Disturbance effects on population growth in two invasive grasses in a California grassland

Background/Question/Methods

Invasions by introduced species pose a threat to biodiversity globally. Low-density population growth rate is a key in determining the initial success and spread of an invading population. Demographic rates can be affected by extrinsic disturbance factors including grazing by livestock. Furthermore, grazing regime could affect growth rates differently depending on the density of individuals in a patch, for example by disrupting plant-litter feedbacks (when the invader is at high density) and altering the composition of competing species (when the invader at low density). In this project, we quantified population growth rates of two invasive species (Bromus hordeaceus and Elymus caput-medusae) using a seed addition experiment where seeds were added at varying densities in an annual-dominated California grassland. Population growth was followed for two years. We ask whether grazing removal and population size influence population growth rates, and whether these factors interact to create variation in how growth rates are affected by density. 

Results/Conclusions

We show population growth rates of two invasive species are characterized by negative density dependence. Growth rates are also influenced by extrinsic factors such as the exclusion of livestock grazing, but this effect depended on population size and differed between the two focal species. In B. hordeaceus, livestock exclusion reduced the population growth rate at low densities, but had no effect on the high density growth rates. In E. caput-medusae, low density growth rates were unaffected, but grazing removal exacerbated negative density dependence at high population densities making growth rates much more negative when populations were large. These differences should affect invasion dynamics. In the first species, removing livestock grazing would act to reduced growth rates when rare and slow spread into new areas; in the second species, removing livestock would elicit strong negative density dependence and thus reduce the size already established populations but encourage spread. We show that disturbance factors can control the density dependence of growth rates, affecting either the establishment or the spread phase of invasion. These results have important implications for when grazing or grazing removal would be useful as a management tool to control invasive populations.