Dryland communities such as aeolian sand communities are becoming more vulnerable to changing climate and invasions, which are altering plant community composition and threatening the flora and fauna that depend on these specialized harsh environments. These annual plant dominated communities are often characterized by high inter-annual variability of abiotic factors. In a system that is heavily dependent on disturbance from wind movement and limited by water, opportunities for invasion may arise through temporal fluctuations in disturbance and resource availability. Hence, It is critical to understand how temporal patterns of abiotic factors influence invasion dynamics. Here, we used a long-term study to investigate the relationship between abiotic factors and invader abundance in a dryland system to understand their impacts on native community dynamics. We collected rainfall, temperature and wind data, as well as species abundance (i.e. density) of plants growing along 81, 100 meter belt transects annually from 2003-2016 at the Coachella Valley National Wildlife Refuge in Palm Desert, California. We used regression analysis to test the predictions that 1) patterns of species abundance are influenced by temporal variability in abiotic factors 2) invader abundance is increased when additional resources (i.e. rainfall) are supplied into the community.
Results/Conclusions
Annual plant communities in this system were heavily influenced by precipitation. Years of above average annual rainfall generally increased the overall abundance of annual plants, although the strength of the effect of annual precipitation varied according to the origin of the species. Invasive species displayed a boom-bust pattern of abundance over the 13 year study. This pattern was strongly driven by one particular invasive species, Brassica tournefortii, Sahara mustard, whose abundance was highly correlated with years of above average rainfall. Native species abundance also increased during years of above average rainfall, however their abundance was consistently lower than invasive species throughout the study. Wind speed and average temperature were not found to have a strong effect on plant abundance for either natives or invasives, likely because these factors did not exhibit high temporal variability in contrast to the high inter-annual variability of precipitation. Our results suggest that in dryland systems, plant communities are highly influenced by variability in abiotic factors, and that invasive species are better adept at taking advantage of resource pulses compared to the natives. With climate change models projecting increases in precipitation variability, more conducive environments for invasion may emerge highlighting the importance for understanding invasion dynamics.