Condition‐specific competition: implications for the altitudinal distribution of stream fishes

The occupation of adjacent, nonoverlapping positions along environmental gradients by closely related and ecologically similar species has drawn considerable attention from many ecologists over the past decades. Condition‐specific competition, wherein competitive superiority varies with the abiotic environmental gradient, has been proposed as the major structuring force behind such distributions. However, few studies have elucidated the underlying mechanisms, such as behavioral and demographic processes. We conducted laboratory experiments to examine the effects of temperature on interspecific competition between two stream salmonid fishes, Salvelinus malma and S. leucomaenis. The two species have a largely allopatric altitudinal distribution on Hokkaido Island, Japan, proposed to be the result of temperature‐mediated competition. We tested predictions that at a higher temperature (12°C), S. leucomaenis would dominate over S. malma in aggressive interactions, foraging performance, growth, and survival, but become subordinate at a lower temperature (6°C). Indeed, S. leucomaenis initiated a greater number of aggressive acts, attained greater food intake and greater growth, and finally excluded S. malma at the higher temperature. Although the two species initiated a similar number of aggressive acts and foraged equally well at the lower temperature, S. leucomaenis achieved a higher growth rate than S. malma; however, the latter eventually became numerically dominant. Clear competitive release in allopatry occurred for S. malma only at the higher temperature, providing direct evidence of condition‐specific asymmetric competition. The lower distribution boundary of S. malma in Hokkaido streams may therefore be determined by temperature‐mediated condition‐specific competition. However, mechanisms determining the upper distribution boundary of S. leucomaenis could not be fully explained by the competitive results at lower temperature, but required an understanding of how effects of competition interacted with species‐specific physiological traits. Thus, species distributions along an environmental gradient cannot be solely explained by a simple model of condition‐specific competition without considering mechanistic linkages among behavioral and physiological responses to the environment, resource use, and demographic processes.