with the emergence of a new species maybe two hundred thousand years ago. The species does not yet have a name—nothing does—but it has the capacity to name things. As with any young species, this one’s position is precarious. Its numbers are small, and its range restricted to a slice of eastern Africa. Slowly its population grows, but quite possibly then it contracts again—some would claim nearly fatally—to just a few thousand pairs. The members of the species are not particularly swift or strong or fertile.
They are, however, singularly resourceful. Gradually they push into regions with different climates, different predators, and different prey. None of the usual constraints of habitat or geography seem to check them. They cross rivers, plateaus, mountain ranges. In coastal regions, they gather shellfish; farther inland, they hunt mammals. Everywhere they settle, they adapt and innovate. On reaching Europe, they encounter creatures very much like themselves, but stockier and probably brawnier, who have been living on the continent far longer. They interbreed with these creatures and then, by one means or another, kill them off.
The end of this affair will turn out to be exemplary. As the species expands its range, it crosses paths with animals twice, ten, and even twenty times its size: huge cats, towering bears, turtles as big as elephants, sloths that stand five metres tall. These species are more powerful and often fiercer. But they are slow to breed and are wiped out. Although a land animal, our species—ever inventive—crosses the sea. It reaches islands inhabited by evolution’s outliers: birds that lay thirty-centimetre-long eggs, pig-sized hippos, giant skinks. Accustomed to isolation, these creatures are ill-equipped to deal with the newcomers or their fellow travelers (mostly rats). Many of them, too, succumb.
The process continues, in fits and starts, for thousands of years, until the species, no longer so new, has spread to practically every corner of the globe. At this point, several things happen more or less at once that allow Homo sapiens, as it has come to call itself, to reproduce at an unprecedented rate. In a single century the population doubles; then it doubles again, and then again.
Vast forests are razed. Humans do this deliberately, in order to feed themselves. Less deliberately, they shift organisms from one continent to another, reassembling the biosphere. Meanwhile, an even stranger and more radical transformation is under way. Having discovered subterranean reserves of energy, humans begin to change the composition of the atmosphere. This, in turn, alters the climate and the chemistry of the oceans. Some plants and animals adjust by moving. They climb mountains and migrate toward the poles. But a great many—at first hundreds, then thousands, and finally perhaps millions—find themselves marooned.
Extinction rates soar, and the texture of life changes. No creature has ever altered life on the planet in this way before, and yet other, comparable events have occurred. Very, very occasionally in the distant past, the planet has undergone change so wrenching that the diversity of life has plummeted. Five of these ancient events were catastrophic enough that they’re put in their own category: the so-called Big Five. In what seems like a fantastic coincidence, but is probably no coincidence at all, the history of these events is recovered just as people come to realize that they are causing another one. When it is still too early to say whether it will reach the proportions of the Big Five, it becomes known as the Sixth Extinction.
The story of the Sixth Extinction, at least as I’ve chosen to tell it, comes in thirteen chapters. Each tracks a species that’s in some way emblematic—the American mastodon, the great auk, an ammonite that disappeared at the end of the Cretaceous alongside the dinosaurs. The creatures in the early chapters are already gone, and this part of the book is mostly concerned with the great extinctions of the past and the twisting history of their discovery, starting with the work of the French naturalist Georges Cuvier.
The second part of the book takes place very much in the present—in the increasingly fragmented Amazon rainforest, on a fast-warming slope in the Andes, on the outer reaches of the Great Barrier Reef. I chose to go to these particular places for the usual journalistic reasons—because there was a research station there or because someone invited me to tag along on an expedition. Such is the scope of the changes now taking place that I could have gone pretty much anywhere and, with the proper guidance, found signs of them.
One chapter concerns a die-off happening more or less in my own backyard (and, quite possibly, in yours). If extinction is a morbid topic, mass extinction is, well, massively so. It’s also a fascinating one. In the pages that follow, I try to convey both sides: the excitement of what’s being learned as well as the horror of it. My hope is that readers of this book will come away with an appreciation of the truly extraordinary moment in which we live.
The Sixth Extinction
The town of El Valle de Antón, in central Panama, sits in the middle of a volcanic crater formed about a million years ago. The crater is six kilometres wide, but when the weather is clear you can see the jagged hills that surround the town like the walls of a ruined tower. El Valle has one main street, a police station, and an open-air market. In addition to the usual assortment of Panama hats and vividly colored embroidery, the market offers what must be the world’s largest selection of golden-frog figurines.
There are golden frogs resting on leaves and golden frogs sitting up on their haunches and—rather more difficult to understand—golden frogs clasping cell phones. There are golden frogs wearing frilly skirts and golden frogs striking dance poses and golden frogs smoking cigarettes through a holder, after the fashion of FDR. The golden frog, which is taxicab yellow with dark brown splotches, is endemic to the area around El Valle. It is considered a lucky symbol in Panama; its image is (or at least used to be) printed on lottery tickets.
As recently as a decade ago, golden frogs were easy to spot in the hills around El Valle. The frogs are toxic—it’s been calculated that the poison contained in the skin of just one animal could kill a thousand average-sized mice—hence the vivid color, which makes them stand out against the forest floor. One creek not far from El Valle was nicknamed Thousand Frog Stream. A person walking along it would see so many golden frogs sunning themselves on the banks that, as one herpetologist who made the trip many times put it to me, “it was insane—absolutely insane.”
Then the frogs around El Valle started to disappear. The problem—it was not yet perceived as a crisis—was first noticed to the west, near Panama’s border with Costa Rica. An American graduate student happened to be studying frogs in the rainforest there. She went back to the States for a while to write her dissertation, and when she returned, she couldn’t find any frogs or, for that matter, amphibians of any kind. She had no idea what was going on, but since she needed frogs for her research, she set up a new study site, farther east.
At first the frogs at the new site seemed healthy; then the same thing happened: the amphibians vanished. The blight spread through the rainforest until, in 2002, the frogs in the hills and streams around the town of Santa Fe, about eighty kilometres west of El Valle, were effectively wiped out. In 2004, little corpses began showing up even closer to El Valle, around the town of El Copé. By this point, a group of biologists, some from Panama, others from the United States, had concluded that the golden frog was in grave danger. They decided to try to preserve a remnant population by removing a few dozen of each sex from the forest and raising them indoors.
But whatever was killing the frogs was moving even faster than the biologists had feared. Before they could act on their plan, the wave hit. I first read about the frogs of El Valle in a nature magazine for children that I picked up from my kids. The article, which was illustrated with full-color photos of the Panamanian golden frog and other brilliantly colored species, told the story of the spreading scourge and the biologists’ efforts to get out in front of it. The biologists had hoped to have a new lab facility constructed in El Valle, but it was not ready in time. They raced to save as many animals as possible, even though they had nowhere to keep them. So what did they end up doing? They put them “in a frog hotel, of course!”
