The phrase mass extinction usually brings to mind events sparked by dramatic environmental change, such as the asteroid impact that led to the demise of the dinosaurs and many other species 65 million years ago. In fact, five such large-scale extinctions have been identified in the fossil record, and according to findings presented on Friday at the annual meeting of the Society of Vertebrate Paleontology in Mexico City, another is under way. This time the cause is nothing so dramatic as a giant asteroid or a radical shift in climate. Rather, it appears, human pressure is to blame.

Like the other mass extinctions, says University of Michigan paleontologist Catherine Badgely, the current crisis is worldwide, affecting a broad range of species. Certain species of vertebrates (animals with backbones) are particularly vulnerable, she reports, especially those with small geographic ranges or narrow subsistence requirements. The numbers are alarming. One quarter of all mammals are endangered or extinct, as are 15 percent of birds. In both groups the larger species are in the most trouble.

The human pressures threatening these creatures include habitat destruction and modification, overhunting, introduction of foreign species, and the increase in carbon dioxide concentrations. Still, extinction of the animals currently designated as endangered is not inevitable, Badgely says. But in order to preserve them, there needs to be a massive change in human actions.