Choose Your Top Poison

By Carl Zimmer | April 21, 2009 12:46 pm

Yesterday I wrote about how conservation biologists are debating the value of moving species to protect them from climate-change-driven extinction. As a follow-up (or an antidote), check out “Blood for no oil: Our obsession with climate change is killing off animals left and right.” in Slate.

Brendan Borrell, biologist turned journalist, argues that climate change poses a genuine threat to biodiversity, but “it does not come close to the immediate, irreparable damage caused by the destruction of habitat.” Good old chainsaws are still the big danger, he argues, because two-thirds of the world’s biodiversity is in the tropics, where deforestation is happening fast and the effects of climate change may not be as dramatic as they’ll be closer to the poles.

I wrote about similar skepticism in 2007 in an article for Science. Some critics point out that we have yet to discover some of the important factors in how species respond to shifting climates. Borrell also points out that one of the most famous examples of an extinction caused by climate change–the golden toad of Costa Rica–looks now as if it may have had other manmade causes (see this study, for example). And in trying to cope with the dangers of climate change, Borrell observes that we’ve already seen some disasters. The feel-good desire for biofuels is driving a catastrophic loss of rainforest in Indonesia for palm oil plantations. And, as I point out in my piece yesterday in Yale Environment 360, moving species to save them from climate change driven extinction could, theoretically at least, turn them into dangerous invasives.

That being said, I think Borrell’s argument is thin. Maybe he didn’t have enough room to make his full argument in a slender Slate piece. But the evidence he lays out doesn’t justify his big claims.

Borrell claims, for example, that the most dire projections about extinctions from global warming should not be trusted because they came from studies that ignored the tropics, which are home to so much biodiversity. But it’s not as if there was some sort of anti-tropics malice involved; it’s just a historical fact that the most careful studies of range shifts due to climate have been done in places like England and the United States. And the same researchers who issued those warnings went on to look at climate change in the world’s biodiversity hotspots, many of which are in the tropics. They concluded that “estimated global-warming-induced rates of species extinctions in tropical hotspots in some cases exceeded those due to deforestation, supporting suggestions that global warming is one of the most serious threats to the planet’s biodiversity.” Borrell may not like this study, but he shouldn’t act as if it doesn’t even exist.

To make the case for a minor risk from climate change, Borrell cites a 2007 study that indicates that birds will suffer many more extinctions from the destruction of habitat than climate change. It’s certainly an important study from leading experts. But it was not the last word on the subject. For example, in 2008 researchers developed a model for bird extinctions that took into account the elevation at which the birds live. (A warmer climate may push some species upslope.)  They concluded that climate may hit birds (including tropical birds) hard. They warn of 400 to 550 extinctions and 2150 additional species put at risk of extinction by 2100.

Stuart Pimm, a conservation biologist at Duke University, reviewed both these bird studies in a piece called “Climate Change or Habitat Loss — Which Will Kill More Species?” He concluded, “Large numbers of species, thus-far largely unaffected by human actions, are in danger of extinction from climate change.”

There are actually a lot of other potential threats that global warming poses to biodiversity. For example, the carbon dioxide we’re pumping into the air is also acidifying the oceans. Coral reefs may be endangered as a result–along with the vast biodiversity they shelter. But Borrell doesn’t even mention these factors.

After trying (unsuccessfully, I think) to downplay the risk of climate change, Borrell then fails to persuade me that concerns about climate change are actually causing extinctions. While the palm oil plantations have been disastrous, it’s not fair to imply that conservation biologists who worry about climate change were responsible for them. I have yet to encounter somebody who says that we don’t have to worry about deforestation and need to push all conservation efforts into dealing with climate change. And Borrell doesn’t present any examples.

Deforestation is clearly harming biodiversity right now, and there’s a fair amount of evidence that global warming will have an important effect too in decades to come, although there’s lots of uncertainty about just how big a role it will have. And the two forces will not be working independently: climate change may make some species more vulnerable to the loss of their habitat, and vice versa. What’s more, there may be some solutions that address both threats at the same time, such as creating networks of preserves. Not only do endangered species get the habitat where they are now; they potentially get a way to move to better habitat if the climate makes their current one unsuitable.

Casting debates about extinctions in simple either-or terms may be good for attracting web traffic, but nature is more complex. And our solutions should be too.

