Providing long-term suppression of invasive species is a central challenge in applied ecology. One promising approach is to suppress invasive species by planting natives with similar niches. However, if not given priority, invasives with overlapping niches may suppress native growth. Additionally, native species’ ability to suppress invasion may depend on patch structure, as invasives may flourish at the edge of restoration treatments and then reinvade into restored sites. California grasslands provide an ideal case study for these challenges, as they are naturally heterogeneous and have experienced differing rates of invasion. In this study, the following treatments were planted in fall 2007: invasive grasses (goatgrass and medusahead), native plants (mix of 8 grasses/forbs), naturalized plants (4 species that dominate California grasslands), and all possible mixes of these three communities, with 8 replicates/treatment. In the 10th growing season, we assessed: (1) In mixed community plots, which groups dominated (e.g. natives vs. invasives)? (2) To what extent did natives resist invasive encroachment? To what extent did natives establish in other community plots? (3) How do the community patterns differ at the center of the plots compared to the edges (e.g. were edges of native plots refuges for invasive species?).
Results/Conclusions
When planted in a mixture, invasive grasses dominated for the first few years, but natives suppressed them to under 20% cover after 5 growing seasons. After 10 growing seasons, the native and invasive grasses coexisted at 15-20% cover each. Treatments originally seeded with only native species had a very low presence of invasive grasses, while treatments seeded with the invasive grasses had low presence of invading natives. This suggests that the two groups not only are direct competitors, but that temporal priority is a crucial factor in community assemblage. Native species established in the center as well as the edges of the invasive plots, though at low densities. In native plots, invasive cover was similar across edges and center even though the dominant natives varied in cover throughout the plot. This demonstrates that the edges of native grass restoration plots do not harbor many invasive species, and are resistant to invasion from areas outside the restoration site. However, natives cannot easily invade areas with high medusahead or goatgrass. This study suggests that in California grasslands, native species restoration is a promising way to suppress invaders over the long-term, particularly after invaders are controlled at the start of the restoration efforts.