Local and landscape correlates of non-native species invasion in restored wetlands

Vulnerability of natural communities to invasion by non-native plants has been linked to factors such as recent disturbance and high resource availability, suggesting that recently restored habitats may be especially invasible. Because non-native plants can interfere with restoration goals, monitoring programs should anticipate which sites are most susceptible to invasion and which species are likely to become problematic at a site. Restored sites of larger area and those with high rates of propagule input should have higher species richness of both natives and non-natives, leading to a positive correlation between the two. However, in restored wetlands, urbanization, riparian landscape settings, and nitrogen enrichment likely favor non-native relative to native species. We sampled 28 restored wetlands in Illinois, USA, modeled the responses of native richness, non-native richness and non-native cover to local and landscape predictors with linear regression, and modeled the presence/absence of 21 non-native species with logistic regressions. Unexpectedly, native and non-native richness were uncorrelated, suggesting different responses to environmental factors. Native richness declined with increasing available soil nitrogen and urbanization in the surrounding landscape. Non-native richness, the richness of non-natives relative to natives, and the likelihood of invasion by several individual invasive species decreased with increasing distance from the city of Chicago, likely in response to decreasing non-native propagule pressure. Total cover of non-natives, however, as well as cover by non-native Phalaris arundinacea, increased with nitrogen availability. Our results indicate that although non-native richness was better predicted by factors related to propagule pressure, non-native species dominance was more closely related to local abiotic factors. Non-native richness in restoration sites may be beyond the control of restoration practitioners, and furthermore, may be of limited relevance for conservation goals. In contrast, limiting the relative dominance of non-natives should be a restoration priority and may be achievable through management of nutrient availability.