Joshua Trees Threatened by Climate Change

Joshua tree-yucca moth-climate changeA unique desert plant with a life cycle made famous in ecology textbooks could disappear from most of its current range because of climate change.  According to a study conducted by the US Geologic Survey, the Joshua tree could disappear from 90% of its range over the next six to nine decades as the planet warms.  If conservationists intervene and help Joshua trees “migrate” to new habitat, it may be possible to save the species.  Even so this plant that has frequently been featured by ecology books provides a stark example of how climate change threatens species and ecosystems across the US and around the world.

Though it is more closely related to the lilies you might plant in the garden, the Joshua tree looks something like a cross between a palm tree and a cactus.  The largest member of a group of lilies called yuccas, Joshua trees can grow up to fifty feet tall with twisted branches that culminate in clusters of spiky leaves.  The species lives only in the Mojave Desert of California, Arizona, Nevada, and Utah, and is adapted to survive in the harsh habitat known as high desert.  These ancient-looking trees have clung to existence in the desert for hundreds of thousands of years, but as the climate warms they will likely be unable to survive in most of their current range.

Ken Cole, lead author of the recent Geologic Survey study on Joshua trees, says the species has had to shift its range north before during previous periods of climate change.  However most climate shifts in the past have occurred much more slowly than today’s warming caused by human activity.  In addition the species had a dependable means of migrating to new areas where the climate was hospitable: its seeds provided food for giant ground sloths, which ranged over large areas and distributed Joshua tree seeds in their droppings.

About 13,000 years ago giant sloths in the Mojave went extinct, perhaps partly due to hunting by early humans.  According to Cole, this left the Joshua tree unable to respond as quickly to shifts in climate.  About a thousand years later just such a shift occurred—and though it didn’t go extinct, the Joshua tree’s range was severely limited.  Now another change in climate, this time caused by human activity, threatens to shrink the range of the species even more.  Cole predicts that Joshua trees won’t be able to keep pace with the shift in suitable habitat if the current trend of global warming continues.

Losing Joshua trees from the American Southwest would also mean the loss of a relationship that has evolved over thousands of years between the trees and a unique desert insect.  Larvae of the yucca moth Tegeticula paradoxa, a small gray insect, feed only on the seeds of Joshua trees.  The adult moths in turn are the only insects that pollinate Joshua tree flowers.  This case of “mutualism,” where two species evolve to benefit one another, illustrates a certain type of evolutionary adaptation so well that it is cited in countless ecology textbooks as an example of mutualism. 

Now Joshua trees and the moths that depend on them have also become a symbol of how global warming threatens to change local climates faster than species can adapt or move to more hospitable habitat.  Today Joshua tree seeds are distributed by small mammals like pack rats that do not wander as far as giant ground sloths, so saving the species may depend on human intervention. 

By transporting seeds to cooler climates and planting new populations of Joshua trees there, conservationists could preserve the species as the climate warms.  Unfortunately this won’t be practical for countless other plants and animals that are also affected by climate change, but which haven’t been studied as thoroughly or are simply impossible to move and establish in a new area.  The survival of many species, each with its own unique evolutionary history and relationship to natural ecosystems, will depend on limiting future climate change as much as possible. 

Photo credit: Jimmy Harris

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