Temperature-dependent competition predicts contrasting outcomes of adjacent secondary contact zones in darters (Percidae: <i>Etheostoma</i> )

Abstract Understanding how abiotic conditions interact with species interactions to shape outcomes of secondary contact remains a central challenge in evolutionary biology. Environmental stress may limit persistence through physiological constraints, but it can also alter competitive dynamics, determining whether closely related species can coexist. We investigated these mechanisms in two long-studied secondary contact zones between the darter species Etheostoma pulchellum and E. radiosum spp. in southeastern Oklahoma that have experienced different fates: long-term coexistence in the spring-fed Blue River versus the disappearance of E. pulchellum from Little Glasses Creek in the warmer, more variable Washita River drainage. Using critical thermal maximum (CTmax) assays, field temperature records, and standardized feeding trials in parental species, we tested whether contrasting outcomes in these systems are better explained by differences in physiological tolerance or by temperature-mediated shifts in competitive interactions. Although CTmax varied among rivers, we found no consistent species-level differences in thermal tolerance across drainages. In contrast, exploitative competition was strongly temperature dependent, with E. pulchellum dominating under cooler conditions characteristic of the Blue River, whereas E. radiosum spp. outcompeted E. pulchellum under elevated temperatures matching summer conditions in Little Glasses Creek. These results show that warming can flip competitive hierarchies, providing a mechanistic explanation for contrasting secondary contact zone trajectories and underscoring how environmental variability shapes species persistence following secondary contact.