The “incredible frog hotel”—really a local bed and breakfast—agreed to let the frogs stay (in their tanks) in a block of rented rooms. “With biologists at their beck and call, the frogs enjoyed first-class accommodations that included maid and room service,” the article noted. The frogs were also served delicious, fresh meals—“so fresh, in fact, the food could hop right off the plate.”
Just a few weeks after I read about the “incredible frog hotel,” I ran across another frog-related article written in a rather different key. This one, which appeared in the Proceedings of the National Academy of Sciences, was by a pair of herpetologists. It was titled “Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians.” The authors, David Wake, of the University of California-Berkeley, and Vance Vredenburg, of San Francisco State, noted that there “have been five great mass extinctions during the history of life on this planet.” These extinctions they described as events that led to “a profound loss of biodiversity.”
The first took place during the late Ordovician period, some 450 million years ago, when living things were still mainly confined to the water. The most devastating took place at the end of the Permian period, some 250 million years ago, and it came perilously close to emptying the earth out altogether. (This event is sometimes referred to as “the mother of mass extinctions”or “the great dying.”) The most recent—and famous—mass extinction came at the close of the Cretaceous period; it wiped out, in addition to the dinosaurs, the plesiosaurs, the mosasaurs, the ammonites, and the pterosaurs.
Wake and Vredenburg argued that, based on extinction rates among amphibians, an event of a similarly catastrophic nature was currently under way. Their article was illustrated with just one photograph, of about a dozen mountain yellow-legged frogs— all dead—lying bloated and belly-up.
I understood why a kids’ magazine had opted to publish photos of live frogs rather than dead ones. I also understood the impulse to play up the Beatrix Potter-like charms of amphibians ordering room service. Still, it seemed to me, as a journalist, that the magazine had buried the lede. Any event that has occurred just five times since the first animal with a backbone appeared, some five hundred million years ago, must qualify as exceedingly rare.
The notion that a sixth such event would be taking place right now, more or less in front of our eyes, struck me as, to use the technical term, mind-boggling. Surely this story, too—the bigger, darker, far more consequential one—deserved telling. If Wake and Vredenburg were correct, then those of us alive today not only are witnessing one of the rarest events in life’s history, we are also causing it. “One weedy species,” the pair observed, “has unwittingly achieved the ability to directly affect its own fate and that of most of the other species on this planet.” A few days after I read Wake and Vredenburg’s article, I booked a ticket to Panama.
The El Valle Amphibian Conservation Center, or EVACC (pronounced “ee-vac”), lies along a dirt road not far from the open-air market where the golden frog figurines are sold. It’s about the size of a suburban ranch house, and it occupies the back corner of a small, sleepy zoo, just beyond a cage of very sleepy sloths. The entire building is filled with tanks. There are tanks lined up against the walls and more tanks stacked at the center of the room, like books on the shelves of a library.
The taller tanks are occupied by species like the lemur tree frog, which lives in the forest canopy; the shorter tanks serve for species like the big-headed robber frog, which lives on the forest floor. Tanks of horned marsupial frogs, which carry their eggs in a pouch, sit next to tanks of casque-headed frogs, which carry their eggs on their backs. A few dozen tanks are devoted to Panamanian golden frogs, Atelopus zeteki.
Golden frogs have a distinctive, ambling gait that makes them look a bit like drunks trying to walk a straight line. They have long, skinny limbs, pointy yellow snouts, and very dark eyes, through which they seem to be regarding the world warily. At the risk of sounding weak-minded, I will say that they look intelligent. In the wild, females lay their eggs in shallow running water; males, meanwhile, defend their territory from the tops of mossy rocks.
In EVACC, each golden frog tank has its own running water, provided by its own little hose, so that the animals can breed near a simulacrum of the streams that were once their home. In one of the ersatz streams, I noticed a string of little pearl-like eggs. On a white board nearby someone had noted excitedly that one of the frogs “depositó huevos!!” EVACC sits more or less in the middle of the golden frog’s range, but it is, by design, entirely cut off from the outside world. Nothing comes into the building that has not been thoroughly disinfected, including the frogs, which, in order to gain entry, must first be treated with a solution of bleach. Human visitors are required to wear special shoes and to leave behind any bags or knapsacks or equipment that they’ve used out in the field. All of the water that enters the tanks has been filtered and specially treated.
The sealed-off nature of the place gives it the feel of a submarine or, perhaps more aptly, an ark mid-deluge.
A Panamanian golden frog, Atelopus zeteki.
EVACC’s director is a Panamanian named Edgardo Griffith. Griffith is tall and broad-shouldered, with a round face and a wide smile. He wears a silver ring in each ear and has a large tattoo of a toad’s skeleton on his left shin. Now in his mid-thirties, Griffith has devoted pretty much his entire adult life to the amphibians of El Valle, and he has turned his wife, an American who came to Panama as a Peace Corps volunteer, into a frog person, too. Griffith was the first person to notice when little carcasses started showing up in the area, and he personally collected many of the several hundred amphibians that got booked into the hotel. (The animals were transferred to EVACC once the building had been completed.)
If EVACC is a sort of ark, Griffith becomes its Noah, though one on extended duty, since already he’s been at things a good deal longer than forty days. Griffith told me that a key part of his job was getting to know the frogs as individuals. “Every one of them has the same value to me as an elephant,” he said. The first time I visited EVACC, Griffith pointed out to me the representatives of species that are now extinct in the wild. These included, in addition to the Panamanian golden frog, the Rabbs’ fringe-limbed tree frog, which was first identified only in 2005. At the time of my visit, EVACC was down to just one Rabbs’ frog, so the possibility of saving even a single, Noachian pair had obviously passed. The frog, greenish brown with yellow speckles, was about ten centimetres long, with oversized feet that gave it the look of a gawky teenager.
Rabbs’ fringe-limbed tree frogs lived in the forest above El Valle, and they laid their eggs in tree holes. In an unusual, perhaps even unique arrangement, the male frogs cared for the tadpoles by allowing their young, quite literally, to eat the skin off their backs. Griffith said that he thought there were probably many other amphibian species that had been missed in the initial collecting rush for EVACC and had since vanished; it was hard to say how many, since most of them were probably unknown to science. “Unfortunately,” he told me, “we are losing all these amphibians before we even know that they exist.” “Even the regular people in El Valle, they notice it,” he said. “They tell me, ‘What happened to the frogs? We don’t hear them calling anymore.’
When the first reports that frog populations were crashing began to circulate, a few decades ago, some of the most knowledgeable people in the field were the most skeptical. Amphibians are, after all, among the planet’s great survivors. The ancestors of today’s frogs crawled out of the water some 400 million years ago, and by 250 million years ago the earliest representatives of what would become the modern amphibian orders—one includes frogs and toads, the second newts and salamanders, and the third weird limbless creatures called caecilians—had evolved.
This means that amphibians have been around not just longer than mammals, say, or birds; they have been around since before there were dinosaurs. Most amphibians—the word comes from the Greek meaning “double life”—are still closely tied to the aquatic realm from which they emerged. (The ancient Egyptians thought that frogs were produced by the coupling of land and water during the annual flooding of the Nile.)
