Mechanisms of local coexistence in juvenile congeneric salmonids in small tributaries with different thermal regimes.

Understanding how ecologically similar species coexist remains a central question in ecology. Shifts in species composition along environmental gradients offer opportunities to explore underlying mechanisms, such as condition-specific competition-where competitive dominance reverses across environments. However, broad-scale gradients often involve multiple co-varying abiotic factors, making causal inference difficult, and experimental evidence for competition reversal remains limited. Although temperature is widely regarded as a key driver of salmonid distributions, its mechanistic role remains unclear. We investigated the roles of temperature and species interactions for the coexistence of juvenile Dolly Varden (Salvelinus curilus, colder-adapted) and white-spotted charr (S. leucomaenis, warmer-adapted) in multiple replicated tributaries that minimized spatial-autocorrelation and co-varying physiochemical factors. Structural equation modeling revealed that water temperature significantly influenced Dolly Varden density but not that of white-spotted charr. In addition, Dolly Varden density negatively affected white-spotted charr density, while the reverse effect was not detected, contradicting prior findings of white-spotted charr competitive dominance. A transplant experiment showed that white-spotted charr persisted and grew well in cold tributaries where Dolly Varden numerically dominated. These results suggest that white-spotted charr avoid cold habitats not due to thermal limitations, but due to biotic interactions with Dolly Varden, potentially mediated by food availability. Thus, although temperature shapes species distributions, temperature-specific competition reversal is unlikely to be the dominant mechanism. Because Dolly Varden typically establish themselves in these habitats earlier and attain numerical dominance, we propose that prior residence effects-where early occupants gain competitive advantages-may better explain local coexistence patterns in this system.