Anthropogenic expansion of invasive species into marginal and suitable habitats
Understanding what controls the spread of invasive species is essential to identify areas susceptible to invasion and address causal factors contributing to spread. Most models used to predict invader spread focus only on habitat suitability; however anthropogenic factors may be key in promoting spread in two major ways, as dispersal vectors and as disturbance. Furthermore, the importance of anthropogenic factors may vary across the landscape, for example dispersal and disturbance may only increase spread if the climate in a particular area is suitable. Here we test how anthropogenic factors, in addition to habitat suitability, affect the spread of 11 rangeland weeds in California (2 grasses, 5 asteraceous forbs, 4 leguminous shrubs). Using logistic regression in a Bayesian framework, we first fit suitability models including mean annual temperature, annual precipitation, soil texture, slope, and aspect. We then fit three additional models including anthropogenic factors (road density and cattle density) 1) in all sites, 2) only in suitable habitat, 3) only in marginal habitat. We used DIC for model selection, and D-squared to quantify the additional deviance explained by adding anthropogenic factors to the habitat model. We hypothesize that anthropogenic factors will promote spread for all species, particularly in suitable habitat.
Anthropogenic factors were important in increasing invasive spread for all 11 species (using DIC); the increase in deviance explained ranged from 1% to 8%. For eight of the 11 species, anthropogenic factors increased spread into both marginal and suitable habitat. For one grass species anthropogenic factors were only important in marginal habitat, and for two forbs anthropogenic factors were only important in suitable habitat. Road density had a stronger effect on invasion than cattle density; road density promoted spread of ten species, while cattle density increased spread in six species. Overall, we see a strong anthropogenic signature on the distributions of invasive species in both marginal and suitable habitats in California. This suggests that limiting dispersal vectors and disturbance associated with roads and cattle will help to prevent spread of invaders across the landscape.