Their eggs, which have no shells, must be kept moist in order to develop. There are many frogs that, like the Panamanian golden frog, lay their eggs in streams. There are also frogs that lay them in temporary pools, frogs that lay them underground, and frogs that lay them in nests that they construct out of foam. In addition to frogs that carry their eggs on their backs and in pouches, there are frogs that carry them wrapped like bandages around their legs. Until recently, when both of them went extinct, there were two species of frogs, known as gastric-brooding frogs, that carried their eggs in their stomachs and gave birth to little froglets through their mouths. Amphibians emerged at a time when all the land on earth was part of a single expanse known as Pangaea. Since the breakup of Pangaea, they’ve adapted to conditions on every continent except Antarctica.
Worldwide, just over seven thousand species have been identified, and while the greatest number are found in the tropical rainforests, there are occasional amphibians, like the sandhill frog of Australia, that can live in the desert, and also amphibians, like the wood frog, that can live above the Arctic Circle. Several common North American frogs, including spring peepers, are able to survive the winter frozen solid, like popsicles.
Their extended evolutionary history means that even groups of amphibians that, from a human perspective, seem to be fairly similar may, genetically speaking, be as different from one another as, say, bats are from horses. David Wake, one of the authors of the article that sent me to Panama, was among those who initially did not believe that amphibians were disappearing. This was back in the mid-nineteen-eighties. Wake’s students began returning from frog-collecting trips in the Sierra Nevada empty-handed.
Wake remembered from his own student days, in the nineteen-sixties, that frogs in the Sierras had been difficult to avoid. “You’d be walking through meadows, and you’d inadvertently step on them,” he told me. “They were just everywhere.” Wake assumed that his students were going to the wrong spots, or that they just didn’t know how to look. Then a postdoc with several years of collecting experience told him that he couldn’t find any amphibians, either. “I said, ‘OK, I’ll go up with you, and we’ll go out to some proven places,’ “Wake recalled. “And I took him out to this proven place, and we found like two toads.”
Part of what made the situation so mystifying was the geography; frogs seemed to be vanishing not only from populated and disturbed areas but also from relatively pristine places, like the Sierras and the mountains of Central America. In the late nineteen-eighties, an American herpetologist went to the Monteverde Cloud Forest Reserve in northern Costa Rica to study the reproductive habits of golden toads.
She spent two field seasons looking; where once the toads had mated in writhing masses, a single male was sighted. (The golden toad, now classified as extinct, was actually a bright tangerine color. It was only very distantly related to the Panamanian golden frog, which, owing to a pair of glands located behind its eyes, is also technically a toad.)
Around the same time, in central Costa Rica, biologists noticed that the populations of several endemic frog species had crashed. Rare and highly specialized species were vanishing and so, too, were much more familiar ones. In Ecuador, the Jambato toad, a frequent visitor to backyard gardens, disappeared in a matter of years. And in northeastern Australia the southern day frog, once one of the most common in the region, could no longer be found.
The first clue to the mysterious killer that was claiming frogs from Queensland to California came—perhaps ironically, perhaps not—from a zoo. The National Zoo, in Washington, D.C., had been successfully raising blue poison-dart frogs, which are native to Suriname, through many generations. Then, more or less from one day to the next, the zoo’s tank-bred frogs started dropping. A veterinary pathologist at the zoo took some samples from the dead frogs and ran them through an electron scanning microscope. He found a strange microorganism on the animals’ skin, which he eventually identified as a fungus belonging to a group known as chytrids.
Chytrid fungi are nearly ubiquitous; they can be found at the tops of trees and also deep underground. This particular species, though, had never been seen before; indeed, it was so unusual that an entire genus had to be created to accommodate it. It was named Batrachochytrium dendrobatidis—batrachos is Greek for “frog”—or Bd for short.
The veterinary pathologist sent samples from infected frogs at the National Zoo to a mycologist at the University of Maine. The mycologist grew cultures of the fungus and then sent some of them back to Washington. When healthy blue poison-dart frogs were exposed to the lab-raised Bd, they sickened. Within three weeks, they were dead. Subsequent research showed that Bd interferes with frogs’ ability to take up critical electrolytes through their skin. This causes them to suffer what is, in effect, a heart attack.
The Chytrid fungus
EVACC can perhaps best be described as a work-in-progress. The week I spent at the center, a team of American volunteers was also there, helping to construct an exhibit. The exhibit was going to be open to the public, so, for biosecurity purposes, the space had to be isolated and equipped with its own separate entrance. There were holes in the walls where, eventually, glass cases were to be mounted, and around the holes someone had painted a mountain landscape very much like what you would see if you stepped outside and looked up at the hills.
The highlight of the exhibit was to be a large case full of Panamanian golden frogs, and the volunteers were trying to construct a metre-high concrete waterfall for them. But there were problems with the pumping system and difficulties getting replacement parts in a valley with no hardware store.
The volunteers seemed to be spending a lot of time hanging around, waiting. I spent a lot of time hanging around with them. Like Griffith, all of the volunteers were frog lovers. Several, I learned, were zoo-keepers who worked with amphibians back in the States. (One told me that frogs had ruined his marriage.) I was moved by the team’s dedication, which was the same sort of commitment that had gotten the frogs into the “frog hotel” and then had gotten EVACC up and running, if not entirely completed. But I couldn’t help also feeling that there was also something awfully sad about the painted green hills and the fake waterfall.
With almost no frogs left in the forests around El Valle, the case for bringing the animals into EVACC has by now clearly been proved. And yet the longer the frogs spend in the center, the tougher it is to explain what they’re doing there.
The chytrid fungus, it turns out, does not need amphibians in order to survive. This means that even after it has killed off the animals in an area, it continues to live on, doing whatever it is that chytrid fungi do. Thus, were the golden frogs at EVACC allowed to amble back into the actual hills around El Valle, they would sicken and collapse. (Though the fungus can be destroyed by bleach, it’s obviously impossible to disinfect an entire rainforest.)
Everyone I spoke to at EVACC told me that the center’s goal was to maintain the animals until they could be released to repopulate the forests, and everyone also acknowledged that they couldn’t imagine how this would actually be done. “We’ve got to hope that somehow it’s all going to come together,” Paul Crump, a herpetologist from the Houston Zoo who was directing the stalled waterfall project, told me. “We’ve got to hope that something will happen, and we’ll be able to piece it all together, and it will all be as it once was, which now that I say it out loud sounds kind of stupid.” “The point is to be able to take them back, which every day I see more like a fantasy,” Griffith said.
Once chytrid swept through El Valle, it didn’t stop; it continued to move east. It has also since arrived in Panama from the opposite direction, out of Colombia. Bd has spread through the highlands of South America and down the eastern coast of Australia, and it has crossed into New Zealand and Tasmania. It has raced through the Caribbean and has been detected in Italy, Spain, Switzerland, and France. In the U.S., it appears to have radiated from several points, not so much in a wavelike pattern as in a series of ripples. At this point, it appears to be, for all intents and purposes, unstoppable.
