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Sloth dung points to stark future for Joshua Trees

By Martin Leggett - 25 Mar 2011 12:2:1 GMT
Sloth dung points to stark future for Joshua Trees

Image Credit: Ken Cole, USGS.

There are some bizarre tricks up scientists sleeves, when it comes to looking at how climate change will impact life on the planet. But few sound weirder than turning to fossilized sloth and pack-rat dung to see how the iconic Joshua Tree will fare, as temperatures rise. That is what a research team, bought together under the umbrella of the US Geological Survey (USGS), have done for a recent paper in Ecological Applications. And the tale from the ancient poop is worrying for the Mojave desert-dwelling tree.

The first part of the puzzle was laid by careful analysis of sloth dung, as well as the middens of pack-rats, which can contain remains of Joshua trees. Shasta ground sloths, living in the Southwest US over 20,000 years ago, were particularly fond of Joshua trees - and their dung is often peppered with its seeds and fruits. Pack-rats, by contrast, were keen on the leaves of the Joshua tree, and packed their burrows with them.

By collecting and dating numerous samples across the region, the team were able to map out the much-wider geographical spread of Joshua trees, thousands of years ago. They were especially interested in what happened following a sudden rise in temperatures around 12,000 years ago. That warming appears similar to the current projections for temperature rises this century, due to global warming.

The survey results showed that the extinction of the ground sloth, and other large mammals, just prior to the 12,000 year-old warming episode, caused major problems for the Joshua tree. It relied on these wide-ranging animals to distribute its seeds. As temperatures warmed, it was unable to shift out of areas that were no longer suitable enough for it - and so its range shrank.

When this information was plugged into a computer model, looking at the likely climate of the Southwest, 90 years hence, the Joshua tree was similarly caught out. The model predicted a loss of up to 90% of the tree's current range - with just a small rump of Joshua trees in the far north left. When it comes to Joshua tree seed dispersal, it seems that the current crop of small animals munching on Joshua tree fruits, such as squirrels, just can't cut it compared to sloths.