Fire, rodent herbivory, and plant competition: implications for invasion and altered fire regimes in the Mojave Desert

Biological invasions are responsive to changing wildfire regimes related to human activities that are altering biological communities. Our objective was to investigate how fire, rodent activity, and competition among plant species modify plant community structure, invasion patterns, and vulnerability to altered fire regimes. We imposed experimental fires, and reduced rodent density using fencing in a full factorial design and quantified competitive interactions among plant species in the northeast Mojave Desert that has experienced dramatic increases in plant invasion and fire in recent years. Vegetation surveys were conducted in the experimental plots to determine plant density, cover, and biomass of herbaceous plants over a 5-year period. Rodent exclusion increased the density, cover, and biomass of Bromus rubens, an invasive annual grass, and density of forb species. In contrast, rodent exclusion decreased the density, cover, and biomass of Schismus spp. another dominant annual invader. Fire increased Schismus spp. and forb species density, cover, and biomass but decreased B. rubens density. Negative spatial correlation between B. rubens and Schismus spp., and forbs indicated interspecific competition among the dominant plant species. Fire reduced rodent community diversity (Shannon's) 2.5-fold, which was correlated with increases in B. rubens cover and biomass, and native forb diversity. Fire, high rodent diversity, and competition from the other plant species may decrease fire potential in our study system by reducing the density and biomass of B. rubens, which because of its taller growth form tends to ignite and carry fire better than Schismus spp. and forbs.