The same way acoustical engineers speak of “background noise” biologists talk about “background extinction.” In ordinary times—times here understood to mean whole geologic epochs—extinction takes place only very rarely, more rarely even than speciation, and it occurs at what’s known as the background extinction rate. This rate varies from one group of organisms to another; often it’s expressed in terms of extinctions per million species-years. Calculating the background extinction rate is a laborious task that entails combing through whole databases’ worth of fossils.
For what’s probably the best-studied group, which is mammals, it’s been reckoned to be roughly .25 per million species-years. This means that, since there are about fifty-five hundred mammal species wandering around today, at the background extinction rate you’d expect—once again, very roughly—one species to disappear every seven hundred years.
Mass extinctions are different. Instead of a background hum there’s a crash, and disappearance rates spike. Anthony Hallam and Paul Wignall, British paleontologists who have written extensively on the subject, define mass extinctions as events that eliminate a “significant proportion of the world’s biota in a geologically insignificant amount of time.”
Another expert, David Jablonski, characterizes mass extinctions as “substantial biodiversity losses”that occur rapidly and are “global in extent.” Michael Benton, a paleontologist who has studied the end-Permian extinction, uses the metaphor of the tree of life: “During a mass extinction, vast swathes of the tree are cut short, as if attacked by crazed, axe-wielding madmen.” A fifth paleontologist, David Raup, has tried looking at matters from the perspective of the victims: “Species are at a low risk of extinction most of the time.” But this “condition of relative safety is punctuated at rare intervals by a vastly higher risk.” The history of life thus consists of “long periods of boredom interrupted occasionally by panic.”
The Big Five extinctions, as seen in the marine fossil record, resulted in a sharp decline in diversity at the family level. If even one species from a family made it through, the family counts as a survivor, so on the species level the losses were far greater. In times of panic, whole groups of once-dominant organisms can disappear or be relegated to secondary roles, almost as if the globe has undergone a cast change.
Such wholesale losses have led paleontologists to surmise that during mass extinction events—in addition to the so-called Big Five, there have been many lesser such events—the usual rules of survival are suspended. Conditions change so drastically or so suddenly (or so drastically and so suddenly) that evolutionary history counts for little. Indeed, the very traits that have been most useful for dealing with ordinary threats may turn out, under such extraordinary circumstances, to be fatal.
A rigorous calculation of the background extinction rate for amphibians has not been performed, in part because amphibian fossils are so rare. Almost certainly, though, the rate is lower than it is for mammals. Probably, one amphibian species should go extinct every thousand years or so. That species could be from Africa or from Asia or from Australia. In other words, the odds of an individual’s witnessing such an event should be effectively zero.
Already, Griffith has observed several amphibian extinctions. Pretty much every
herpetologist working out in the field has watched several. (Even I, in the time I spent researching this book, encountered one species that has since gone extinct and three or four others, like the Panamanian golden frog, that are now extinct in the wild.) “I sought a career in herpetology because I enjoy working with animals,” Joseph Mendelson, a herpetologist at Zoo Atlanta, has written. “I did not anticipate that it would come to resemble paleontology.”
THE SIXTH EXTINCTION. An Unnatural History.
by Elizabeth Kolbert.
get it at Amazon.com
ARE WE IN THE MIDST OF THE SIXTH MASS EXTINCTION?
A view from the world of amphibians.
David B. Wake and Vance T. Vredenburg.
Many scientists argue that we are either entering or in the midst of the sixth great mass extinction. Intense human pressure, both direct and indirect, is having profound effects on natural environments.
The amphibians—frogs, salamanders, and caecilians—may be the only major group currently at risk globally. A detailed worldwide assessment and subsequent updates show that one-third or more of the 6,300 species are threatened with extinction. This trend is likely to accelerate because most amphibians occur in the tropics and have small geographic ranges that make them susceptible to extinction.
The increasing pressure from habitat destruction and climate change is likely to have major impacts on narrowly adapted and distributed species. We show that salamanders on tropical mountains are particularly at risk.
A new and significant threat to amphibians is a virulent, emerging infectious disease, chytridiomycosis, which appears to be globally distributed, and its effects may be exacerbated by global warming. This disease, which is caused by a fungal pathogen and implicated in serious declines and extinctions of >200 species of amphibians, poses the greatest threat to biodiversity of any known disease.
Our data for frogs in the Sierra Nevada of California show that the fungus is having a devastating impact on native species, already weakened by the effects of pollution and introduced predators. A general message from amphibians is that we may have little time to stave off a potential mass extinction.
Biodiversity is a term that refers to life on Earth in all aspects of its diversity, interactions among living organisms, and, importantly, the fates of these organisms. Scientists from many fields have raised warnings of burgeoning threats to species and habitats. Evidence of such threats (e.g., human population growth, habitat conversion, global warming and its consequences, impacts of exotic species, new pathogens, etc.) suggests that a wave of extinction is either upon us or is poised to have a profound impact.
The title of our article, suggested by the organizers, is an appropriate question at this stage of the development of biodiversity science. We examine the topic at two levels. We begin with a general overview of past mass extinctions to determine where we now stand in a relative sense. Our specific focus, however, is a taxon, the Class Amphibia. Amphibians have been studied intensively since biologists first became aware that we are witnessing a period of their severe global decline. Ironically, awareness of this phenomenon occurred at the same time the word “biodiversity” came into general use, in 1989.
FIVE MASS EXTINCTIONS
It is generally thought that there have been five great mass extinctions during the history of life on this planet (1, 2). [The first two may not qualify because new analyses show that the magnitude of the extinctions in these events was not significantly higher than in several other events (3).] In each of the five events, there was a profound loss of biodiversity during a relatively short period.
The oldest mass extinction occurred at the Ordovician–Silurian boundary (≈439 Mya). Approximately 25% of the families and nearly 60% of the genera of marine organisms were lost (1, 2). Contributing factors were great fluctuations in sea level, which resulted from extensive glaciations, followed by a period of great global warming. Terrestrial vertebrates had not yet evolved.
The next great extinction was in the Late Devonian (≈364 Mya), when 22% of marine families and 57% of marine genera, including nearly all jawless fishes, disappeared (1, 2). Global cooling after bolide impacts may have been responsible because warm water taxa were most strongly affected. Amphibians, the first terrestrial vertebrates, evolved in the Late Devonian, and they survived this extinction event (4).
