Abiotic and biotic resistance to grass invasion in serpentine annual plant communities

Biological invasions severely impact native plant communities, causing dramatic shifts in species composition and the restriction of native species to spatially isolated refuges. Competition from resident species and the interaction between resource limitation and competition have been overlooked as mechanisms of community resistance in refugia habitats. We examined the importance of these factors in determining the resistance of California serpentine plant communities to invasion by three common European grasses, Avena barbata, Bromus diandrus, and Hordeum murinum. We added seeds of each of these grasses to plots subjected to six levels of resource addition (N, P, Ca, H₂O, all resources together, and a no-addition control) and two levels of competition (with resident community present or removed). Resource limitation and competition had strong effects on the biomass and reproduction of the three invaders. The addition of all resources together combined with the removal of the resident community yielded individual plants that were fourfold to 20-fold larger and sixfold to 20-fold more fecund than plants from control plots. Competitor removal alone yielded invaders that were twofold to sevenfold larger and twofold to ninefold more fecund. N addition alone or in combination with other resources led to a twofold to ninefold increase in the biomass and fecundity of the invaders. No other resource alone significantly affected native or invader performance, suggesting that N was the key limiting resource during our experiment. We found a significant interaction between abiotic and biotic resistance for Bromus, which experienced increased competitive suppression in fertilized plots. The threefold increase in resident biomass with N addition was likely responsible for this result. Our results confirm that serpentine plant communities are severely N limited, which, in combination with competition from resident species, promotes the resistance of these systems to invasions. Our work suggests that better understanding the relative sensitivities of invaders and residents to the physical environment is critical to predicting how abiotic and biotic factors interact to determine community resistance.