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:
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:
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.
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.
One of the most powerful approaches for understanding biological invasions by non-native species is to examine ecological patterns and processes in both the native and non-native ranges of invasive species. Here’s a great article on the subject:
The number of articles published on biological invasions has increased exponentially over the last 20 years, but biogeographically explicit studies replicated in the native and non-native ranges of invasive species are still VERY rare. This hampers our mechanistic understanding of the invasion process and therefore our ability to explain, predict, and manage biological invasions.
Bromus rubens (i.e., red brome) invasion in the Mojave Desert provides a great opportunity to address this knowledge gap. We are planning to examine the individual and joint effects of shrub facilitation and post-dispersal seed predation on the abundance of B. rubens in its native (Israel) and non-native (California and Nevada) ranges. This experiment is broadly interesting because it allows us to test the relative importance of the effects of two fundamental biotic interactions on two continents. Here’s a cartoon of our experimental design:
Solid circles represent functional exclosures that effectively exclude seed predators; dashed circles represent non-functional exclosures that admit seed predators. Note the control treatment that monitors recruitment from seed banks. This is a full-factorial design that crosses shrub facilitation (open vs. shrub microsites) with seed predation (functional vs. non-functional exclosures). Pretty cool.
We will replicate this setup at 5 shrub-open pairs per site at 6 sites across the Mojave (GPS coordinates are preliminary):
We will replicate the experiment at 5 sites in the Negev Desert of Israel with the help of Dr. Merav Seifan of the Ben Gurion University of the Negev. She rocks! Site locations and GPS coordinates in Israel are forthcoming.
The biogeographic contrast of the effects of seed predation can be considered a test of the enemy release hypothesis, which has only been examined once in the context of seed predation:
Bromus tectorum, one of North America’s most problematic invasive plant species, may owe at least part of its explosive success to escaping the effects of generalist consumers in the non-native range relative to the native range.
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.
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.
The enemy release hypothesis (ERH) of plant invasion asserts that translocation to novel communities allows exotic plants to escape population controls imposed by natural enemies in native communities. The ERH predicts that 1) invader densities are greater in non-native communities than native communities, 2) natural enemies impose strong negative effects on invader abundance in the native range but in not the non-native range. These predictions are straightforward, but testing them involves conducting parallel vegetation surveys and enemy exclusion experiments in both the native and non-native ranges of invaders. Due to logistic challenges, very few studies have done this.
As part of an international team of collaborators from the USA, Canada, and Poland, we are explicitly testing the predictions above with respect to the prickly cucumber, Echinocystis lobata (fruit pictured below). This climbing vine is native to North America but invasive in Poland, where it can dominate local communities and extirpate native competitors.
So far, our surveys indicate that E. lobata is much more abundant in Poland than anywhere examined in N. America, and that E. lobata plants are larger and more fecund in Poland than in N. America. It also seems that physical defenses aimed at protecting seeds from generalist granivores are present at much higher frequencies in Poland than in N. America, which is very cool! We look forward to results from enemy exclusion experiments.