Climate-driven range shift prompts species replacement

Climate change prompts warm-tolerant species upward and poleward to either displace or replace cold-tolerant species. Warm-tolerant species may replace cold-tolerant individuals with upward migration, or cold-tolerant genes if the species hybridize. We examined genetic and morphological differences between low elevation, warm-tolerant (Aphaenogaster rudis) and high elevation cold-tolerant (A. picea) ant species that form an upward-shifting ecotone in the southern Appalachian Mountains (USA). The A. picea/A. rudis ecotone shifted upward ca. 200 m between the decades 1970 and 2010, and characteristic morphological traits appeared muddled where the species met, suggesting hybridization. However, we found no evidence of genetic hybridization, and the trait most associated with species identity, pigmentation, remained so across the environmental gradients. Conversely, femur length did not differentiate well between species identities, and it shifted across the environmental gradients. These results suggest that the cold tolerant A. picea, associated with high-elevation and high-latitude, was replaced by the warm-tolerant, low elevation A. rudis species. As such, these results suggest that less competitive cold-tolerant species may be replaced by more competitive cold-intolerant species with climate warming.