A set of studies in Tanzania has added strong support to a theory that ecologists have long believed but have had difficulty proving: that species are more likely to become extinct in a smaller habitat. An analysis in this month's issue of Conservation Biology found that populations of antelope and several other hoofed mammals were unable to persist in small parks in Tanzania. The finding suggests that conservation managers must step up efforts to protect vulnerable species in shrinking habitats.
A team led by University of Utah conservation biologist William Newmark spent 8 years surveying six Tanzanian parks, between 100 and 20,000 square kilometers in area, for the presence of antelope, monkeys, giraffes, and other large animals. These species were known to have flourished when the parks were established, between 1913 and 1961. Newmark's team found that populations of six species of hoofed mammals were more likely to have gone extinct in the smaller parks. These animals, however, were still flourishing in the larger reserves.
The findings support a theory first developed to account for how many species can coexist on an island. Scientists have sought to apply island biogeography theory to mainland habitat "islands," but these efforts have been controversial because it has been unclear whether colonization and competition among island species serves as a good model for interactions among mainland species. Newmark, however, says his results "indicate that island biogeography theory is clearly applicable to the dynamics of large mammal populations in [mainland] nature reserves."
The Tanzanian extinctions suggest that habitat fragmentation and increasing isolation of nature reserves will result in more local extinctions elsewhere too, says Michael Gilpin, a conservation biologist at the University of California, San Diego. "In a truly small reserve, almost everything will eventually go extinct" within 10,000 years, says Gilpin. Indeed, some biologists estimate that a minimum area necessary for long-term survival of large mammal species is at least 1,000,000 square kilometers. If this is the case, Newmark says, more steps should be taken to protect vulnerable populations in parks and to establish wildlife corridors connecting parks.