Can consumer effectiveness limit the diversifying effect of intraspecific competition?

Background/Question/Methods

Competition among members of the same population is thought to be an important promoter of variation within natural populations. However, little work explores how consumer traits affect the relationship between competition and diversification. Consumer effectiveness, or the per-capita interaction strength of a consumer, is potentially important in that it may limit the ability of competition to promote diversification of key characteristics like niche breadth. Here, we link comparative and experimental work to test the idea that an effective consumer will limit its own ability to diversify in response to competition. First, we compared the dietary breadth and specialization of juvenile alewife (Alosa pseudoharengus) from wild populations across a range of densities. For this, we utilized short-term direct diet data as well as long-term stable isotope data. Second, we manipulated the density of juvenile alewife in large-scale field mesocosms and again compared the dietary metrics. Third, we manipulated resource availability and alewife density in a similar mesocosm experiment to alter the ability of alewife to structure their prey communities, and ultimately to determine if this modifies the relationship between competition and diversification.

Results/Conclusions

In the whole lake comparison and first mesocosm experiment manipulating just alewife density, we found decreasing population total niche breadth with increasing competition. Further, we found that competition has no significant effect on measures of individual specialization. In the second mesocosm experiment manipulating both resource availability and alewife density, we found that altering the ability of alewife to shape their prey resources affects the relationship between competition and niche width. Together, our results indicate that consumer traits play a central role in determining how intraspecific competition affects intraspecific variation. Generally this suggests that increased competitive interactions are unlikely to drive dietary diversification in highly effective consumers. To test this idea we searched the literature for data with which to compare the effect of competition on dietary variation in a variety of species. Our results from this comparison support the idea that consumer effectiveness limits the diversifying effects of competition in a range of taxa.