- In desert ecosystems, plant species coexist by growing and flowering at different times, cued by rainfall and other triggers. This diversity strengthens community stability.
- The invasive Sahara mustard arrived in California a century ago, likely via the palm trade. But until now, its impacts on desert ecology have been difficult to measure.
- Scientists recently used long-term data to understand the mustard’s effects on native plant communities on sand dunes in the Mojave Desert.
- The study found that Sahara mustard can overtake desert plant communities, making them less diverse and less stable in an increasingly unpredictable environment.
In southern California’s Coachella Valley, pale sand dunes sprawl under the desert sun, shifting and re-forming in the wind. In dry years, lone shrubs dot the dunes. But the landscape bursts with color and greenery after a wet winter. That’s because seeds of desert plants may wait in the soil for years, even decades, for the right conditions to sprout.
But an introduced mustard species that arrived a century ago is having an unexpected impact. The Sahara mustard (Brassica tournefortii) is creating less stable plant communities by outcompeting native species in the Coachella Valley’s Mojave Desert, according to a 17-year study published in the journal Ecology. These findings raise questions about how well native plants will persist in an increasingly unpredictable environment.
Across ecosystems, “asynchrony” – species responding differently to weather and other cues – increases diversity. In the Mojave Desert, many plant species coexist by germinating at different times, or even in different years.
Generalist species such as Sahara mustard can upset this balance. “The invasive species that come in…can germinate year-round, even though our natives wait because they’re used to their germination cues,” said Clarissa Rodriguez, a plant ecologist at the University of San Diego who led this research. Although the region’s spring bloom of native plants has ended for this year, she said, “If you go out right now to the field, you can probably see baby Sahara mustards coming up while a bunch of our native [plants] are dying down already.”
This flexibility gives Sahara mustards an advantage because they will sprout at the first rain. By the time the winter rainy season gets into full swing, they’ve already taken over patches of desert, blocking other species from growing.
Sahara mustard, an annual plant, grows up to about 1 meter (3 feet) tall. It has broad leaves, blooms tiny yellow flowers, and produces long, narrow seed pods. Native to the Mediterranean and Middle East, it most likely arrived in California when the U.S. began importing date palms to the Coachella Valley at the turn of the last century. Since then, it has spread across arid lands in the U.S. and Mexico.
Desert ecologists previously struggled to measure the impacts of nonnative species such as the Sahara mustard. Asynchronous desert plant communities behave very differently over time, but research projects generally only lasts a few years, making it hard to capture those longer-term dynamics.
For a time, scientists believed that nonnative species couldn’t thrive in the Mojave Desert’s inhospitable sand dunes, but this research shows otherwise. “These deserts were thought to be more resistant to invasion because it’s so hot, there’s limited resources, there’s no way [introduced species] can outcompete natives,” Rodriguez said. “But now we’re seeing that that’s not the case.”
Rodriguez analyzed 17 years of data collected at the Coachella Valley National Wildlife Refuge to learn how Sahara mustards were impacting plant diversity over time. Researchers returned each year at peak flowering season and surveyed 228 plots. Despite harsh conditions, the sand dunes support more than 90 native plant species, including the federally endangered Coachella Valley milkvetch, a small member of the pea family with purple flowers and feathery leaves.
As Sahara mustard expanded across the study area, the overall community stability – the number and prevalence of different plant species – declined.
Although this research did not include desert animals, Rodriguez said that these changes to plant diversity also may impact fauna. For example, “Ants like to forage on various seeds, but when Sahara mustard comes in, they reduce that foraging availability,” she said. “And then the lizards who prey on the ants have less to eat, and then it’s just a cycle.”
Meanwhile, native plants are ever more stressed. “[They’re] getting hit with so much. They’re responding to changing climates, but also now they’re getting more competitors that are really good at acquiring resources,” Rodriguez said.
The connections between decreasing desert plant diversity and decreasing community stability make sense, said desert ecologist Scott Ferrenberg, a professor at the University of Montana’s Department of Ecosystem and Conservation Sciences who was not involved with the study. He noted that Rodriguez’s findings support the idea that species asynchrony increases community stability.
Ferrenberg said he was surprised that the amount of Sahara mustard in a study plot impacted other plant species more than how frequently it appeared in that location. It was just the “sheer magnitude” of the mustard that mattered most, he said.
Sahara mustard may be benefitting from human-caused climate change to the detriment of native species. “You get this one-two punch that you have a climate system that’s changing and potentially decreasing the abundance of native species that are otherwise moderate- to low-abundance, and you have this invader that’s affecting the overall stability of the system,” Ferrenberg said.
The Sahara mustard and its potential impacts on community stability are not limited to the Mojave Desert’s wind-whipped sand dunes. Rodriguez recently began a new position at the University of California, San Diego, where she said she was “mortified” to encounter the plant. “I’m like, ‘Wait, so you’re really good at growing in desert, but also really good at growing on the coast?’” It’s further evidence of how a generalist species like Sahara mustard can thrive in a changing world.
Banner image: Sahara mustard plants in California. Image by thesafarihiker2 via iNaturalist (CC BY-NC 4.0).
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Citation:
Rodriguez, C., Sweet, L., Davis, M., Heacox, S., Barrows, C., Larios, L. (2025). Temporal invasion regime attributes influence community synchrony and stability in an arid land system. Ecology, 106(4), e70081. doi:10.1002/ecy.70081
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