Are urban systems beneficial, detrimental, or indifferent for biological invasion?

Urban environments are often seen as unique or degraded habitats that both present hardships for some sensitive species and provide opportunities to others. Non-indigenous species (NIS) are commonly referenced in the latter group, and are comprised of species that can tolerate the unique conditions or capitalize on the opportunities found in urban environments. Moreover, these urban beneficiaries may be those that normally cannot overcome competitive interactions in intact native communities, but find opportunity to flourish in urban habitats. We ask the question: do NIS benefit from urbanization? We answer this question using three strategies. First, we explore the problem conceptually, using community assembly theory. Second, we perform a broad literature review. Finally, we analyze studies with sufficient information using a meta-analysis. We show that the available evidence supports the proposition that NIS benefit from urbanization, with NIS obtaining higher abundances and greater diversity in more urbanized habitats. There were only 43 studies that measured NIS abundance and diversity while adequately quantifying the degree of urbanization surrounding plots, and effect sizes (measured by Hedge's D) reveal that NIS obtain higher abundances in more urbanized habitats, and especially for invertebrates. Despite the intense interest in NIS dynamics and impacts, we note a general dearth of robust studies that adequately quantify 'urbanization', and we end with a general call for more detailed research.