The Permian–Triassic extinction (≈ 251 Mya) was by far the worst of the five mass extinctions; 95% of all species (marine as well as terrestrial) were lost, including 53% of marine families, 84% of marine genera, and 70% of land plants, insects, and vertebrates (1, 2). Causes are debated, but the leading candidate is flood volcanism emanating from the Siberian Traps, which led to profound climate change. Volcanism may have been initiated by a bolide impact, which led to loss of oxygen in the sea. The atmosphere at that time was severely hypoxic, which likely acted synergistically with other factors (5). Most terrestrial vertebrates perished, but among the few that survived were early representatives of the three orders of amphibians that survive to this day (6, 7).
The End Triassic extinction (≈199–214 Mya) was associated with the opening of the Atlantic Ocean by sea floor spreading related to massive lava floods that caused significant global warming. Marine organisms were most strongly affected (22% of marine families and 53% of marine genera were lost) (1, 2), but terrestrial organisms also experienced much extinction. Again, representatives of the three living orders of amphibians survived.
The most recent mass extinction was at the Cretaceous–Tertiary boundary (≈65 Mya); 16% of families, 47% of genera of marine organisms, and 18% of vertebrate families were lost. Most notable was the disappearance of nonavian dinosaurs. Causes continue to be debated. Leading candidates include diverse climatic changes (e.g., temperature increases in deep seas) resulting from volcanic floods in India (Deccan Traps) and consequences of a giant asteroid impact in the Gulf of Mexico (1, 2). Not only did all three orders of amphibians again escape extinction, but many, if not all, families and even a number of extant amphibian genera survived (8).
A SIXTH EXTINCTION?
The possibility that a sixth mass extinction spasm is upon us has received much attention (9). Substantial evidence suggests that an extinction event is underway.
When did the current extinction event begin? A period of climatic oscillations that began about 1 Mya, during the Pleistocene, was characterized by glaciations alternating with episodes of glacial melting (10). The oscillations led to warming and cooling that impacted many taxa. The current episode of global warming can be considered an extreme and extended interglacial period; however, most geologists treat this period as a separate epoch, the Holocene, which began ≈11,000 years ago at the end of the last glaciation. The Holocene extinctions were greater than occurred in the Pleistocene, especially with respect to large terrestrial vertebrates. As in previous extinction events, climate is thought to have played an important role, but humans may have had compounding effects. The overkill hypothesis (11) envisions these extinctions as being directly human-related. Many extinctions occurred at the end of the Pleistocene, when human impacts were first manifest in North America, in particular, and during the early Holocene. Because naive prey were largely eliminated, extinction rates decreased. Extinctions were less profound in Africa, where humans and large mammals coevolved. Most currently threatened mammals are suffering from the effects of range reduction and the introduction of exotic species (12). In contrast to the overkill hypothesis, an alternative explanation for the early mammalian extinctions is that human-mediated infectious diseases were responsible (13).
Many scientists think that we are just now entering a profound spasm of extinction and that one of its main causes is global climate change (14–16). Furthermore, both global climate change and many other factors (e.g., habitat destruction and modification) responsible for extinction events are directly related to activities of humans. In late 2007, there were 41,415 species on the International Union for Conservation of Nature Red List, of which 16,306 are threatened with extinction; 785 are already extinct (17). Among the groups most affected by the current extinction crisis are the amphibians.
AMPHIBIANS IN CRISIS
Amphibians have received much attention during the last two decades because of a now-general understanding that a larger proportion of amphibian species are at risk of extinction than those of any other taxon (18). Why this should be has perplexed amphibian specialists. A large number of factors have been implicated, including most prominently habitat destruction and epidemics of infectious disease (19); global warming also has been invoked as a contributing factor (20). What makes the amphibian case so compelling is the fact that amphibians are long-term survivors that have persisted through the last four mass extinctions.
Paradoxically, although amphibians have proven themselves to be survivors in the past, there are reasons for thinking that they might be vulnerable to current environmental challenges and, hence, serve as multipurpose sentinels of environmental health. The typical life cycle of a frog involves aquatic development of eggs and larvae and terrestrial activity as adults, thus exposing them to a wide range of environments. Frog larvae are typically herbivores, whereas adults are carnivores, thus exposing them to a wide diversity of food, predators, and parasites. Amphibians have moist skin, and cutaneous respiration is more important than respiration by lungs. The moist, well vascularized skin places them in intimate contact with their environment. One might expect them to be vulnerable to changes in water or air quality resulting from diverse pollutants. Amphibians are thermal-conformers, thus making them sensitive to environmental temperature changes, which may be especially important for tropical montane (e.g., cloud forest) species that have experienced little temperature variation. Such species may have little acclimation ability in rapidly changing thermal regimes. In general, amphibians have small geographic ranges, but this is accentuated in most terrestrial species (the majority of salamanders; a large proportion of frog species also fit this category) that develop directly from terrestrial eggs that have no free-living larval stage. These small ranges make them especially vulnerable to habitat changes that might result from either direct or indirect human activities.
Living amphibians (Class Amphibia, Subclass Lissamphibia) include frogs (Order Anura, ≈5,600 currently recognized species), salamanders (Order Caudata, ≈570 species), and caecilians (Order Gymnophiona, ≈175 species) (21). Most information concerning declines and extinctions has come from studies of frogs, which are the most numerous and by far the most widely distributed of living amphibians. Salamanders facing extinctions are centered in Middle America. Caecilians are the least well known; little information on their status with respect to extinction threats exists (18).
Amphibians are not distributed evenly around the world. Frogs and caecilians thrive in tropical regions (Fig. 1). Whereas caecilians do not occur outside the tropical zone, frogs extend northward even into the Arctic zone and southward to the southern tips of Africa and South America. Salamanders are mainly residents of the North Temperate zone, but one subclade (Bolitoglossini) of the largest family (Plethodontidae) of salamanders has radiated adaptively in the American tropics. The bolitoglossine salamanders comprise nearly 40% of living species of salamanders; ≈80% of bolitoglossines occur in Middle America, with only a few species ranging south of the equator.
Global amphibian species diversity by country visualized using density-equalizing cartograms. Country size is distorted in proportion to the total number of amphibian species occurring in each country relative to its size. (Inset) Baseline world map. Brazil (789 species) and Colombia (642) have the largest number of species. China (335) has the largest number of species in the Old World. The Democratic Republic of the Congo (215) has the largest number from continental Africa. However, 239 species are recorded from Madagascar. Australia has 225 species, and Papua New Guinea has 289. In North America, Mexico has the largest number of species (357). There are 291 species in the United States. Prepared by M. Koo.
The New World tropics have far more amphibians than anywhere else. Fig. 1 shows the number of species in relation to the size of countries (all data from ref. 21). The Global Amphibian Assessment completed its first round of evaluating the status of all then-recognized species in 2004 (18), finding 32.5% of the known species of amphibians to be “globally threatened” by using the established top three categories of threat of extinction (i.e., Vulnerable, Endangered, or Critically Endangered); 43% of species have declining populations (17). In general, greater numbers as well as proportions of species are at risk in tropical countries (e.g., Sri Lanka with 107 species, most at risk; nontropical New Zealand has an equivalent proportion, but has only 7 species) (Fig. 2). Updates from the Global Amphibian Assessment are ongoing and show that, although new species described since 2004 are mostly too poorly known to be assessed, >20% of analyzed species are in the top three categories of threat (22). Species from montane tropical regions, especially those associated with stream or streamside habitats, are most likely to be severely threatened.
