How Worrysome is Habitat Loss?

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Razib’s link to the discovery of a new mammal species in Madagascar makes the following point:

This species is probably the carnivore with one of the smallest ranges in the world, and likely to be one of the most threatened. The Lac Alaotra wetlands are under considerable pressure, and only urgent conservation work to make this species a flagship for conservation will prevent its extinction.

In the case of this particular species, that makes sense. This cat/rat looking thing appears to inhabit a threatened microclimate, and so faces a high risk of extinction.

You hear statements like this all of the time, and not always with respect to animals inhabiting odd niches. For instance, we are told that ongoing destruction of the Amazon jungle is a bad thing, as it will result in the extinction of more species.

I can’t vouch for anyone else, but my general impression in reading these press releases is that the relationship between habitat destruction and animal extinction is roughly linear. The more habitat we destroy, the more animals go extinct.

But if you think about this, that doesn’t quite make sense. A little more habitat destruction in, say, the Amazon will only erode a bit of jungle at the margins. Many animals should be able to migrate elsewhere; immobile species should still presumably have counterparts elsewhere. The only way a bit of additional logging will result in species extinction is if the extremity in question happens to be a unique microclimate home to species not found elsewhere.

And that seems to be an unreasonable model for the Amazon as a whole. To be sure, there may be certain areas of extreme biodiversity. But the Amazon jungle in general appears to be a reasonable homogenous environment, home to species that are reasonable spread out throughout the breadth of the area. To the extent species are not; this may reflect the presence of subspecies with little differentiation.

So I might instead expect habitat destruction to follow a non-linear pattern with respect to extinction in a unique environment. The first few trees you chop down may do little damage; the last several may eliminate the last survivors of a number of species.

Bjorn Lomborg has made similar arguments in the past — that large amounts of climate degradation (even in the range of 98-99 percent in the cases of Puerto Rico and the Eastern US) does not result in substantial animal extinctions, as long as some of the environment remains intact. Though I understand this remains a hotly disputed claim. The “unknown unknown” problem is bad here — we don’t know what species we are unknowingly killing off.

Meanwhile, the continued existence of various species even in the event of large natural climate disasters has been taken as evidence for the existence of various refuga that sustain whole ecosystems. For instance, glaciation appears to have disrupted various tropical forests, yet many animals survived — though with odd geographic dispersion patterns. The Toba volcano incident ~70k years ago resulted in massive layers of ash spread throughout South and South East Asia; yet the nearby Mentawai islands appear to have suffered no species loss. On the other hand, the introduction of humans (even when it does not result in massive environmental change) seems to be clearly linked to mass megafauna extinction. That would seem to suggest that “poaching” is far more destructive than cutting down a few trees.

If this is true, than ongoing destruction of Amazon, say, would not by itself be particularly troubling on the grounds of animal extinction. As long as we keep sufficiently large patches of the original jungle (or several different patches if we think there are a few microclimates), we should be able to keep the vast majority of the existing species. Even if some large fraction of the jungle is required to maintain a water cycle; presumably that is less forest than currently exists. We already recognize that some areas are more important conservation-wise than others (ie, coral reefs v. Russian forests). It may also be more important to maintain the existence of unique refuga environments (even in sharply reduced form) than the quantity of overall protected space.

But I realize this isn’t my area of speciality, and would like to hear back from more knowledgable commenters. Hopefully, you’re smart enough to see that my point isn’t to support rainforest destruction. I’d also like to hear back from anyone who can tell me how much a species is “worth.” While I’m opposed to animal extinction, I don’t have a good sense why other than a vague sense that it’s wrong, and a desire to see more National Geographic specials. Yes, I understand that there are some pharmaceutical applications, but that doesn’t seem to be a good reason by itself to maintain large non-human occupied environments. Tourism is more plausible, but presumably only redirects tourist dollars from some alternate attraction.


  1. preserving the amazon is essential, as it will prevent brazil from continuing along its “ascension graph”!

    though seriously, from what i recall it isn’t the amazon which is threatened. it’s only been singed on the margins. OTOH, the atlantic rainforests of brazil are almost gone, and southeast asian forest is rapidly being denuded.

  2. Here is an actual attempt to estimate extinction rates,

    Willis Eschenbach does it for birds (because they are so visible) and concludes that only 129 species have gone extinct in the last 100 years or so.

    There may be something wrong with the Macarthur/Wilson species/area theory.

    More importantly, Wilson’s own estimate of extinction rates was a top of the head guesstimate made in the middle of an interview and had no empirical basis.

    To my limited knowledge, Eschenbach’s attempt has not be duplicated or peer-critiqued, so there may be significant errors in it. But, it does put in question almost all the discussion of habitat loss and extinction.

    This is important enough for someone knowledgeable to sort out.

  3. Those who lament the loss of species, it seems to me, aren’t primarily concerned about economic value. Rather, they regard each species as having its own unique spiritual value– just as every child is “special” on Sesame Street– and they see a value in diversity itself.

    In traditional religious terms, this is analogous to the Biblical Flood. God made sure that “every” animal species was preserved, and then he promised never to extinguish life on Earth– a promise that by implication covered all of the species on the Ark.

