The mechanism of ecological invasion has been the focus of many investigations, but questions remain regarding the primary drivers of this process. Areas rich in native species were once considered to be resistant to the invasion of exotic plants; however, studies at large scales find evidence to the contrary. An explanation for this continues to elude ecologists. Stohlgren et al. (2003) conducted a nationwide, US-based analysis that found a correlation between native and invasive plant species richness across counties within states. We expanded this by including additional potential variables (area, human population density, productivity, and habitat heterogeneity along with exotic species richness) in a mixed linear model to analyze the entire country at once, with state as a categorical variable to eliminate previous biases.
Results/Conclusions
The best model ranked by AIC included all variables, yet native species richness remained the most important of the numerical predictors, indicating that the relationship is not driven by mutual correlations with any of the other variables. Human population density is next in importance to native species richness, making the relationship between anthropogenic activities and invasion ever more clear. Residuals of the full model reveal county-level hotspots of invasion that appear correlated in part with ports and major river systems. As long as a clear mechanism for the direct influence of native plant species richness on plant invasibility remains elusive, a strong justification for either promoting or downplaying biodiversity as a defense against exotic species must wait. For now, emphasis should be placed on standardizing the collection of plant data between states and minimizing the human activities that introduce and spread the exotic species.