Percentage of amphibian fauna in each country in the top three categories of threat (Critically Endangered, Endangered, and Threatened) (22). (Inset) Baseline world map. Visualization based on density-equalizing cartograms prepared by M. Koo.
We present a case study from our own work to explore the reasons underlying declines and extinctions of amphibians.
RANA IN THE SIERRA NEVADA OF CALIFORNIA
One of the most intensively studied examples of amphibian declines comes from the Sierra Nevada of California. The mountain range spans thousands of square kilometers of roadless habitat, most of which is designated as National Park and Forest Service Wilderness Areas, the most highly protected status allowable under U.S. law. The range contains thousands of high-elevation (1,500- to 4,200-m) alpine lakes, as well as streams and meadows, that until recently harbored large amphibian populations. Biological surveys conducted nearly a century ago by Grinnell and Storer (23) reported that amphibians were the most abundant vertebrates in the high Sierra Nevada. Because large numbers of specimens were collected from well documented localities by these early workers, the surveys provide a foundation on which current distributions can be compared. Of the seven amphibian species that occur >1,500 m in the Sierra Nevada, five (Hydromantes playcephalus, Bufo boreas, B. canorus, Rana muscosa, and R. sierrae) are threatened. The best studied are the species in the family Ranidae and include the Sierra Nevada Yellow-legged Frog (R. sierrae) and Southern Yellow-legged Frog (R. muscosa) (24). In the1980s, field biologists became aware that populations were disappearing (25), but the extent of the problem was not fully appreciated until an extensive resurvey of the Grinnell-Storer (23) sites disclosed dramatic losses (26). Especially alarming was the discovery that frogs had disappeared from 32% of the historical sites in Yosemite National Park. Furthermore, populations in most remaining sites had been reduced to a few individuals.
The yellow-legged frogs, which had been nearly ubiquitous in high-elevation sites in the early 1980s, are ideal subjects for ecological study. Their diurnal habits and their use of relatively simple and exposed alpine habitats make them readily visible and easy to capture. Typically these frogs occurred in large populations, and rarely were they found >2 m from the shores of ponds, lakes, and streams. Censuses throughout the Sierra Nevada began in the early 1990s and intensified in this century. Although most of the frog habitat in this large mountain range is protected in national parks and wilderness areas, yellow-legged frogs are now documented to have disappeared from >90% of their historic range during the last several decades (24). The most recent assessment lists them as Critically Endangered (18). Factors implicated in the declines include introduced predatory trout (27), disease (28), and air pollution (29, 30). Experiments that extirpated introduced trout led to rapid recovery of frog populations (31). Thus, for a time, there was hope that, simply by removing introduced trout, frog populations would persist and eventually spread back into formerly occupied habitat. Curiously, multiple attempts at reintroduction in the more western parts of the range clearly failed (32). Hundreds of dead frogs were encountered at both reintroduction and many other sites in the western part of the range (28), and it became apparent that predation was not the only factor affecting the frogs’ survival.
In 2001, chytridiomycosis, a disease of amphibians caused by a newly discovered pathogenic fungus [Batrachochytrium dendrobatidis (Bd)] (33) was detected in the Sierra Nevada (34). Subsequently, a retrospective study disclosed that Bd was found on eight frogs (R. muscosa, wrongly identified as R. boylii) collected on the west edge of Sequoia and Kings Canyon National Parks in 1975 (35). Infected tadpoles of these species are not killed by Bd. When tadpoles metamorphose, the juveniles became reinfected and usually die (36). However, tadpoles of yellow-legged frogs in the high Sierra Nevada live for 2 to 4 years, so even if adults and juveniles die, there is a chance that some individuals might survive if they can avoid reinfection after metamorphosis.
The disease is peculiar in many ways (37, 38). Pathogenicity is unusual for chytrid fungi, and Bd is the first chytrid known to infect vertebrates. The pathogen, found only on amphibians, apparently lives on keratin, present in tadpoles on the external mouth parts and in adults in the outer layer of the skin. The life cycle includes a sporangium in the skin, which sheds flagellated zoospores outside of the host. The zoospores then infect a new host or reinfect the original host, establishing new sporangia and completing the asexual life cycle. Sexual reproduction, seen in other chytrids, is unknown in Bd (39). Much remains to be learned about the organism (38). For example, despite its aquatic life cycle, Bd has been found on fully terrestrial species of amphibians that never enter water, and the role of zoospores in these forms is uncertain. No resting stage has been found, and no alternative hosts are known. Vectors have not been identified. It is relatively easy to rid a healthy frog of the fungus by using standard fungicides (40). Yet the fungus is surprisingly virulent. Finally, and importantly, how the fungus causes death is not clear, although it is thought to interfere with oxygen exchange and osmoregulation (41).
With associates, we have been studying frog populations in alpine watersheds within Yosemite, Sequoia, and Kings Canyon National Parks for over a decade. We recently showed that yellow-legged frogs are genetically diverse (24). Mitochondrial DNA sequence data identified six geographically distinct haplotype clades in the two species of frogs, and we recommended that these clades be used to define conservation goals. Population extinctions, based on historical records, ranged from 91.3% to 98.1% in each of the six clades, so challenges for conservation are daunting. In the last 5 years, we have documented mass die-offs (Fig. 3) and the collapse of populations due to chytridiomycosis outbreaks (28). Although the mechanism of spread is unknown, it may involve movements of adult frogs among lakes within basins or possibly movements of a common, more vagile, and terrestrial frog, Pseudacris regilla (on which Bd has been detected), ahead of the Rana infection wave. Mammals, birds, or insects also are possible vectors. We have followed movements of R. muscosa and R. sierrae using pit tags and radio tracking from 1998 to 2002 (42), and we believe that movement between local populations may be spreading the disease. The environment in this area (2,500–3,300 m) is harsh for amphibians, with isolated ponds separated by inhospitable solid granite that lacks vegetation. Small streams join many of the lakes in each basin. The maximum movement of frogs, (≈400 m) was in and near streams; most movements are <300 m. Our results are compatible with those of another study (43), which included a report of a single overland movement event. If chytridiomycosis sweeps through the Sierra Nevada the way it has through Central America (44), then population and metapopulation extinctions may be a continuing trend; we may be on the verge of losing both species.
Distribution of the critically endangered yellow-legged frogs in California. Chytridiomycosis outbreaks have had devastating effects (Rana muscosa photographed in Sixty Lake Basin, August 15, 2006).
