Earth Could Lose Thousands Of At-Risk Species: Studies Indicate Extinctions Are Not Random Events

Fariss Samarrai -- (804) 924-3778; samarrai@virginia.edu

A new study indicates that thousands of at-risk bird and mammal species world-wide could eventually become extinct due to the non-random nature of extinctions. The findings will be reported in the April 14 issue of the journal Science.

"Our findings suggest that extinction events result in a further loss of biodiversity, possibly even the extinction or threatening of thousands of additional species of animals, including large, charismatic ones such as rhinos and chimpanzees," says John L. Gittleman, associate professor of biology at the University of Virginia, one of the study's authors.

"We have identified a number of characteristics that make certain species prone to greater extinction risks, and we have determined that extinctions do not occur randomly. Species that are biologically prone to certain risk factors are far more likely to go extinct than suggested by models of random extinction. The resulting potential loss of biodiversity is enormous."

Gittleman says the biological characteristics that make a mammal species more likely to become extinct include living at a high trophic level, low population density, long gestation length, and especially small geographic range size.

"When we looked at 1,000 simulated phylogenies [evolutionary trees representing the branching order of ancestral relations among species], we found that the chances of large numbers of species going extinct were, in real terms, much greater than in simulated models of random extinction. We believe that the threat of extinction for many species is real and because of a cluster effect could result very quickly in further loss of biodiversity and genetic history," Gittleman says.

Species that have many close relatives are the most likely to be saved from extinction, Gittleman says, but species that are of a unique lineage with few relatives, such as the giant panda, are at severe risk. Once lost, there would be a significant loss of evolutionary history as well.

"You can't simply pluck species randomly from the tree of life in a computer model and come up with a realistic account of what is happening in nature," Gittleman says. "Many factors affect the likelihood of extinction risk for any given species, and these factors are not random. Our findings suggest that we must take seriously the threat of biodiversity loss that is likely occurring in reality because of the nonrandom nature of extinction risk."

Gittleman points out that in addition to the particular biological characteristics that make some species prone to extinction, there are several environmental factors that also can accelerate risk.

"Many species become endangered because of human activity such as habitat destruction for agriculture or development purposes, and by exploitation of species through fishing and hunting. Many species also become threatened by the introduction of exotic species and disease. The reality is, extinctions are not random events."

The other authors of the study are Andy Purvis and Paul-Michael Agapow of the Department of Biology, Imperial College, U.K.; and Georgina M. Mace, Institute of Zoology, the Zoological Society of London.