Even more species may become extinct in the next few decades than previously feared, according to a study published today in Science. By tallying up parasites and other known "affiliates" that depend on species on the endangered list, the authors estimate that another 6,300 species will disappear along with the endangered ones.

Some affiliated species, including many parasites, are adapted to a single host species. Thus, if the host becomes extinct, so does the parasite. Other species--for example, butterflies that rely on plants to host their larvae--can switch to another host if their preferred one dies out. But as more and more host species disappear, the affiliate may run out of alternatives. To estimate the importance of this effect, the researchers looked at a wide variety of cases where they knew which affiliates depended on which hosts, including pollinating wasps, butterflies, and various parasites. They found that how many host species an affiliate could choose from mathematically determined how sensitive it was to extinction of the hosts. This relationship should allow a better understanding of the global extinction rate.

Scientists have recognized the importance of such "co-extinction" for some time. "What's new is that we've invented a way to quantify it that can be applied to any other group that has sufficient data," says Robert Colwell of the University of Connecticut, one of the authors of the study. He notes that the method could be extended to other hosts and affiliates, and that many more species are likely to be at risk. Remarks Colwell: "The more we look, the more we find relationships." --Don Monroe