Species provenance and traits mediate establishment and performance in an invaded grassland

In many invaded grasslands, dominant exotic species can produce large amounts of litter that modify local abiotic conditions and species' interactions. These novel conditions can reduce native species abundance and promote the persistence of exotic species, yet the strength of this disparity may be influenced by how consumer pressure interacts with litter accumulation. Consumers may exacerbate this disparity by preferentially targeting native species or by promoting heterogeneity in microhabitats due to their movement and small-scale ground disturbances that favours fast growing exotic species. How species respond to litter accumulation and consumer pressure may depend on either evolutionary differences, whereby exotics species may benefit from a lack natural predators, or by functional differences, in which species' physiological traits may confer fitness advantages to low-light conditions or herbivory or granivory pressure. We examined the impact of litter presence and small mammal herbivory on the establishment and reproduction of functionally diverse exotic versus native species seeded across sites that naturally vary in resource availability in an annual invaded California grassland. We assessed whether seed mass and leaf nitrogen content (LNC) were predictive of successful establishment and reproduction. Litter accumulation affected exotic and native species differently, with litter significantly decreasing native recruitment and reproduction, while exotics were largely unaffected. Small mammals had a slight positive effect on the establishment of native species when litter was present but did not influence exotic species. Regardless of species provenance, larger seeded species established at a higher density while species with lower leaf nitrogen content had a higher density of reproductive individuals. Native species that successfully established and reproduced were functionally different in LNC than the resident community, while successful exotic species were functionally more similar to the resident community in LNC. Our study demonstrates that exotic species outperformed native species regardless of the presence of litter or herbivory pressure. Without the removal or thinning of litter, it is likely that exotic species will continue to dominate, resulting in positive feedback that further favours the persistence of exotic species within this invaded grassland system. Read the free Plain Language Summary for this article on the Journal blog.