UPDATE Apr. 7, 2011: I've been corrected by Dr. Ken Cole in that the sloth dung are not coprolite fossils. Indeed, fossilization would imply a conversion into inorganic, mineral materials. "It still smells," Cole says of the 13,000 year old sloth dung. You can learn more in this 2008 NPR interview with Cole: http://www.npr.org/templates/story/story.php?storyId=17628032


The strangely shaped Joshua tree (Yucca brevifolia) is one of the main attractions of the American Southwest. But in a new study from USGS, researchers say temperature increases resulting from climate change will likely eliminate Joshua trees from 90 percent of their current range over the next century.

And clues to the tree's historical and predicted population decline came from a rather unique source -- from studying the fossilized dung of an extinct giant sloth.

You see, Joshua trees were one of the favorite food items of the Shasta ground sloth (Nothrotheriops shastensis), a bear-sized vegetarian with giant claws that became extinct about 13,000 years ago. Of course, wherever Shasta sloths went about and foraged on Joshua trees, they also defecated the remains and seeds of the Joshua tree.

So wherever you can find sloth dung fossils with Joshua tree remains and seeds, it's likely that Joshua trees were present in the landscape at that point in prehistory. Create a map of sloth dung fossils plus other locations with Joshua tree remains, and you can extrapolate the geographic distribution of Joshua trees during the time of the sloths.



What does this mean for today's Joshua trees? It turns out that around 12,000 years ago, the planet experienced a climate warming event similar to the climate change projections for the 21st century. That's also when the Joshua tree's geographic distribution shrank drastically, with only a few fragmented populations remaining.

By comparing the warming event and Joshua tree decline of 12,000 years ago with the today's climate change projections and Joshua tree temperature and habitat preferences, researchers can estimate which regions Joshua trees are likely to disappear from, and which regions may become suitable for Joshua trees to spread to.

Creating maps like this could help national parks and other resource agencies protect and manage Joshua tree ecosystems. Unlike prehistoric times when giant sloths were around to disperse Joshua tree seeds across great distances and help start new populations, today's Joshua trees only have the help of small rodents, which don't travel as far. If both climate change and Joshua tree declines unfold as forecasted, it will be up to humans to identify optimal locations to establish new Joshua tree refuges and to help manage the seed dispersal and survival of Joshua trees.

The research was led by Ken Cole at our sister center -- the USGS Southwest Biological Science Center -- in collaboration with Northern Arizona University, NOAA, University of Arizona, Lawrence Livermore National Laboratory and USDA Forest Service.

-- Ben Young Landis

Joshua tree image courtesy of National Park Service. Shasta sloth illustration courtesy of Natural History Museum of Los Angeles County/Pat Ortega.

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