The impact of native vertebrates on enemy release and plant functional traits during community assembly

Vertebrate consumers are known to influence plant recruitment and community assembly based on the species they prefer to eat. The enemy release hypothesis (ERH) predicts that invasive introduced plant species may avoid consumption due to a lack of coevolved predators. Herbivores and granivores may also prefer species due to their functional traits, irrespective of species origin. We hypothesized that in a highly invaded plant community, invasive plants would benefit from enemy release and animal consumers would likely decrease plant diversity if they avoided invasive species in favor of native species (Hypothesis 1). Animals were expected to filter the species pool based on preference for palatable functional traits (Hypothesis 2). A factorial experiment combining cage and native seed addition treatments was established in the Voorhis Ecological Reserve, a California sage scrub ecosystem in Southern California. Seed and seedling preference trials were conducted to gather additional evidence of consumer preference at the study site. Although we found little influence of plant functional traits on consumer selection of species, animals created more functionally similar communities. In preference trials, native rodents and birds were found to prefer native plant species and avoid dominant invasive species, and the effects of animal preference were observed in experimental plant communities. Small vertebrates encouraged the success of invasive plants while limiting recruitment of native species, consistent with the ERH. Thus, birds and small mammals may function as an important filter in sage scrub habitat by reducing community diversity and promoting the dominance of competitively superior invasive plant species.