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Facilitation is usually considered an ecosystem service — positive interactions between species can increase biodiversity and ecosystem function. But what happens when the beneficiary species is an exotic invader that degrades ecosystems?

This spring, I sampled the annual plant community using a paired shrub-open microsite contrast at six sites situated along an aridity gradient from Mesquite, NV to Panoche Hills, CA.

At each study site, I found that Bromus rubens, an annual grass species native to Eurasia and northern Africa but highly invasive across southwestern North America, associated strongly with native shrubs. Specifically, Bromus abundance, biomass, and fecundity were consistently greater under shrubs than in the open. Interestingly, the strength of facilitation did not depend on the size or species of foundation shrubs, but it did depend on aridity — Bromus-shrub associations were strongest in the most arid environments.

A halo of beefy Bromus rubens plants growing under an Ephedra shrub in the Cuyama Valley, CA. This pattern was typical at all study sites.
Bromus was facilitated in abundance at all sites, but Bromus-shrub associations were strongest in the most arid environments.

These findings suggest that invasive B. rubens can associate with native foundation shrubs across a large portion of the non-native range, and that positive interactions mediated by native shrubs can potentially exacerbate B. rubens invasion by increasing abundance, fitness (i.e., fecundity), and competitive ability (i.e., size). The positive relationship between aridity and facilitation strength suggests that the threat of facilitated invaders may be greatest in the most arid conditions.

Oh, and look what else I found:

An endangered desert tortoise chillin’ under a shrub…amongst the Bromus. Don’t see that every day!