The interacting influences of rainfall and biotic resistance on introduced plant invasion in an annual serpentine grassland

Non-native species have established in nearly every environment on Earth, and when they become invasive, can cause a host of issues, including ecosystem degradation and extirpation of native species. Global change may shift how invasive species spread through changes in climatic means and variability, shifting ranges and how species compete. Resident species in an environment can resist invasion through competition with the invader for space, water, or other resources, called ‘biotic resistance’. This thesis empirically examines how changes in rainfall, which is expected to shift due to global change, and biotic resistance interact to limit the success of a non-native grass, Bromus hordeaceus. In an annual grassland in northern California, I conducted a field study using rain shelters and neighbouring plant removal to test the strength of biotic resistance under three rainfall treatments and examined how rainfall influenced the effects of biotic resistance on invader fecundity. I used Bayesian modeling to analyze my field data and found that biotic resistance was strongest in the rainfall treatments that most benefited the invader, leading to no net effect of rainfall on invasion success. I was also able to decouple two key factors of biotic resistance: plant abundance and per-capita interaction strength. My results showed that because the resident community was highly diverse, strong biotic resistance could be maintained under multiple rainfall scenarios as different rainfall treatments benefited different species in distinct ways (abundance or per-capita effects). Overall, my thesis provides unique insights into how biotic resistance and climate interact, including decoupling drivers of biotic resistance on a species level (which, to my knowledge, has not been done before), and finding that small changes in rainfall may not provide opportunities for invasion if the resident community and invader respond similarly to environmental change. These findings may be useful for invasive species management or for modeling species spread under various climate scenarios.