In last week's Nature* there was a news report about the removal of highly threatened frogs from their habitats to captive breeding facilities.
Besides relentless habitat loss, one of the biggest threats to frogs is the often fatal fungus Batrachochytrium dendrobatidis (did I spell that correctly?). Captive breeding programs are expected to protect the highly vulnerable species from the fungus and the other dangers they face in their habitats. According to the article, the International Union for Conservation of Nature has a plan "to transfer 500 members of 500 [frog] species into protective custody within five years." Curiously, though, a brief search on the IUCN site did not find any relevant information.
Anyway, what attracted my attention in the Nature article was the following cautionary note.
A problem of equal complexity is what will happen to an amphibian’s native ecosystem in the amphibian’s absence. It may be that the things it ate will immediately increase in number and the animals that fed on it will become fewer. Such adjustments may lead to other changes in an unpredictable cascade through the ecosystem. Depending on how long the amphibian is gone, its ecological niche might not be there when it returns.I suppose the removal of a major predator could significantly influence the future population levels of its prey. But if the frogs have already declined in numbers and if their prey (insects, etc.) are abundant anyway, would the frogs' removal would make any difference in the numbers of their prey? Probably not. The same argument also applies to the numbers of frogs' predators. If a species is on the brink of extinction, its influence on its environment has probably already dropped to an insignificant level.
Nevertheless, I think the point raised in the Nature article should be kept in mind. Most ecosystems have complex, intricate organizations that make it almost impossible to predict the outcome of the modifications we impose on them.
*Emma Marris, 26 March 2008, Nature, 452:394-395.