It might be possible to arrest an epidemic. Laboratory treatments have shown that infected animals can be cleared of infection within days (40); if the dynamics of the disease can be altered or if animals can survive long enough to mount an immunological defense, then survival might be possible. Survival of infected frogs after an apparent outbreak has been seen in Australia (45), but is unknown in the Sierra Nevada frogs. The yellow-legged frogs of the Sierra Nevada are an ideal species in which to test this because they live in discreet habitat patches, are relatively easy to capture, and are highly philopatric.
COMMON THEMES IN AMPHIBIAN DECLINES
In the early 1990s, there was considerable debate about whether amphibians were in general decline or only local fluctuations in population densities were involved (46, 47). A definitive 5-year study that involved daily monitoring of a large amphibian fauna at the Monteverde Cloud Forest Preserve in Costa Rica showed that 40% (20 species of frogs) of the species had been lost (48). These instances involved some extraordinary species, such as the spectacularly colored Golden Toad (Bufo periglenes) and the Harlequin Frog (Atelopus varius). Particularly striking about this case is the highly protected status of the Preserve, so habitat destruction, the most common reason for species disappearances in general, can be excluded. The start of this decline was pinpointed to the late 1980s. At about the same time, disappearances of species from protected areas in the Australian wet tropics were recorded (49). Both species of the unique gastric brooding frogs from Australia (Rheobatrachus) disappeared. Declines in other parts of the world included most species of the generally montane, diurnal frogs of the genus Atelopus from South and lower Central America, and species of Bufo and Rana from the Sierra Nevada of California (20, 25, 44). At first all of these declines were enigmatic, but eventually two primary causal factors emerged: the infectious disease chytridiomycosis and global warming (20, 44).
Chytridiomycosis was detected almost simultaneously in Costa Rica and Australia (33). From the beginning, it was perceived as a disease with devastating consequences. It quickly swept through Costa Rica and Panama, leaving massive declines and local extinctions in its wake (44). More than half of the amphibian species in lower montane forest habitats suffered declines on the order of 80%, and several disappeared. This extinction event had been predicted on the assumption that chytridiomycosis would continue its sweep southward from Monteverde, in northwestern Costa Rica (see below), to El Cope in central Panama (44). Attention is now focused on eastern Panama and northwestern Colombia, where chythridiomycosis has yet not had evident impact.
Carcasses of animals from the Monteverde extinction event are not available, and it is not known whether Bd was responsible for frog deaths. However, Bd has been detected in many preserved specimens that were collected at different elevations along an altitudinal transect in Braulio Carrillo National Park in 1986 (50). The park is in northern Costa Rica ≈100 km southeast of Monteverde. Given the high prevalence of Bd in the specimens surveyed, it seems reasonable to assume that Bd also was present at Monteverde. Of course, there are many more species present in tropical areas (67 at El Cope, Panama) (44) than in the Sierra Nevada (seven at high elevations, but three most commonly, only two of which are aquatic), and hence there are many more opportunities for the spread of Bd among tropical species. The average moisture content of the air in the tropical environments is doubtless much higher, on average, in Central America than in the Sierra Nevada, where a characteristic dry summer rainfall pattern prevails and where there is no forest canopy because of the altitude and substrate. Although we do not know the mechanism of spread, conditions in Central America appear more suitable for the spread of an aquatic fungus.
Amphibians tend to have broader ranges in temperate regions than in the tropics. Despite many population extinctions in temperate regions, there have been few extinctions. Accordingly, the tropical species of amphibians are more at risk, but not just because of their typically small geographic ranges. Because they occur in rich, multispecies communities, the species become infected simultaneously.
Climate change has been implicated in declines since the documentation of disappearances at Monteverde (51, 52). Unusual weather conditions were initially implicated with amphibian declines. Large increases in average tropical air and sea surface temperatures were associated with El Niño events in the late 1980s; substantial warming had already occurred since the early 1970s. Temperature increases were correlated with increases in the height at which clouds formed at Monteverde and consequent reductions in the deposition of mist and cloud water critical for maintenance of cloud forest conditions during the dry season (20). Simulations using global climate models showed that greenhouse warming could have the effect of raising the cloud line by as much as 500 m at Monteverde during the dry season (20, 52).
A more general effect of climate change has been proposed for the disappearance of 100 species of tropical montane frogs of the genus Atelopus, which is widespread in southern Central and northern South America. A detailed correlational analysis revealed that ≈80 species were last seen immediately after a warm year (20). Several species disappeared from Ecuador during 1987–1988, which included the most extreme combination of dry and warm conditions in 90 years (53). Authors of this article document that the mean annual temperature in the Ecuadorian Andes has increased by ≈2°C during the last century.
Pounds and coworkers (20) hypothesized that climate change, precipitation, and increased temperature have acted synergistically in favor of the growth of the infectious chytrid fungus. They argue that global warming has shifted temperatures closer to the presumed optimal conditions for B. dendrobatidis at Monteverde and the other intermediate elevation areas of the Central and South American highlands, where most of the extinctions of Atelopus have occurred. Warming has increased cloud cover in these areas, which had the effect of elevating already higher nighttime temperatures, thus favoring fungal growth. The hypothesis has yet to be tested.
IS GLOBAL WARMING A REAL EXTINCTION THREAT?
The Intergovernmental Panel on Climate Change (IPCC) reached consensus that climate change is happening and that it is largely related to human activities (15). Estimates of global warming during the next century vary, but generally fall in the range of 2°C to 4°C, whereas rises as high as 7°C are projected for much of the United States and Europe, with even higher temperatures expected in northern Eurasia, Canada, and Alaska (15). Such rises would have devastating effects on narrowly distributed montane species, such as cloud forest and mountain-top salamanders and frogs in Middle and South America. The physiology of ectotherms such as amphibians and their ability to acclimate also are important considerations for these species (54). With climate change (already 2°C changes in temperature have been recorded in montane Ecuador) (53), altitudinal limits of plant and animal communities will shift upward and amphibians must either move with them or acclimate until adaptation occurs. Even small increases in temperature lead to significant metabolic depression in montane salamanders (55). Impacts of the different warming scenarios are all dramatic and severe (see fig. TS.6 in ref. 15). The first event predicted by the IPCC panel, “Amphibian Extinctions Increasing on Mountains,” is now an empirical fact.
