At various times in its past, the Earth has succeeded in killing off most of its inhabitants. Although the impact that killed the non-avian dinosaurs and many other species gets most of the attention, the majority of the mass extinctions we're aware of were driven by geological processes and the changes in climate that they triggered.

Unfortunately, based on the current rate at which animals are vanishing for good, we're currently in the midst of another mass extinction, this one driven by a single species: humans. (And many of the extinctions occurred before we started getting serious about messing with the climate.) This week's edition of Science contains a series of articles tracking the pace of the extinction and examining our initial efforts to contain it.

Extinction and “defaunation”

Estimating the total number of animal species is a challenging task, but numbers range from roughly five to 10 million. Of those, we seem to be exterminating about 10,000 to 60,000 every year. Up to a third of the remaining vertebrate species are thought to be threatened or endangered. Amphibians have it even worse, with over 40 percent of species considered threatened.

Those are grim numbers, but the authors of one review of the data point out that vanishing species aren't the only problems. "From an abundance perspective," the authors state, "vertebrate data indicate a mean decline of 28 percent in number of individuals across species in the past four decades." In other words, in just over the past decade, species that still exist have seen their membership decline drastically. The authors use the term "defaunation," a play on deforestation, to describe the plunging number of animals alive today.

Although extinction and defaunation are distinct issues, the underlying causes are the same. The authors mention overexploitation of species used for food, habitat destruction, and the impact of invasive species, all of which have played a role for centuries; the first two factors in particular help explain why tropical species tend to be faring the worst. More recently, the rapid pace of climate change is adding to the stresses felt by many species.

Why should we care about the vanishing species? The authors provide one obvious example: insect pollinators are needed for nearly 75 percent of the world's food crops and are estimated to be worth 10 percent of the global agricultural output. But other species provide a variety of what are called ecosystem services, such as dispersing seeds, controlling pests, enhancing the cycling of nutrients, and enhancing water quality. There's a Policy Forum on Science's website that discusses how the shrinking number of animals can create or enhance conflicts among human populations.

Desperate and photo-friendly measures

This long list of problems has helped convince people of the need to do something to reverse species' declines. The challenge is deciding what to do. As the authors of a second review note, that decision is anything but simple. Most species require a significant habitat of a specific type in order to maintain a stable population. The problem is that many of these habitats simply don't exist anymore. Beyond the obvious issue—human occupation of the former habitat—invasive species and climate change may have irrevocably altered habitats that have otherwise been left undisturbed.

Habitat restoration has been done successfully, though. But the authors say that even this poses difficult questions: "Attempts to return a system to some historical condition make somewhat arbitrary decisions about how far back in time to go." It might make sense to try to restore a patch of North America to a pre-European arrival state, but it makes less sense to try for a pre-human arrival state, as that would mean attempting to establish ice-age habitats in a far warmer world.

Regardless of the hard decisions, attempts have been made to reintroduce species to areas where local populations have been exterminated, both in order to re-establish a robust ecosystem and as a way of protecting species that are at risk of extinction because they only exist in a limited range. These translocation projects are booming. Among vertebrates, there had been over 400 attempts between 1990 and 2005, with the majority of them after 1998.