Do native shrubs facilitate exotic species equally??

I just returned from some exciting desert fieldwork! Last year, I sampled the annual plant community under shrubs and in the open at six sites across the Mojave and San Joaquin Deserts, and here’s the gist of what I found:

RII values (negative values indicate negative associations with native shrubs; positive values indicate positive associations) for nine vegetation measures taken at six study sites across the Mojave (Mesquite, Mojave, and Vegas) and San Joaquin (Panoche, Cuyama, Carrizo) Deserts. Panels are arranged in ascending order of relative aridity. Data currently in review at Diversity and Distributions.

Spatial association with native foundation shrubs strongly and consistently increased the abundance, biomass, cover, and fitness of the dominant invader Bromus rubens but not the native annual community. This is interesting because positive interactions mediated by native species are seldom invoked to explain the success of exotic invaders. Very cool, but what about the system’s many other exotic, invasive species? Is facilitated invasion species-specific?

To tackle this question, I returned to the desert and sampled the annual plant community at nine study sites scattered across the Mojave and San Joaquin Deserts. Six are repeats from last year, three (Yuc, Cna, Hea) are new:

The three leftmost sites are in the San Joaquin Desert. The six rightmost sites are in the Mojave Desert.

At each site, I sampled the annual plant community at 20 pairs of shrub and open microsites with a 0.5m x 0.5m quadrat. Shrub microsites were the area immediately beneath the canopies of foundation shrubs, and open microsites were areas >1m from any shrub canopy. In each microsite, I estimated the abundance of the native annual community as a whole and the exotic annual community as a whole. I collected species-specific measurements for the abundance, biomass, and fitness of the following exotic plant species: Bromus rubens, B. tectorum, B. diandrus, Erodium cicutarium, Schismus spp., and Brassica tournefortii. Each of these exotic invaders can contribute to biodiversity loss and diminished ecosystem function.

Strong facilitation of Bromus rubens near Las Vegas, NV. Bummer, but cool.

We’ll have to wait for the official stats, but it seemed that B. rubens, B. tectorum, and B. diandrus formed strong and consistent positive associations with native shrubs. Each of the other exotic species ( E. cicutarium, Schismus spp., and B. tournefortii ) associated with native shrubs more sporadically. The most interesting observation was this: it seemed that B. rubens controlled the game — when it was super abundant under shrubs, nothing else (except other bromes) strongly associated with shrubs. When it was less abundant, other exotic (and sometimes native!) species apparently associated with shrubs more strongly. This suggests a competitive hierarchy in which exotic bromes, and especially B. rubens, rule the understory, followed by other exotic species, followed by native species. Again, the stats will give us the official story, but I think that’s what I saw! Very cool.

More cool stuff:

There was a crazy flash flood on Tues, April 14. This is the road I was driving on!
That’s 1/2″ of hail. Did it hurt? Hail yes (sorry).
Sunday, April 12 was a glorious morning! Cholla and Fouquieria near Turtle Mountain, NV. Did I mention glorious?