Impacts and interactions of multiple human perturbations in a California salt marsh.

Multiple disturbances to ecosystems can influence community structure by modifying resistance to and recovery from invasion by non-native species. Predicting how invasibility responds to multiple anthropogenic impacts is particularly challenging due to the variety of potential stressors and complex responses. Using manipulative field experiments, we examined the relative impact of perturbations that primarily change abiotic or biotic factors to promote invasion in coastal salt marsh plant communities. Specifically we test the hypotheses that nitrogen enrichment and human trampling facilitate invasion of upland weeds into salt marsh, and that the ability of salt marsh communities to resist and/or recover from invasion is modified by hydrological conditions. Nitrogen enrichment affected invasion of non-native upland plants at only one of six sites, and increased aboveground native marsh biomass at only two sites. Percent cover of native marsh plants declined with trampling at all sites, but recovered earlier at tidally flushed sites than at tidally restricted sites. Synergistic interactions between trampling and restricting tidal flow resulted in significantly higher cover of non-native upland plants in trampled plots at tidally restricted sites. Percent cover of non-native plants recovered to pre-trampling levels in fully tidal sites, but remained higher in tidally restricted sites after 22 months. Thus, perturbations that reduce biotic resistance interact with perturbations that alter abiotic conditions to promote invasion. This suggests that to effectively conserve or restore native biodiversity in altered systems, one must consider impacts of multiple human disturbances, and the interactions between them.