Knowing how invasive grasses compete could help preserve native grasslands, according to Stanford researchers

Rolling golden hills are an iconic landscape in California, but these golden grasses aren’t native to the Golden State. As invasive European grasses swept through California, their numbers quickly surpassed that of native species.

Stanford researchers monitor plant growth at Jasper Ridge Biological Preserve to learn about native and invasive species. (Image credit: L.A. Cicero)

Although California’s grasslands are yellow with invasive species, some native bunch grasses survived. The question is how best to protect those that remain. To find answers, Erin Mordecai, assistant professor of biology at Stanford University, turned to grasslands near campus to understand how native and invasive grasses compete.

“We’re interested in the plants and animals of California because it’s an unusually diverse place,” Mordecai said. “A lot of these species only occur here, which is why it’s important to protect them.”

Mordecai and her research team monitored plant growth in plots at Jasper Ridge Biological Preserve, located in the foothills near Stanford campus, and extrapolated into the future with computational models. The team found that invasive species generally outcompeted native ones, and the first invaders tended to dominate the landscape. The study was published January 10 in The American Naturalist.

Rock, paper, scissors

With the help of Jasper Ridge docents, Mordecai and her team planted two native and three invasive grass species, in various combinations and densities, in experimental plots. Then they waited.

Erin Mordecai is an assistant professor of biology at Stanford. (Image credit: L.A. Cicero)

At the end of the growing season, they counted the species that remained. Lawrence Uricchio, a postdoctoral scholar working with Mordecai, built a computer model that predicted how grasslands would continue changing over time, based on their data. The fieldwork ended, but the experiment continued in the computer simulation.

Traditionally, ecologists try to understand species competition by pitting two species against each other. It’s like playing rock, paper, scissors. Paper always beats rock; species A always beats species B. But in real grassland ecosystems, outcomes are much more complicated.

“If we just observe the plots, it can be hard to figure out the long-term outcomes,” Mordecai said. “To really observe population dynamics, we need to observe for decades. That’s where the mathematical models are powerful.”

Protecting native grasses

In general, the invasive grasses possessed fitness advantages that allowed them to dominate over native grasses in the ecosystem. However, Mordecai and her team found that which invasive species predominated depended on which species arrived first.

“The earlier arriving species almost always win in the competition in our model, which recapitulates what we’ve observed in Jasper Ridge,” Uricchio said.

Most ecology research focusing on competition between species is conducted in undisturbed grasslands, where there are few exotic species. The team members were excited to conduct their experiment in Jasper Ridge, where both native California bunch grasses and invasive European grasses reside.

Mordecai speculates that species competition in grasslands with both native and invasive species may be different than pristine or protected areas. Interactions between native and invasive species are common in ecosystems that have been disturbed by paving, fencing, trampling and overgrazing. Disturbed ecosystems are susceptible to a loss of biodiversity, so ecologists like Mordecai want to understand how to conserve these habitats and the species that live there.

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Funding for this research was provided by the Kennedy Endowment at Jasper Ridge. Experimental fieldwork was completed with the help of volunteers and docents at Jasper Ridge Biological Preserve.