Elevated human impact on islands increases the introduction and extinction status of native insular reptiles

Species ranges are changing in the Anthropocene, the ranges of introduced species are expanding, while extinction-prone species are contracting. Introductions and extinctions are both caused by how species respond to human impacts, but it is unknown why the ranges of some species expand and some contract. Here, we test that this opposite response of human impact is due to introduced and extinction-prone species falling at opposite ends of geographic, evolutionary, or ecological trait continua. We constructed a database of native range maps, traits, phylogenetic relationships, and the introduction and extinction-prone status of squamate reptiles with ranges native to the Western Hemisphere. Across >3,000 snake and lizard species (88% of known native squamates), 142 had been introduced elsewhere and 483 were extinction-prone (i.e., extinct, vulnerable, threatened). To explain variation in status, we first tested if the same human-impacted regions in the Americas contained the native ranges of species of either status. Second, we tested for phylogenetic signal in species status. Finally, we tested the explanatory power of multiple trait continua. The native ranges of introduced and extinction-prone reptiles were clustered in island regions with high human impact vs. mainland regions with lower human impact. Phylogenetic signal was weak for status, but introduced and extinction-prone species were clustered in different clades. All geographic and ecological traits that explained both statuses supported the opposite ends hypothesis. Introduced species had larger, edgier ranges, while extinction-prone species had smaller, simpler ranges. Introduced species were mostly herbivorous/omnivorous, while extinction-prone species were mostly carnivorous. Introduced species produced larger clutches, while extinction-prone species were smaller in body size. In the Anthropocene, the naive ranges of introduced and extinction-prone species are in the same human-impacted regions where trait continua, having opposite effects, determine whether species ranges expand or contract in the continuing face of global change.