Morphological and dietary differences between individuals are weakly but positively correlated within a population of threespine stickleback

Background: Many theoretical models of speciation and niche evolution assume that the ecological similarity between conspecific individuals depends on their phenotypic similarity. Thus, competition between individuals is expected to depend on their phenotypic similarity. Theoretical models often assume that this intraspecific competition function is Gaussian. Questions: Are morphological similarity and diet similarity positively correlated? If so, is this relationship non-linear? Data: Stomach contents, stable isotope ratios (δC and δN), and trophic morphology (standard length, gape width, body width, gill raker number, and gill raker length) for 265 threespine sticklebacks (Gasterosteus aculeatus) from a single population from a lake in British Columbia. Analysis: We calculated the diet similarity and morphological similarity between all pairs of individuals in our sample. We examined the correlation between diet and morphological similarity, and tested whether the relationship exhibits any non-linearity. Conclusions: Similarity in trophic morphology is correlated with dietary similarity between individuals. However, both body size and trophic morphology explained remarkably small percentages of the variance in diet overlap. Also, we found no evidence of curvature in the intraspecific competition function.