Substitutable habitats? The biophysical and anthropogenic drivers of an exotic bird's distribution

The spread and distribution of exotic species depends on a number of factors, both anthropogenic and biophysical. The importance of each factor may vary geographically, making it difficult to predict where a species will spread. In this paper, we examine the factors that influence the distribution of monk parakeets (Myiopsitta monachus), a parrot native to South America that has become established in the United States. We use monk parakeet observations gathered from citizen-science datasets to inform a series of random forest models that examine the relative importance of biophysical and anthropogenic variables in different regions of the United States. We find that while the distribution of monk parakeets in the southern US is best explained by biophysical variables such as January dew point temperature and forest cover, the distribution of monk parakeets in the northern US appears to be limited to urban environments. Our results suggest that monk parakeets are unlikely to spread outside of urban environments in the northern United States, as they are not adapted to the climatic conditions in that region. We extend the notion of substitutable habitats, previously applied to different habitats in the same landscape, to exotic species in novel landscapes (e.g., cities). These novel landscapes provide resources and environmental conditions that, although very different from the species' native habitat, still enable them to become established. Our results highlight the importance of understanding the regionally-specific factors that allow an exotic species to become established, which is key to predicting their expansion beyond areas of introduction.