Update 4/27: Brendan is back from South Africa and has left a lengthy–and well-considered–comment below. Definitely check it out. I still have some problems with his argument, but it’s so much more satisfying to discuss these issues with somebody who takes the scientific research seriously than someone who just wants to quote-mine.


Comments (16)

  1. Mike Lemonick

    Borrell misses an even bigger point: since deforestation, with the resulting loss of biodiversity is a major CAUSE of climate change, it’s kind of clueless to talk about worrying about one or the other.

  2. I agree with all that has been said. I also am willing to give Borrell the benefit of the doubt and say he had a more nuanced argument in mind and even that he may not have loved the title the editors gave him.
    But one part rings true and that is our turning away from habitat loss. Environmentalism is all about defining priorities. While few are willing to encourage ignoring one environmental problem altogether, they do push priorities around. If that is the case, then in the hurly burly of battling climate change, let’s not forget about the “good old fashioned chainsaw.”

  3. Borrell could have made his point better if he had pointed that habitat destruction creates sick, fragmented ecosystems subject to total collapse, and that healthy ecosystems might be able to deal with the stresses of climate change. As a marine scientist I immediately think of coral reefs – while polluted, overfished, trampled coral reefs have no hope of surviving climate change, happy protected reefs with lots of grazers and predators might make it.

    However, I understand his frustration. Science & conservation funding IS a zero-sum game – there’s only so much money to go around. And habitat destruction frequently has relatively simple & cheap solutions (parks, conservation easements) compared to climate change (overhaul world economy).

  4. 220mya

    Borell should read Heatstroke by Tony Barnosky. It lays out a pretty effective argument about why protecting habitat may be irrelevant in the long run if we don’t take on climate change.

  5. Don

    Ecologists and environmentalists are well aware that climate change and habitat destruction are not independent but synergistic contributors to species loss (as Carl indicates in his piece). Borrell could have written a piece that pointed this out, but it would have had less less journalistic zip and would have appealed to more thoughtful, but fewer, readers. I see that his comments at SLATE are dominated by anti-global warming types and Al Gore bashing. We should keep our eye on Borrell; he may be an embryonic Lomborg.

  6. CG

    These two problems are intimately connected and not remotely dichotomous. Animal populations respond to climate change (past and contemporary) by genetically adapting, going extinct, or range movements. Rapid evolution occurs but is rare. We shouldn’t count on populations rapidly adapting to climate change; we can only hope that some do. We can all agree that extinction is undesirable. There are many, many examples of populations following their climate niches in response to shifting climates and persisting in climatically suitable refuges. These range movements will be much more difficult now than in the past due to massive habitat loss and fragmentation. The root cause of these impending extinctions is climate change and emissions need to be curbed. However, due to the inertia of climate change even if emissions were cut today, it seems that ensuring sufficient and connected habitat might be the most appropriate conservation approach to climate change.

    See. Problems are one and the same. Reducing habitat loss can be justified from multiple angles.


  7. To echo Mike’s comment: the land use use patterns responsible for habitat degradation (including deforestation, overgrazing, industrial ag, urban development etc.) generate greenhouse gases and tend to disrupt natural carbon sinks (though by how much is still a matter of major scientific debate). Creating a false dichotomy between habitat loss and climate change is not likely to be helpful, anymore than agonizing over whether I should stop smoking or start exercising.

  8. We’re discussing this very issue on my blog, SouthernFriedScience. I’d be curious to hear what you have to say, Carl.

  9. There was an article, not front page, in the NYT roughly two months ago that noted an astonishing resurgence in Central America tropical rainforest acreage over the last ten or so years. IIRC, it was due to more productive farming leading to fewer cultivated acres.

    Just like has happened in the US over the last century.

    Also, you might have heard the FAA is now releasing aircraft bird-strike information. Over the last five years, flight hours have been, at best, flat.

    But reported bird strikes are way up. Why might that be?

  10. Sorry for the delayed reply, but I’ve been traveling in one of those spectacular hotspots that is bound to get nailed by climate change — Africa’s Cape Floral Region aka the Fynbos. I’d like to thank Carl for the thoughtful critique, and tell Dr. Don Strong that calling me an “embryonic Lomborg” is a bit of a low blow! Yes, those denialists may be sending me creepy fan mail, but I’m hardly cut from the same cloth.

