How Long Does It Take Species to Go Extinct? Longer Than Previously Thought

Article excerpt

Habitat destruction drives species extinct more slowly than previously thought, according to a new model described in this week's Nature. 'We have bought a little time for saving species,' says scientist.

The pace at which plants and animals are vanishing from the planet as their habitats shrink may be overstated by as much as 160 percent or more, according to new research.

An approach widely used to estimate extinctions from habitat loss is conceptually flawed, says a study, set for publication in the May 19 issue of the journal Nature. The researchers involved say that their new method more accurately reflects the interplay of shrinking habitats and the populations that rely on them.

The research is one of at least two new studies highlighting scientists' efforts to sharpen the tools needed to track the scope of the species-extinction problem - and to design approaches for dealing with it.

Another challenges the theory that there is a 'magic number' of organisms, below which the species may not be able to recover.

In both cases, the issues are more than academic, specialists say. Projections of the impact of shrinking habitat on extinctions can feed into local wildlife management decisions on how much land to conserve. They also affect long-term projections of the extinctions traceable to global warming and its effects on habitats worldwide.

Additionally, the use of a single benchmark for a population's viability may prevent conservation managers from nurturing species whose numbers fall below that threshold but still may be recoverable, explains Steven Beissinger, an ecologist at the University of California at Berkeley who took part in the study.

The development of a new tool for estimating extinctions "is welcome news, in the sense that we have bought a little time for saving species," says Stephen Hubbell, an ecologist at the University of California at Los Angeles and one of two scientists who performed the analysis.

"But it's unwelcome news," he adds, "because we have to redo a whole bunch of research" performed with the previous method.

New model a better match to real ecosystems?

Many ecologists have realized for years that the technique they relied on for estimating extinctions could overestimate the losses. When scientists visited shrinking habitats, they found that the endangered plants and animals were not going extinct as fast as the estimations suggested they should be.

"Nobody could figure out why," says Fangliang He, the Nature paper's lead author and a researcher who specializes in modeling biodiversity and landscape processes at the University of Alberta in Edmonton.

Some suggested that the gap may be due to what became known as "extinction debt," where endangered plants or animals may linger after losing a significant portion of their habitat, but whose eventual disappearance has become inevitable.

Now, Drs. He and Hubbell say they have found the reason for the gap and offer up a method that more closely matches extinction rates seen in the field. While the new approach doesn't rule out the possible existence of an extinction debt, the duo says their method doesn't need it to explain the gap.

Mass extinctions then and now

The world still faces a significant extinction problem, the two emphasize. They cite a study published in March, for instance, that compares extinction rates written in the fossil record with those recorded by the International Union for the Conservation of Nature, the world's scorekeeper on threatened, endangered, and extinct species today.

The analysis, by a team led by Anthony Barnosky of the University of California at Berkeley, concluded that species extinctions over the past few thousand years are higher than the typical rate seen in the fossil record.

If currently threatened species become extinct within the next 100 years and the pace continues unchecked, within 240 to 540 years extinctions will rise to a level not seen since the last five major mass extinctions in Earth's history, Barnosky found. …