Beloved California desert plants threatened by climate change, thirsty animals – Desert Sun
Veteran desert biologist Jim Cornett was astonished to see a bright yellow and black caterpillar munching on a spiny ocotillo plant one late March day. Normally the razor-spiked plants would not be considered fine dining — or dining at all — for those sphinx moth larvae and other creatures.
But the more he looked at the stand of gangly, twisted ocotillo, best known for their fluttering red springtime blooms, the more caterpillars he saw. It was a tiny but telltale sign of the unmistakable decline of those iconic plants and others in California’s deserts due to global warming, he and fellow experts say. And animals desperate for moisture are likely playing a role.
In a pair of recently published research articles, Cornett, of JWC Ecological Consultants, describes how ocotillos and another botanical giant of the southwest desert, Washington fan palms, are succumbing to the impacts of drier, hotter weather. Joshua trees, a keystone species that is important to many animals, are also in trouble, they said.
“Our beloved desert plants are declining,” Cornett said. One less understood but likely reason, he said, is animals devouring the plants as a last-ditch effort to survive themselves. During droughts, bighorn sheep are also now known to eat ocotillo leaves, and antelope squirrels strip away their bark. Black-tailed jackrabbits remove Joshua tree bark to get at the tree’s moist interior, and desert woodrats consume their leaves.
“None of these animals were recorded to have attacked these plants before climate change,” he said. “In short, cascading indirect impacts may be more damaging than the direct effects of heat and drought.”
By his calculations, ocotillo in Anza Borrego Desert State Park are dying at a rate of about 1% a year, meaning in a century, they’ll be wiped out. Although he hasn’t calculated similar data for ocotillo trees he studies in Joshua Tree National Park, the rates of loss are even higher there based on his observations, he said.
While it is now “widely known” that extreme heat and longer, more frequent droughts “place great burdens on species, particularly plant species,” he said the role of animals desperate for moisture — like the sphinx moth caterpillars tucking into the ocotillo in March 2017 — could be the tipping point that kills an already declining plant or population. That’s the subject of his note in the Pan Pacific Entomologist, published online in December.
“Less known is the indirect impact of all kinds of wildlife, particularly in deserts where animals struggle to find sufficient water,” he said. “The article describes how, apparently for the first time, the larvae of white-lined sphinx moth began eating ocotillo leaves to survive drought.”
Earlier that March, Cornett had spied the bright black and yellow larvae busily munching on brown primrose, their preferred main meal in that area. But it had been a hot, dry spring, and when he returned 11 days later, every primrose was gone or wilted to the ground. Instead the larvae, as the juvenile stage of moth species are scientifically known, were busy devouring ocotillo leaves.
“It’s the first time that’s ever been seen,” said Cornett, who has tracked 500 ocotillo plants across the Southwest each spring since 2009. “But I don’t think it will be the last.”
He said the ability of the sphinx moth to switch quickly to another food source means it is faring fine so far as weather shifts, but its consumption of ocotillo leaves “places another burden on an already stressed iconic and keystone plant” in the state park.
Elsewhere in slightly cooler, wetter spots in the southwest, including Big Bend National Park in Texas and Carlsbad Canyons National Park in New Mexico, the ocotillo stands which Cornett studies are doing somewhat better.
Another desert biologist said the findings confirm work her team had done about possible future threats to ocotillo due to climate change.
“With a decrease in the consistency of rainfall that we’ve seen with the recent drought and potentially in the future, it could make it hard for the ocotillo stands to sustain themselves,” said Lynn Sweet, director of UC Riverside Palm Desert’s Center for Conservation Biology, in an email. “Past studies in our office have shown that this species may face a decrease in range with climate change that is significant, and this species ranked as moderately vulnerable … Less vulnerable than the Joshua tree or black brush or jojoba (a fellow Colorado Desert species) but still fairly vulnerable due to the predicted range reduction.”
Both scientists said Joshua trees are in trouble due to climate change, and Cornett suspects thirsty animals again are a culprit.
“An established Joshua tree can likely withstand several successive years of increased heat and drought, possibly even wildfire, but not if it is losing its bark to jackrabbits and leaves to woodrats,” he said. “It is the accumulation of all impacts, both direct and indirect, that hastens the impact of climate change.”
As for the Washington fan palms, which are unique to southwest deserts of North America, older plants are dying as traditional sources of water underground shrivel, and no new plants are taking root. That’s the finding of Cornett’s second paper, recently published in Palms, the journal of the International Palm Society.
He focused on the Seventeen Palms population of Washingtonia filifera, also in Anza Borrego Desert State Park, which has been analyzed by scientists for 104 years.
“A dramatic shift in structure from juvenile to adult dominance was recorded,” he writes. “With only two tentative recruits to the population since 1984, W. filifera seems likely
to be extirpated at Seventeen Palms if existing conditions persist.”
In other words, the fan palms for which the site is named are dying out and no new baby palms are taking their place, “most likely due to diminished spring flow. A decline in precipitation, increase in frequency and duration of drought and crustal
displacement from earthquakes are possible factors in the reduction of
moisture availability. Rising temperatures contribute to the moisture deficit.”
Sweet said Cornett’s research provides important clues as they and others try to determine what is changing due to a warming climate, and what is due to more typical changes in what has always been a dynamic system.
“Long-term data sets like his repeat look at Palm density is the type of evidence we need to understand what’s happening,” she wrote. “It’s true that you need small young palms to get large, older palms, and any reduction in the number of young palms can spell trouble for the sustainability of a group of palms.”
She explained, “These oases are isolated in the landscape, and that poses a challenge to this species. Having very large leaves, these plants are strongly tied to water sources, because they evaporate so much water while doing photosynthesis. They are not a classic desert plant in this sense. … Any change in hydrology or recharge for the water sources will affect this species.”
Because it’s a plant that structures wildlife habitat and “is of interest and use to humans across its range,” she said, “it’s an important species to monitor.”
Sweet and Cornett agree it’s important to determine whether there are trends or declines in several oases to definitively tie the cause to climate changes, since changes in a single stand might also be tied to human withdrawal of water from that site, or a shift in an earthquake fault that shuts off previously available subterranean water. He’s studying a second major fan palm stand as part of his research for a book on 100 years of changes in desert plants, including his own observations over 37 years.
Overall, Cornett said he’s been “shocked” to see “wholesale reduction of plant life” in what were “beautiful desert, vegetated flatlands.”
Watering wild plants in the desert or other direct measures are likely to fail, he said. What’s needed is recognition of the loss, and taking action to finally slash dangerous greenhouse gases in the atmosphere. The solutions are not easy or immediate, he said, but they’re critical.
“I think one strategy is to show people who are surrounded by ocotillo and fan palms, people who love these things and use them on their business stationery, that this is not something being done by China halfway around the world,” he said. “It’s something happening right here today, right where we live.”
Cornett will give a Zoom talk on imperiled Joshua trees on Wednesday, Jan. 26 from 3:30 to 4:30 p.m. via the Palm Springs Library.
Janet Wilson is senior environment reporter for The Desert Sun, and co-authors USA Today’s Climate Point newsletter. You can sign up to get Climate Point for free in your inbox here: She can be reached at email@example.com or @janetwilson66 on Twitter