    As for Mike Lemonick’s comment: I do think these types of mental exercises are important. Battling climate change and saving biodiversity may not always be achieved via the same means and I think it is quite clear that they are often in conflict with one another. No doubt the conflicts will become more serious in the future and folks who care about biodiversity must think carefully about how they want to craft their policies and spend their dollars.

    I stand behind the central point of the Slate essay, but I’ll add a bit of nuance to the discussion here. We can all agree that in terms of extinctions, the effects climate change and habitat loss are of the same order of magnitude (i.e. factor of 10). But after taking a close look at worldwide estimates for species loss, my money is still on habitat loss.

    The two Con Bio papers that Zimmer cites deserve to be included in the debate, but they are hardly any more definitive than the analyses I mentioned in the Slate piece. For instance, in their discussion, Malcolm et al write, “The deforestation average [extinction rate] was 0.24%, which was more than twice as great as our overall SAR-based [climate] average of 0.11%. Under certain scenarios, however, the estimates associated with climate change exceeded those with deforestation.” The scenarios they describe in which climate change beats out habitat loss are, frankly, far-fetched.

    The Sekercioglu et al (2008) paper is a nice look at how climate is going to force many bird species up mountains — and sometimes mountains don’t go high enough. But its not the “last word” on the bird extinction story, and the authors even write in the abstract “our climate-induced extinction estimates are broadly similar to those of bird species at risk from other factors, but these estimates largely involve different sets of species.” I also know from speaking with Jetz that he’s not about to temper his own findings, which fall strongly in favor of habitat loss as the biggest extinction culprit.

    So where does this leave us? Eager for more data, certainly. But it should also be a reminder that “old school” conservation strategies still matter in a very big way, which is something I think many folks (and eco-conscious companies) have forgotten with all the talk of carbon offsets. Yes, we may need to tweak our land protection schemes to take into account shifting biomes, and we may want to keep north-south corridors intact for climate-related migration. But you can still burnish your environmental credibility by saving a chunk of land.

  11. Brendan: Thanks for leaving a comment, and thanks for focusing on the science (as opposed to inciting flame wars or engaging in disingenuous quote-mining the way these conversations sometimes go). As I indicated in my post above, it’s very clear that projections of extinctions due to global warming have a fairly wide range of uncertainty at the moment. Global warming might well prove to cause fewer extinctions than habitat destruction. But what evidence is there that the new concern for global warming has reduced efforts to save habitat? We’ve long known that biodiversity can be threatened by combinations of factors, such as pollution and invasive species, but conservation biologists have not simply picked out a number one threat and ignored the rest. Are they really doing so now?

  12. I think there are couple of really interesting issues here. First off, I agree that there are only a handful cases where climate change policy has negatively influenced biodiversity — the oil palm plantations I cited before are the most vivid example. But consider the effect of the alternative energy infrastructure — wind farms, solar power, and dams — on biodiversity. Birds and bats are slamming into turbine blades. Rivers are being choked for hydropower. I don’t know the numbers on these, but someone should do the calculations before they start crafting policy. Right?

    While a good conservationist will hedge their bets, they should generally be looking to save, for instance, the greatest number of species per dollar. And from the consumer’s perspective, a dollar spent on a hybrid vehicle may not be spent on a year’s supply of shade grown coffee.

    The verdict on habitat loss vs. climate may not be in, but judging from these and other comments I’ve read — that alone is news to people.

  13. Carl,

    I believe you’re correct to point out that conservation biologists are not suddenly placing all their chips on climate change as the number one threat.

    But generally speaking, current environmental debates both in the blogosphere and in mainstream media center around climate change politics and policy. I well understand that this reflects the topicality of climate change in the news–from Copenhagen to George Will to recent congressional hearings and proposed legislation.

    Still, I wonder if Brendan has put his finger on something larger, a whole-scale shift in the way ecologists and environmentalists view threats to ecosystems and biodiversity. Of course this is not quantifiable. (Time for a survey?)

    So while there may be, as Brendan admits, “only a handful of cases where climate change policy has negatively influenced biodiversity,” what’s more notable to me is how the media and many environmentalists have taken to viewing big, longstanding ecological issues, as I wrote here (, “through a climate change lens.”

    Lastly, Brendan, I doubt you’ll get much traction with your cost-benefit argument. In fact, it’ll probably only reinforce the whacky idea advanced by some, that you’re an “embryonic Lomborg.”


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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.


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