An icon of the Mojave Desert, the Joshua tree may be found only at its highest or most northerly outposts by the end of the century. A new study suggests its range may shrink by 90 percent in the next 60 to 90 years as a consequence of global warming.
Ken Cole of the U.S. Geological Survey in Flagstaff, Ariz., and an interdisciplinary group of colleagues used information about the current distribution of Joshua trees combined with climate models to predict where the trees may be by 2070 to 2099.
But they also mined data from the ancient past, using fossilized dung from the extinct Shasta ground sloth -- a shaggy, Volkswagen-beetle-sized creature that consumed Joshua tree fruit as a central part of its diet. The researchers also looked at the fossilized scrap piles of packrats alive more than 10,000 years ago. With this data, the team could characterize how the tree's range changed at the time of a similar climate warming around 11,700 years ago.
The extreme dry climate of the desert has preserved sloth dung -- chock full of Joshua tree leaf fibers and fruit and seed remains -- in caves throughout the sloth's former range for at least 40,000 years. The sloths' waste accumulated in undisturbed layers over generations until the sloth went extinct 12,900 years ago. The topmost sample "represents the last dungball," Cole said.
The dry climate has similarly preserved the little scrap piles of waste and collected plants made by the ancestors of today's packrats.
Taking inventory of which plants are found in the dungballs and scrap piles, formally called middens, and dating the specimens allows researchers to trace the range of prehistoric Joshua trees. The findings add support to the team's predictions for the future.
The climate in the Southwest warmed about 7 degrees Fahrenheit in less than a century during the warming 11,700 years ago, Cole said. "Coincidentally, that happens to be just about the rate of change and magnitude of change we're projecting for the 21st century."
"As the climate warmed at the end of the ice age, it appears that the formerly widely distributed Joshua tree was suddenly only in about 10 percent of its range," Cole said. "It was suddenly eliminated from everywhere except the northernmost parts."
The habitat shrinkage is similar to what the researchers predict for coming decades. But they also note a key difference in Joshua tree range since the extinction of the Shasta ground sloth.
"Formerly, it seemed to be much more widely distributed," Cole said. The sloth was probably a key disperser of Joshua tree seeds, and it was capable of traveling longer distances than the modern-day seed couriers -- squirrels and packrats that carry the seeds no more than 100 feet.
Records suggest that the trees have crept northward at a pace of only about three to six feet per year since the sloth's extinction. This means the trees will have a tough time outpacing the heat to reach suitable habitat for warmer times.
"We mapped the areas where we think, at least at the end of the century, would be good for Joshua trees and that have suitable public lands," Cole said.
Some are areas that the trees could possibly reach on their own. Other pockets of habitat, however, could be suitable only if humans decided to act like ground sloths and port the seeds to the new areas via so-called assisted migration, a controversial proposal among ecologists, some of whom worry about tinkering with ecosystems.
The Joshua tree also relies on a single moth species, the yucca moth, for pollination so the moths would have to migrate, too.
"It really brings together a number of different lines of evidence to bring to bear on the problem of what is going to happen under global warming," said Claudio Latorre Hidalgo of Universidad Catolica de Chile and the Institute of Ecology and Biodiversity in Santiago, who also studies ancient climate using middens in the Atacama desert.
The modeling results are supplemented by the fossil evidence, he notes, which increases confidence in the findings.
"A lot of ecosystems, especially those that are sensitive to higher temperatures, they are really vulnerable to these changes and they could completely disappear," he added. "This is a very charismatic species."