CONSERVATIONISTS who simply follow the number of animals in an endangered population may be failing to spot species that are heading for oblivion. The number of animals remaining from a threatened species may actually rise as it approaches the brink of extinction.
This surprising conclusion comes from a mathematical analysis by theoretical ecologist Peter Abrams. It is unclear whether animal numbers boom then bust this way in the wild, but Abrams says conservationists should take his findings as a warning that they should change the way they monitor the health of populations.
Abrams says the phenomenon can be understood by thinking about a hypothetical island inhabited by lynxes that feed on hares. The population is in balance: a few lynxes control hare numbers, keeping them low. But if environmental conditions take a turn for the worse, everything changes. If the lynxes start to be poisoned by something in the environment, for instance, then their death rate increases and hare numbers go up.
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But this large hare population in turn can support a larger lynx population. So after the initial dip, lynx numbers increase again despite the fact that a higher proportion are being poisoned than before.
When Abrams worked through the mathematics in detail, he found that a species鈥� numbers can remain high even as environmental conditions get steadily worse (American Naturalist, vol 160, p 293). Populations often don鈥檛 start to crash until the death rate of a species is so high that it has very nearly passed the point of no return (see Diagram).
鈥淵ou can think of this as an ecological counterpart of the law of unintended consequences,鈥� says Robert Holt, a theoretical ecologist at the University of Florida in Gainesville. The biology makes sense, but it can be missed if conservationists simply look at the numbers of a species and say: 鈥淗ey, we鈥檙e doing fine.鈥�
Because Abrams鈥檚 conclusions are based on pencil-and-paper calculations, rather than experiments on actual organisms, it remains to be seen how often 鈥� and to what extent 鈥� a species鈥� march to extinction can be masked in this way. Predators should be most at risk, as their numbers are tightly bound up with those of their prey. His findings are more likely to apply to species endangered by subtle threats such as global warming or pollution, rather than those menaced by habitat loss 鈥� the major threat to most species today.
Even so, Abrams鈥檚 results hold a warning for conservationists. 鈥淚t might be good to monitor the effect of the environment on their ability to catch prey, rather than just monitoring population size,鈥� says Abrams. 鈥淭hen you鈥檇 have some warning that things were getting worse, even if you couldn鈥檛 see that from the population density.鈥