In previous publications, we showed that many tropical plethodontid salamanders have very narrow altitudinal limits and are often restricted to single mountains or local mountain ranges (56). With few exceptions, species found above 1,500–2,000 m have narrow distributional limits. We have surveyed extensively a mountainous segment of eastern Mexico from the vicinity of Cerro Cofre de Perote (≈4,000 m) in central western Veracruz in the north to Cerro Pelon (≈3,000 m) in northern Oaxaca in the south. These two peaks, separated by ≈280 km, lie along the eastern crest of the Sierra Madre Oriental, a nearly continuous range that is broken only by Rio Santo Domingo (Fig. 4). Otherwise the crest lies above 1,500 m, with many peaks that rise to ≈2,000 m or higher. There are 18 species of plethodontids on both Cofre de Perote and Pelon, but only two species—widespread lowland members of Bolitoglossa—are shared. To determine the geographical limits of the other species, we have been surveying the entire crest area since the 1970s. We have learned that most of the species on each mountain are endemic to it. When we searched in the intervening region, expecting to expand the known distributional ranges for different species, we instead discovered numerous undescribed species (many since named) almost every time we explored an isolated peak at >2,000 m. On a single short trip just 5 years ago to the Sierra de Mazateca, north of the Rio Santo Domingo (Fig. 4), we discovered two new species of Pseudoeurycea and at least one new species of Thorius (57). We suspect that many species disappeared without ever having been discovered because the area is heavily populated and has experienced extensive habitat modification. Furthermore, the newly discovered species are endangered and survive in what appear to be suboptimal, disturbed habitats or in small fragments of forest. The majority of species along altitudinal transects in this area are found at >2,000 m in cloud forests that are being forced upward by global warming. On Cerro Pelon, eight of the species are found only at >2,200 m, and six of them range to the top of the mountain. Global warming threatens to force them off the mountain and into extinction.
A diagrammatic profile of the Sierra Madre Oriental from north-central Veracruz to northern Oaxaca, Mexico. The range extends in a generally north-northeast to south-southwest direction, but the section from Cofre de Perote to Loma Alta extends mainly east-northeast and has been straightened. This mountain system is home to 17 described and several unnamed species of Minute Salamanders, genus Thorius. Most of the species are clustered between 1,500 and 3,000 m. All of the species that have been evaluated are Endangered (E) or Critically Endangered (CR) and at risk of extinction, and three have been found so infrequently that they are categorized as Data Deficient (DD) (22).
The section of the Sierra Madre Oriental we have been studying is home to 17 named and 3–5 as yet unnamed species of the plethodontid salamander genus Thorius, the Minute Salamanders. All but four of these species occur exclusively at >2,000 m, often on mountains that rise only a little above that level. Of the 17 named species, 11 are listed as Endangered and 3 are listed as Critically Endangered; the remaining 4 species are so rare and poorly known that they can only be listed as Data Deficient (Fig. 4) (18). We consider this region to be a hot spot of extinction, and yet it is still very incompletely known. Based on our studies of altitudinal transects elsewhere in Middle America, we expect that the situation we have described for eastern Mexico is typical of mountainous parts of the entire region.
WHAT WILL WE LOSE?
The amphibians at greatest risk of extinction are likely to be those with relatively few populations in areas undergoing rapid habitat conversion because of human activities. Populations that are already reduced in size are especially susceptible to other stressors, such as introduced species and disease. Tropical montane species are at special risk because of global warming. These already stressed species, reduced to a few populations, also are likely to be hit hardest by Bd. However, a paradoxical fact is that new species of amphibians are being described at an unprecedented rate. In 1985, the first comprehensive account of all amphibian species reported ≈4,000 species (58). That number has now risen to >6,300, and species are being named at a rate exceeding 2% per year. Some of these species are cryptic forms that were found as a result of molecular systematic studies, but the vast majority are morphologically distinctive species mainly from tropical regions (Fig. 5). These biologically unique species often have been found as a byproduct of the heightened interest in amphibians and consequent field research. Field surveys in still relatively unstudied parts of the world (e.g., New Guinea and nearby islands, Madagascar) have resulted in many new discoveries. Among the most spectacular discoveries during this decade are a frog from India that is so distinct that it was placed in a new family (59) and a salamander from South Korea that is the only member of the Plethodontidae from Asia (60). It is impossible to know what has been overlooked or has already been lost to extinction, but there is every reason to think that the losses have been substantial.
Distribution of species of amphibians discovered and named during the period 2004–2007. Color scale bar indicates number of new species per country. (Inset) Baseline world map. Visualization is based on density-equalizing cartograms prepared by M. Koo.
The rate of extinction of amphibians is truly startling. A recent study estimates that current rates of extinction are 211 times the background extinction rate for amphibians, and rates would be as high as 25,000–45,000 times greater if all of the currently threatened species go extinct (61).
Despite these alarming estimates, amphibians are apparently doing very well in many parts of the world, and many thrive in landscapes heavily modified by human activities. Species such as the Cane Toad (Bufo marinus), the American Bullfrog (Rana catesbieana), and the Clawed Frog (Xenopus laevis) have proven to be potent invasive species, and they have not yet been shown to be afflicted by chytridiomycosis. Attempts are being made to mitigate anticipated losses of amphibian species. Promising research on bacterial skin symbionts of amphibians suggests that they may have antifungal properties (62, 63), possibly opening pathways for research on changing the outcomes of fungal attacks. Local extinctions have been so profound and widespread in Panama that a major initiative has been launched to promote in situ as well as ex situ captive breeding programs. Species will be maintained in captivity until solutions to problems such as chytridiomycosis, local habitat destruction, or others can be mitigated, at which time reintroduction programs will be developed (64). Although amphibians are suffering declines and extinctions, we predict that at least some frogs, salamanders, and caecilians will survive the current extinction event on their own or with help, even as their ancestors survived the four preceding mass extinctions.
WHAT IS THE PRINCIPAL CAUSE OF THE PRESENT EXTINCTION SPASM?
Human activities are associated directly or indirectly with nearly every aspect of the current extinction spasm. The sheer magnitude of the human population has profound implications because of the demands placed on the environment. Population growth, which has increased so dramatically since industrialization, is connected to nearly every aspect of the current extinction event. Amphibians may be taken as a case study for terrestrial organisms. They have been severely impacted by habitat modification and destruction, which frequently has been accompanied by use of fertilizers and pesticides (65). In addition, many other pollutants that have negative effects on amphibians are byproducts of human activities. Humans have been direct or indirect agents for the introduction of exotic organisms. Furthermore, with the expansion of human populations into new habitats, new infectious diseases have emerged that have real or potential consequences, not only for humans, but also for many other taxa, such as the case of Bd and amphibians (66). Perhaps the most profound impact is the human role in climate change, the effects of which may have been relatively small so far, but which will shortly be dramatic (e.g., in the sea) (16). Research building on the Global Amphibian Assessment database (18) showed that many factors are contributing to the global extinctions and declines of amphibians in addition to disease. Extrinsic forces, such as global warming and increased climatic variability, are increasing the susceptibility of high-risk species (those with small geographic ranges, low fecundity, and specialized habitats) (67). Multiple factors acting synergistically are contributing to the loss of amphibians. But we can be sure that behind all of these activities is one weedy species, Homo sapiens, which has unwittingly achieved the ability to directly affect its own fate and that of most of the other species on this planet. It is an intelligent species that potentially has the capability of exercising necessary controls on the direction, speed, and intensity of factors related to the extinction crisis. Education and changes of political direction take time that we do not have, and political leadership to date has been ineffective largely because of so many competing, short-term demands. A primary message from the amphibians, other organisms, and environments, such as the oceans, is that little time remains to stave off mass extinctions, if it is possible at all.
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