Predicting plant community responses to biotic and abiotic stressors to understand future impacts on ecosystem functioning has become a foremost task for ecological research. However, given the vast array of stressors faced by plant communities, it is still unclear whether species or community responses are driven by either individual or combinations of stressors. For example, in semi-arid grassland systems, water availability can heavily influence species interactions, however small mammals may alter these interactions via granivory/herbivory pressure. Using invaded annual grassland communities in the Carrizo Plains National Monument (Maricopa, California), we examined whether functional traits can predict species and community level responses to water availability and the presence of small mammals. We measured plant community composition over five years in 320 x 1m2 experimental plots in which the presence of small mammals and water availability (50 % decrease, ambient and 50 % increase) were manipulated. We used a range of functional traits including seed mass, specific leaf area and maximum height that are known to correlate to with resource acquisition, competition and granivory pressures, to predict responses in species richness and vegetation cover under different water availability conditions and small mammal presence.
Results/Conclusions
Overall, species richness and vegetation cover responded differently to water availability and the presence of rodents. Changes in species richness was not influenced by increased water availability, but was reduced in drought plots, with this effect magnified in drier years. The influence of small mammals on species richness was also evident in drier years, and was consistent between drought and water addition plots. Variation in water availability and small mammal presence did not influence changes in vegetation cover. Functional diversity predicted changes in species richness, with low functionally diverse communities displaying decreased richness, regardless of water availability and small mammal presence. Functional diversity was not a good predictor of changes in vegetation cover in response to water availability or the presence of small mammals. Our results suggest that changes in plant community responses to small mammal pressures are magnified in drought conditions and using functional traits to predict community level responses to biotic and abiotic stressor will likely be context dependent.