UW News

January 10, 2001

Past climate change effects on mammals may mirror global warming impact

A thousands-of-years-old peek at the effects of climate change on small mammals in the western United States may provide a snapshot of the future impact of global warming on animal populations.

When the climate of the Great Basin of the western U.S. shifted dramatically and became more arid between 8,300 and 5,000 years ago, the number of small-mammal species plummeted, with some becoming locally extinct, according to University of Washington archaeologist Donald Grayson. In a study published in the current issue of the Journal of Biogeography, Grayson also reported that the population balance among small mammals was substantially altered, with arid-loving kangaroo rats becoming the dominant species.

Grayson’s data are based on analysis of more than 184,000 mammal bones and teeth recovered in stratified layers from Homestead Cave in north-central Utah, near the eastern edge of the Bonneville Salt Flats. It is the largest sample of small mammal fossils from the western U.S. ever studied. The bones and teeth were deposited over a period of about 11,300 years, extending back to the late Pleistocene epoch, by foraging owls that roosted in the cave. The cave is at an elevation of about 4,000 feet.

“This research is important because it is the first detailed analysis of what is likely to happen to mammals in this region under conditions of global warming,” said Grayson. “It has implications of how we deal with global warming and how we set up and manage wildlife preserves. If we don’t intervene the impact on mammals will be tremendous. There will be local extinctions across the valleys of the Great Basin and into the mountains.”

The climate in the Great Basin, which includes most of Nevada and portions of California, Idaho Oregon and Utah, was different in the late Pleistocene and the early Holocene, the epoch which began about 10,000 years ago. The area was much wetter and cooler, and dotted with marshes and shallow lakes, according to Grayson. These bodies of water dried up when the climate became hotter and more arid in the middle Holocene and few of them ever came back. The Great Basin is the 200,000-square-mile region between the Sierra Nevada and Wasatch mountains whose rivers and streams have no outlet to the sea.

Grayson said evidence from the White Mountains of Eastern California shows the temperature rose 2 degrees C in the middle Holocene and he believes the increase was probably higher in other areas of the Great Basin.

Small mammals responded quite differently to the changing climate. Some species such as yellow-bellied marmots and pygmy rabbits became extinct at Homestead Cave and today only can be found in cooler, moister parts of the Great Basin, he said. Others, including bushy-tailed woodrats, Great Basin pocket mice and sage voles, disappeared and did not return to the area until the climate moderated in the late Holocene. Two species of kangaroo rats, which were not abundant earlier, thrived in the warmer, drier conditions and their remains became the most common found in the cave. Their numbers declined when the climate moderated in the late Holocene.

Human response to the changing climate in the middle Holocene mirrored that of many of the small mammals, according to Grayson.

“What happened is not fully understood, but it is very difficult to find human archaeological sites in the Great Basin for the middle Holocene,” he said. “There are human sites dating back to around 11,000 years ago. It appears that when it got hot and dry the human population dropped dramatically and didn’t come back until the late Holocene when the number of sites increase. Human sites are really hard to come by in the middle Holocene.”

Grayson cautioned that there are several caveats in comparing the middle Holocene to current global warming. First there will be an increase in carbon dioxide levels and scientists don’t know how plants will respond to this change. Also the plant community of the Great Basin has changed in the past l25 years with the introduction of exotic species such as cheat grass.

“Homestead Cave is as close as we can get to an analog to global warming in the fossil record over the past 10,000 years,” he said. “It provides a way of predicting which species will become extinct and which will gain with a temperature increase of at least 2 degrees C. Every animal that declines in the Homestead fossil record will be threatened at low elevations throughout the Great Basin in global warming. And there are implications outside the Great Basin in parts of eastern Washington and Oregon where small mammals will be in peril. What happened in the middle Holocene will repeat under global warming conditions. But it may be worse. A lot depends on what happens to the cheat grass.”

The Defense Department’s Legacy Project and Hill Air Force Base funded Grayson’s research.
For more information, contact Grayson at (206) 543-5587 or grayson@u.washington.edu