Environmental stress amplifies competitive asymmetry and drives divergent hybrid zone outcomes
Community persistence depends on the balance between abiotic constraints and biotic interactions. Environmental stress can either sort species by physiological limits or amplify competitive asymmetries, producing coexistence, exclusion, or collapse. We tested these alternatives in two replicate hybrid swarms between orangethroat and orangebelly darters (Etheostoma pulchellum and E. radiosum spp. complex) with contrasting outcomes: long-term coexistence in the Blue River versus collapse in the Washita River. We combined critical-thermal-maximum (CTmax) assays with standardized feeding experiments to evaluate physiological tolerance, competitive exclusion, and stress-amplified competition. CTmax varied with river, sex, and body size but not consistently between species, indicating that local history and demography outweighed intrinsic physiological differences. In contrast, competition trials revealed strong, temperature-dependent asymmetries: E. pulchellum dominated in the cooler, stable Blue River, whereas E. radiosum spp. gained a foraging advantage under high temperatures in the warmer Washita River drainage. These results support the prediction that abiotic stress amplifies competitive asymmetries, flipping dominance and explaining divergent hybrid zone outcomes. More broadly, our study links hybrid zone dynamics to coexistence theory, showing that climate extremes can shift competitive balance and determine whether secondary contact results in persistence or loss.