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.

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.

Global warming poses an increasing threat to biodiversity by elevating the risk of local extinctions. Species distribution models are widely used to forecast extinction risk under future climate s...

Global warming poses an increasing threat to biodiversity by elevating the risk of local extinctions. Species distribution models are widely used to forecast extinction risk under future climate s...