Big risk. Large size significantly ups the odds of extinction for mammals such as elephants and pandas.
August, 2005 Science Magazine
News of the Week
Global Analyses Reveal Mammals Facing Risk of Extinction
Two new studies are helping conservation biologists think big--in the case of one of the studies, as big as one-tenth of the continents.
Conservationists typically set goals and priorities for relatively small regions. Although some have come up with priorities for the planet, these have often been wish lists rather than objectives drawn from rigorous analyses. Now a
team of researchers, led by mammalogist Gerardo Ceballos of the National Autonomous University of Mexico, has conducted the first global analysis of the conservation status of all known land mammals. On page 603, they report that 25% of known mammal
species are at risk of extinction. In order to decrease the risk to mammals worldwide, about 11% of Earth's land should be managed for conservation, the analysis finds.
This is the first time such a global conservation estimate has been calculated for mammals, and although experts are not surprised by these results, they praise the study for its comprehensiveness and detail. "This sets a new standard
for global priority-setting analyses," says Peter Kareiva, lead scientist for The Nature Conservancy.
A second conservation study, reported online by Science this week (www.sciencemag.org/cgi/content/abstract/1116030), finds that large mammals may be more threatened than their smaller relatives. A team led by Georgina Mace of the
Zoological Society of London and Andy Purvis of Imperial College London reports that adult mammals that weigh more than 3 kilograms tend to have biological traits that hike their risk of extinction. "Both of these papers provide us with finer and more
detailed insights into threat patterns and processes," says Thomas Brooks of Conservation Inter-national in Washington, D.C.
The two new analyses rely on massive data sets. Ceballos and his colleagues combed the literature and compiled geographic ranges for all 4795 known species of land mammals. After dividing the world's land into many thousands of cells,
each 10,000 square kilometers, they plugged their range data into a conservation planning model, called MARXAN, that identified the least amount of area--all told, 17,020,000 km2, or 1702 cells--that would conserve at least 10% of the range of each
species. Various population models used by conservation biologists typically specify that threshold as the minimum amount of range needed to sustain a healthy population of a species.
This particular analysis won't be used in specific conservation efforts because the scale is much too coarse, but experts say it reveals important points. For example, the analysis shows that the collection of 1702 cells--11% of the
total--would provide a resilient and flexible strategy, because almost any cell can be replaced by another cell without an overall loss of species diversity. But about 80% of these high-priority cells have already been affected by agriculture, which
destroys natural habitat. "We simply are not going to be able to do conservation without making it compatible with some measure of agriculture," notes Kareiva.
The results from Ceballos's team are only for mammals, whose ranges may not overlap with those of other taxa. Adding birds, amphibians, and reptiles would increase the amount of land needed to be conserved. "We need to do much more,"
says study author Paul Ehrlich, a population biologist at Stanford University. "If you want to add in most biodiversity, you're talking about [conservation of] 30% to 40% of Earth's surface," he speculates. Ehrlich adds that the population size of a
species that can survive by preserving 10% of its former range won't be as effective at providing ecosystem goods and services, such as pollination or bush meat.
Similar results about mammal ranges and conservation, not yet published, will come from John Gittleman, an evolutionary biologist at the University of Virginia, Charlottesville. His group spent 4 years collecting range maps and
biological data for all known land mammals. "There's a nice convergence," he says. "It's very reassuring."
The report by Mace, Purvis, and their colleagues relies on information from Gittleman's group as well as other data sets such as the so-called World Conservation Union's Red List, which ranks mammals according to the extinction threats
they face. Drawing on such information for 4000 mammal species, the authors determined what factors, such as small geographic ranges or large body size, put particular species at higher risks of extinction.
The analysis found that for mammals smaller than 3 kilograms, the main risk factors were environmental, such as proximity to agriculture or human populations. Identifying and conserving habitat is likely to be enough to keep these
species going, the scientists conclude. But larger animals, such as elephants and pandas, face threats magnified by intrinsic biological constraints, such as small litters and long gestation times. Conservation biologists had suspected that larger
mammals face greater extinction risks, but the size of this data set puts the premise on a much stronger footing, Gittleman says.
Mace, Purvis, and their colleagues conclude that the survival of large mammals will likely require a concerted effort tailored to the biology of each species.