Despite being stressful environments, the arid regions of western North America have by now been invaded by non-native plants, notably by annual, Old-world species. Predominately among those are annual grasses that today often play dominant roles in the desert ecosystems. A good example is the red brome (Bromus madritensis rubens) that today dominates many desert shrub-associated annual plant communities. We have been studying the role of this species in the desert plant community and have been documenting its population dynamics at one site in the Mojave Desert in California for the last 15 years in order to investigate the effects of climate variability on the fate of an invader. The main question is whether the suit of high-risk population strategies - as expressed by this originally relatively mesic species - is sustainable in the face of increasing drought. For this we followed the population dynamics of the species on permanent plots and along transects ranging from relatively low to high environmental stress.

Results/Conclusions

Bromus population size and individual fecundity is fluctuating dramatically in response to rainfall fluctuation.  Since the species fails to build up a long-lasting seedbank population, growth is constrained by the conditions of the both the previous and the current year. Starting with very high densities in the 1990s, the current decrease of rainfall in the study region caused the almost complete disappearance of the species in wide parts of the study sites. With buffering seedbanks unavailable, the species can only reinvade from mesic microsites that act locally (washes, roadsides) or regionally (higher elevation in adjacent mountain ranges) as refugia. Using spatial models we show that such reinvasion is only expected under unlikely climate scenarios with increasing rainfall. We document a typical example of a boom-and-bust phenomenon as it is predicted but little described for invasive species. We also demonstrate that abiotic stress can act as one the few natural factors counteracting species invasion.