The Iconic, Vanishing Joshua Tree: Evolutionary Dance
Editor's Note: This is Part 2 of a three-part series about the precarious survival of the Southwest's iconic Joshua Trees, which are threatened by climate change and urban sprawl. Click here for Part 1, and here for Part 3.
It was this unique species interaction that intrigued Smith. The realm of flowering plants is incredibly varied and Smith wondered whether and how pollination helped produce the enormous diversity. Because of the specific relationship between the yucca and its moth, he said, Joshua trees were an ideal plant to study.
Smith and his team study reciprocal evolution, or the ability for two separate species to co-evolve to maximize survival. They hypothesize that the two species of yucca moth were the catalyst for the evolution of the second subspecies of Joshua tree.
“The interaction between them determines the outcome of the [evolution] process,” he said. “They are adapting to one another.”
If you visit Joshua tree flowers on cool spring nights, you can see the female yucca moths at work. Their job isn’t easy. The flowers have a pungent odor, which some scientists describe as similar to blue cheese or chanterelle mushrooms, Smith said. William Trelease, the famed botanist who died in 1945, put it a different way. The usual description of fetid was not entirely accurate, he said, rather the odor was “so oppressive as to render the flowers intolerable in a room.”
The tiny yucca moths are well suited for the job of pollination. Special tentacle-like organs near their mouths evolved to pollinate Joshua tree flowers where the moths lay their eggs, Pellmyr said. The pollination allows the tree to produce seeds that feed moth caterpillars until they mature. The remaining seeds then are dispersed around the desert by animals, such as squirrels and other rodents, that eat Joshua tree fruit.
Although a number of creatures now do this job, some scientists believe one of the most fascinating seed dispersers was the Shasta ground sloth. The cow-sized herbivore lived during the last ice age and went extinct 12,900 years ago, around the time when man first entered ground sloth territory, said Ken Cole, a research ecologist with the United States Geological Survey.
In the 1930s in Gypsum Cave near Las Vegas, scientists discovered fossilized sloth dung full of Joshua tree remnants in the Southwestern desert. Now that the sloths are extinct, however, Cole worries that Joshua trees’ range could be limited as smaller desert animals take over the job.
“There are seed dispersers here and now, like birds and squirrels, but those animals don’t range as far as a [migratory] ground sloth,” Zarki said.
Animals in the desert today can move seeds about half a mile, said Todd Esque, an ecologist for the USGS. It’s a far cry from the several miles that Cole estimates sloths traveled.
Click here for Part 3.