Condition-specific competition: Implications for the altitudinal distribution of stream fishes

  • Taniguchi Y
  • Nakano S
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The occupation of adjacent, nonoverlapping positions along environmental gradients by closely related and ecologically similar species has drawn considerable atten- tion 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 elu- cidated thk underlying-mechanisms, such as behavioral and demographic processes. We conducted laboratorv exueriments to examine the effects of temuerature on intersuecific A . competition between two stream salmonid fishes, Salvelinus mafma and S. leucokaenis. 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 distri- bution boundary of S. malma in Hokkaido streams may therefore be determined by tem- perature-mediated condition-specific competition. However, mechanisms determining the upper distribution boundary of S. leucomaenis could not be fully explained by the com- petitive 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 phys- iological responses to the environment, resource use, and demographic processes. Key

Author-supplied keywords

  • Altitudinal distributions
  • Competition
  • Condition-specific competition
  • Growth rates
  • Mechanistic approaches
  • Size-dependent mortality
  • Stream salmonids
  • Survival analysis
  • Temperature and competitive exclusion

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  • Yoshinori Taniguchi

  • Shigeru Nakano

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