    Bible readers in the ancient world (and some of them today) assumed that all species had been made at the creation and no new ones would ever emerge. Environmentalists with a scientific bent (a minority of the flock) understand that species are born and die in the natural course of evolution, just as individuals do, and they may even relate to that process in a spiritual way.

    But they are really worried that a high RATE of evolution will “impoverish” our planet. Although I personally find environmentalism to be a thin gruel of a religion, this particular theme resonates with me, and probably with most people.

    Of course, there are probably unknown trillions of species on other planets, so not to worry so much.

  4. Razib,

    As per bob’s link; the destruction of the Atlantic forests supports the Lomborg theory. ~90% habitat destruction is associated with only one extinct bird in the wild.


    Excellent link. It seems to suggest that hunting and the introduction of alien pests in island environments is the dominant source of animal extinction — not habitat destruction.

  5. That sounds pretty good, we should not fear global warming or any kind environmental disaster to threaten the human specie.
    And man hunting is now out of fashion.
    Ah! There remains some “alien pests” of the microbial kind, not nice…
    Hey! Wait a minute, the species are not threatened but what about the individuals?
    Not so good after all!

  6. Willis Eschenbach does it for birds (because they are so visible) and concludes that only 129 species have gone extinct in the last 100 years or so.

    Well, that’s over 1% of the ~9,500 full species. How much extinction is acceptable? Admittedly, a lot of those extinguished ones are flightless rails and other flightless birds of small islands that had never had predators before contact with feral animals, but a lot aren’t.

    Also, I might round up the 129 by five or ten, because birds are still being discovered today.

    It’s true that hunting, or being treated as a pest, took out the 7 or 8 full species of US/Canada birds that are gone outside Hawaii. In Hawaii it is certainly higher.

  7. Eric,

    Just 6 of the bird species were Continental, out of ~9k total continental bird species, and some of that is due to hunting.

    Statistically, I don’t think we could reject the idea that no Continental bird species have been driven to extinction via habitat loss!

    The point is not to downplay animal extinction — it’s to isolate the cause.

  8. I didn’t get the line about Brazil’s “ascension graph”.

  9. 1. Eschenbach skips over the fact that extensive surveys are required to declare a species extinct. It’s not surprising to see the extinction rates “drop” in recent years – the surveys haven’t been done to prove extinction. Many anti-environmental websites point to the old Red Lists that required 50 years without a sighting before declaring a species extinct. This is only slightly better.

    2. Eschenbach ignores subspecies extinctions. Those still constitute a permanent loss of genetic diversity.

    3. The Amazon may be more differentiated than Razib acknowledges.

    4. Normative values for the preservation of species can translate into economic values. Many Christians, for example, are disturbed by the implication of extinction as it applies to their religious instruction that man is the steward and not the owner of Creation (this is the “Creation Care” movement). They would place a high economic value therefore to avoid species extinction.

    5. Lomborg didn’t event the idea that it’s the last lost habitat area that maximizes extinction rates. Nor did he invent the idea of misusing that concept to avoid taking sensible action.

  10. The trouble is that habitat destruction can happen very fast. For example, Ohio’s transition from a state that was overwhelmingly covered with forests and swamps, to its present state (with far less than a square mile of virgin forest and almost all of the swamps drained), almost all happened over the course of a century.

    Species like passenger pigeons and the American buffalo were similarly hunted to near extinction in a similar or shorter time period.

    In the Australian megafauna extinction, human presence related fires that destroyed habitats were an important means by which extinction happened.

    Also, you have to look at scale in terms of the food pyramid. Top predators play an outsized role in the health of the entire ecosystem and declines in top predators can counterintuitively deeply impair prey populations. When top predators suffer, the whole food pyramid that they top can suffer a disruptive collapse that can wipe out species that aren’t very secure. But, top predators often have much larger geographic ranges than their prey. For example, there were never more than about 2,000 tigers in all of Siberia. So, it takes a much larger chunk of land to support an ecosystem that can support an endangered species than it might seem looking at the species in isolation.

  11. Brian,

    Perhaps you could supply some different sources then?

    This should be pretty easy to figure out. Take an island; survey all of the wildlife; and then see how many species die off as more and more habitat is destroyed.

  12. Eric Johnson That is a very surprising number for US and Canada bird extinction. Are you adding in subspecies and island species? I’d appreciate seeing the list.

  13. Take an island; etc…

    Very realistic and honest argument!

  14. Something to keep in mind is that when Edward O. Wilson worries about extinction rates, he’s worrying about extinction of beetles and other small, creepy animals. To Wilson, the Creator has a most ordinate fondness for beetles. When most other people hear Wilson worry about extinction rates, they worry about giant pandas and condors and the like.

    But, those are things that need a lot of habitat room.

  15. It’s a question of good taste. The world has no need for the Book of Kells. So, sure, I could use leaves from the Book of Kells to wipe my ass, but that would be idiotic. Nevertheless, people do similar things every day.

    The habitat destruction which extirpated the ivory billed woodpecker was just such an act of idiocy. Needless, short-sighted, and dumb.

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