Understanding patterns of species diversity is a central goal of ecology. At large spatial scales, human activities reduce terrestrial diversity through land transformation and the introduction of nonnative species. These anthropogenic activities can alter resource availability, habitat connectivity, and regional species pools, and thus may all influence plant diversity through their effects on species coexistence mechanisms. Thus, human development may impact species richness by altering the environmental and competitive landscapes in which they live (i.e. their niches). To examine how human development influences patterns of species richness and niche specialization, we synthesized data on vascular plant occurrences, human footprint, and nonnative species across >2000 wetlands in Alberta. We calculated indices of niche specialization based on each species’ occurrence frequency across the full human development gradient. We asked the following questions: (1) How does vascular plant richness vary with human development? (2) Does the niche specialization of plant species change across the human development gradient? (3) How does the occurrence of nonnative species coincident with human development mediate the responses of richness and niche breadth?
Results/Conclusions
We found that both vascular plant species richness and mean niche specialization were significantly related to the proportion of adjacent land developed by human activities (both p<0.001). Species richness peaked around 40% human development and was lower at very low and high development extents. In contrast, niche specialization was highest in wetlands with the lowest and highest human development extents, indicating that species in these wetlands have restricted ranges compared to species inhabiting wetlands surrounded by intermediate levels of human development. Human development extent was positively correlated with the proportion of nonnative species (p<0.001) and best models of species richness included a human development x nonnative species interaction (p<0.001). Since nonnative species may expand from the point of invasion, the observed unimodal relationship between species richness and human development could be contingent upon the time of observation relative to the time since invasion. More broadly, it suggests care should be taken to understand which broad biogeographic patterns are driven by nonnative species, native species, or both.