Fossils of small mammals excavated around a Shasta County cave tell a remarkable story of animal diversity and its loss when the Earth warmed abruptly after the last major Ice Age ended some 12,700 years ago.

The fossils reveal how the changing climate sharply altered the pattern of their distribution as some populations rose to dominate the landscape, while others - once dominant - diminished to virtual obscurity.

Stanford scientists dug inside and around remote Samwell Cave in the Shasta-Trinity National Forest to count the fossils of small animals that lived there and then surveyed the species that remain in the region today.

They found that climate changes in the past caused striking changes in the diversity of those animal populations - a lesson for what could happen as global warming continues now, they say.

Jessica Blois, a former Stanford graduate student now a postdoctoral researcher at the University of Wisconsin, and Elizabeth Hadly, a Stanford professor of biology, published a report on their findings Sunday in the online edition of the journal Nature.

Since the last Ice Age ended and the climate warmed to what it is today, the diversity of all the animal populations in the region has declined by a full 30 percent, they found.

"When we dug up fossils in the cave," Blois said in an interview, "we found that near the end of the Ice Age, populations of small animals like the deer mice and mountain beavers and the Mazama gophers were about equally abundant.

"But today the deer mice are all over the place - they're now by far the dominant population in the region because they're 'generalists' that can eat anything and thrive anywhere apparently in any climate. And while there are still some beavers and gophers around, most have moved away to higher elevations or to cooler and wetter climates."

The population of the deer mice doubled between 16,000 and 13,000 years ago, the two scientists said, and records indicate that since the Ice Age ended, temperatures were rising by 7 to 12 degrees Fahrenheit until the world's climate stabilized about 10,000 years ago.

None of the small animals became extinct during the long period Blois and Hadly studied, they said, and they didn't try to study the extinction of large animals like mammoths and mastodons because extinction alone would not reveal the full story of diversity.

"It's the warming in the past that disrupted the diversity of those animal communities, and I'll expect that to happen again as the world's climate continues to warm," Blois said. "In fact, the degree of climate change that occurred after the Ice Age is very similar to the rise in the world's temperature that has been predicted for the next century."