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.