THE destruction of nature is driving species to extinction - but perhaps not as rapidly as has been thought. While the most widely publicised estimates predict the loss of natural habitat will condemn 18 to 35 per cent of all species to extinction by 2050, these figures could be about twice as high as the actual number - all because of a mathematical error that has gone unnoticed for decades. We still face an extinction crisis, warn Stephen Hubbell of the University of California, Los Angeles, and Fangliang He of Sun Yat-sen University in Guangzhou, China. But the pair's work will allow biologists to more precisely define how habitat destruction leads to extinction.
It is impossible to accurately measure extinction rates. Dozens of new species are identified each year, and counting those that disappear is hard because many are small and live in poorly studied, mainly tropical environments.
Instead, extinction rates are often predicted from a mathematical model based on habitat loss, which is more easily measured. The larger the area you survey, the more species you encounter. Ecologists calculate a curve called the species area relationship (SAR) for an ecosystem by measuring the area they must survey to encounter the first individual of each successive species. To establish the number of extinctions caused by habitat destruction, they run the SAR calculation in reverse.
"We had a feeling there were problems with this, but we could not say why mathematically," Hubbell says. So Hubbell and He checked the method using data from forest plots located all over the world. The pair could calculate the SAR for each plot, and also see what happened to species unique to these plots if they "destroyed" a certain area of each plot in their mathematical model. As the area of destruction widened, these species began to die out. But after each simulated loss of habitat, "more species always remained than were expected from the SAR", says Hubbell.
The pair's analysis explains why. Using the reverse SAR method, biologists have assumed that a species is lost with the destruction of an area of habitat equivalent to the area needed to first encounter it. But in reality, the species is lost only with destruction of the habitat area that includes every individual of the species, which is always larger. Consequently, the SAR method loses species too fast. Read More
