The desert’s harshness breeds strange bedfellows—desert bats fly hundreds of miles in the night to feed on agave, and seedlings often rely on their dead forebears to provide shelter. But those relationships can make it hard to adapt in a changing climate.
Rising temperatures have set the natural world creeping uphill or northwards in search of relief. But ecosystems don’t move in lock-step. The foxes might outpace the tortoises which might outpace the trees. In the process, the ecological deck is shuffled. These changes threaten to unravel the interspecies networks on which desert organisms depend. And the Joshua tree, an international symbol of the American desert, seems to have been dealt a bad hand.
By 2100, scientists predict that Joshua Tree National Park will lose almost all of its Joshua tree habitat to climate change. But the Joshua tree relies on other species to survive, and a new study on the relationship between Joshua trees and a type of symbiotic yucca moth piles on the bad news. According to the study, yucca moths, which the Joshua tree relies on to reproduce, aren’t healthy in the few places where Joshua trees can survive the heat.
“We’re finding that not only does climate change affect species, it affects how species interact with each other,” says Juniper Harrower, the study’s first author, and a graduate student at the University of California Santa Cruz.
Joshua trees and the yucca moth depend on one another in a striking example of co-evolution. If the yucca moths don’t survive in the park’s cold pockets, Joshua trees won’t make it long either, and both species will disappear from the park.
“In the cooler parts of the park,” Harrower explained, “the trees are just not sexually reproducing.” That, she says, will make it impossible for them to move with the changing climate. (Learn more about climate change impacts to popular national parks.)
Of moths and trees
Joshua Tree National Park sits at the extreme southern edge of the Joshua tree’s range, and so trees there are the first to be threatened by climate change. Already, says Harrower, “There are more dead trees than living trees in the hottest part of the park.”
That doesn’t necessarily mean Joshua trees will be completely wiped from the park. Previous studies suggested that trees might survive in high, cool pockets of the park’s north—what scientists call climate refugia.
But scientists had noticed that in some high-altitude refugia, Joshua trees didn’t seem to reproduce normally. “You see these ‘fairy rings,’ a circle of trees with nothing in the middle,” says Chris Smith, a biologist at Willamette University. The fairy rings turn out to be the result of asexual reproduction—the trees can sprout clones from their roots—but it’s not a winning strategy in the long term. Clones are vulnerable to pests, and can’t disperse the species.
Harrower wanted to know if interspecies relationships played into the story. Although the relationship between moths and Joshua trees was well known—“It’s literally in every biology textbook,” she laughs—no one had studied how the trees and moths overlapped in Joshua Tree National Park, or how the insects respond to temperature.
To find out, she surveyed Joshua tree groves in a range of microclimates, from the low, hot southern park, to the cool slopes in the north. At each site, she collected tree flowers and fruits and counted moth populations with sticky traps.
“We wanted to know where the trees were happiest, and if moths had anything to do with that.”
Climate change already seems to have taken a toll on the symbiosis. “We don’t see any moths in the southern edge of the park,” she says. Trees there no longer bear fruit, and some are beginning to die altogether.
But in the high-altitude northern areas of the park, the story wasn’t much better. There weren’t any moths there, either, and few flowers. While there were some new trees, they were all clones. Reproduction seemed to be most successful in a tight band of middling elevations—areas where Joshua trees aren’t predicted to last much longer.
“Farming the trees”
The relationship between Joshua trees and yucca moths is particularly special, even in a desert full of surprising relationships.
“It’s like the moths are farming the trees,” says Harrower.
The yucca moth intentionally fertilizes Joshua trees, using a set of special “tentacles” to carry balls of pollen from flower to flower. And they do it without an immediate nectar reward: Adult yucca moths don’t eat a bite during their adulthood, which lasts only five days.
“On its face, it seems like the moth is doing a ton of work with no payoff,” says Smith. The ball of pollen is huge compared to the moth, “and they sure seem like pathetic fliers. When you disturb them, they just fall to the ground and writhe.”
But the moths need the trees, too. “Yucca moths need to lay their eggs inside of a Joshua tree flower,” explains Harrower. “Their babies develop inside the flower’s ovary.” The resulting seedpod shelters both species’ young. And the tree provides the young moths with food: The larvae chew their way through a good number of the fruit’s seeds on their way to maturity.
Without moths, surviving trees will be more like echoes than survivors. There may be a few scattered locations where both moths and trees manage to hang on, says Harrower, but the overall story remains the same. Joshua trees will be gone from the overwhelming majority of the park bearing their name.
The park’s uncertain future
Joshua trees bear more than the weight of symbolism in the park. “What happens to an ecosystem that loses its main tall tree?” asks Harrower. “All sorts of creatures interact with Joshua trees—insects, kangaroo mice, loggerhead shrikes. They create microhabitats by creating shade in the desert. Young trees germinate nearby. You have a reverberation through the entire ecosystem.”
There are other, immediate threats to Joshua trees as well. Invasive grasses have proliferated, and with them, fires. “These plants didn’t evolve with fire, so they just don’t come back from it,” says David Smith, the park’s superintendent.
The park’s skyrocketing popularity can itself be a problem. Joshua Tree’s annual attendance has doubled in the last four years, to nearly 3 million visitors. The number of rangers, however, has remained constant. “We’re just in crisis mode right now,” Smith says. By the time 2100 rolls around, he says, “There might not be any trees left anyway.”
For many of those studying the trees, there’s also a personal dimension to the loss. Harrower grew up just outside the national park, and the time she spent working in the nearby Big Morongo Canyon Preserve set her on her course as a naturalist. She’s also been the park’s artist in residence, and designed material for the park’s interpretive programs.
Harrower recently shared her research with an audience at a local music festival. She says, “I asked people what it means for them to lose Joshua trees. The thing that kept coming up was their sense of identity and culture. People identify with these trees as part of their home.”
That’s been ringing in her head ever since, because it’s how she feels, too. “The idea of not having these trees around is devastating and strange.”