Alternative states models provide an effective framework for invasive species control and restoration of native communities.
Abstract
Identifying the mechanisms that facilitate invasion is crucial for the design of preventative measures and understanding the invasion process, but not necessarily for the design of effective control strategies. When a novel disturbance is identified as facilitating invasion, the intuitive control strategy is to restore the previous disturbance regime. This recommendation is implicitly based on traditional models of succession, which explain the dynamics of few natural ecosystems. We investigate whether the application and/or manipulation of novel disturbances designed explicitly under an alternative states model are more effective for restoration. Using a field trial, we manipulated a relatively new disturbance (ungulate grazing), a facilitator of invasion, to determine its effects on the invasive grass species, Eragrostis curvula and the rest of the plant community. The split-plot design used excluded and permitted grazing and was combined with reductions in the biomass of the invader using mowing and selective application of herbicides. We also altered soil nutrient conditions and monitored responses over two growing seasons characterized by different rainfall patterns. We found that the control strategy based implicitly on traditional successional models (removing the causal disturbance and killing the invasive grass) was not the most effective option for restoring a desirable native community. While this control strategy reduced the abundance of the invader and increased native grasses, it led to an increase in the abundance of an undesirable exotic forb, Verbena tenuisecta. The most effective control strategy was based on alternative states models and involved the maintenance of grazing and the use of fertilizers to increase the palatability of the invader. With this strategy, the abundance of the invader decreased because grazing pressure increased, native grasses increased and the exotic forb was maintained at low levels. Synthesis and applications. The control strategy based on the alternative states models initially appears at odds with restoration efforts, because livestock grazing and nutrient addition are associated with the degradation of plant communities worldwide. However, for effective restoration, we found approaches that take into consideration the dynamics of the novel invader-dominated system were the most effective in the short term.