He proposed that the risk faced by a species today may depend on the experiences the species had in the past. On many islands, for example, birds are now threatened with extinction. Rats introduced onto the islands from ships attack the birds’ eggs, and they are helpless to do anything about it.

Dr. Balmford observed that some islands have had native species of rodents for thousands of years. Birds on those islands may be resilient to the threat of new rodents, he speculated.

This idea is called the extinction filter hypothesis. The species we see today have passed through a gantlet of challenges over time. Other animals unable to cope with a threat in the past became extinct — filtered out by history, as it were.

The authors of the new study reasoned that an extinction filter might be at work in today’s forest fragments. Some forests were relatively undisturbed until recent decades, while others were split into fragments in ancient times.

In parts of Europe and Asia, farmers were breaking up forests thousands of years ago. Even before, some forests were regularly disturbed by storms or fires. And during each ice age, northern belts of trees were broken into isolated stands.

In the new study, published in Science, the researchers examined 4,493 species in 73 forest regions worldwide. They used several methods to determine how sensitive each species was to forest fragmentation. For instance, they observed whether species could be found at the edges of forests or kept to the core of the fragment.

Some species, such as the cedar waxwing of North America and the rufous-tailed hummingbird of Central America, didn’t seem to mind life in fragments. But other species shied away from forest edges, such as the sun bear of Borneo and the hairy woodpecker of North America.