The introduction of a novel species may increase or decrease the species richness of a community, depending on its effects on native diversity. The ability of an introduced species to establish and its effects on diversity depend on the ecology of both the invader and native species. In a coastal mixed-evergreen forest in central California, we measured how the invasion of an unrepresented functional group, a sprawling C3 grass (Ehrharta erecta), affects native species richness and abundance. This community naturally has an open and sparse understory, and invasive species are typically rare, possibly due to its acidic soils, high shade, and thick duff layer. Since the rapid spread and establishment of E. erecta may be facilitated by the openness of the community, we also tested if invasion by E. erecta could be curbed by increasing native plant cover with native species transplants. Finally, using chemical and mechanical treatments, we measured the community response to E. erecta removal, as well as the feasibility of removal, by comparing community diversity and invader abundance in treatment, control, and reference plots.
Results/Conclusions
Native species did not appear to strongly compete with E. erecta. We found lower percent cover of native species in invaded plots compared to nearby non-invaded plots, but native cover was not correlated with E. erecta abundance. This was surprising considering the high density of E. erecta, which quadrupled total vegetation cover in some plots. Native transplants (Clinopodium douglasii) did not slow regrowth of E. erecta, but did increase total native plant percent cover, though only by increasing C. douglasii. Both these findings demonstrate that, at least in this community, native species can establish and grow in the presence of E. erecta. Twenty-two months following management treatments, both mechanical and chemical treatments significantly reduced E. erecta percent cover. However, chemical removal also significantly reduced native species plant cover compared to manual removal. This research contributes to a growing body of literature indicating that invasion impacts are case dependent and that management of invaders can exacerbate changes to the native community. Greater predictability to identify which invasions result in high biodiversity losses would both allow managers to target rapid response efforts towards high-impact invasions, as well as allow greater ability to devise control and mitigation plans that match management goals.