Predator-prey interaction strengths do not consistently depend on body-size ratios

Background/Question/Methods

The distributions of strong and weak predator-prey interactions in a food web are an important determinant of the stability of ecological communities, both theoretically and empirically. However, measuring interaction strengths for each individual predator-prey pairing in a community is not feasible. Empirically-derived linear scaling relationships between interaction strength and the relative body sizes of predator-prey pairings have been used to create general scaling relationships which can be used to characterize interactions within whole food webs. It is possible however, that species’ traits independent of body size (e.g. morphological defenses) may influence predator-prey interactions and therefore alter the distributions of interaction strengths in food webs.  To investigate the importance of body size in comparison to other traits in determining predator-prey interaction strengths, we carried out a series of stream mesocosm experiments. These experiments included a range of predator sizes (fish and predatory invertebrates) and both protected and unprotected invertebrate prey.

Results/Conclusions

Interaction strength scaled with predator-prey body size ratios, with stronger interaction strengths occurring at larger body size ratios. However, the relationship between all predator-prey pairings and predator-prey body size ratios was not a general linear scaling relationship, but instead showed a limit response, with more variation in interaction strengths at larger predator-prey ratios. The variation in this relationship can be partially explained by the traits of the individual predator-prey pairings. The expected linear relationship between predator-prey body size and interaction strength held when unprotected prey (mayflies) were paired with mobile consumers (trout and galaxiids).  However, the expected relationship was not observed when protected prey (cased caddisflies and snails) were paired with any predator or when small predatory invertebrates were paired with any of the prey. Although predator-prey body size ratios are a useful, easy-to-measure predictor of interaction strength, these results indicate other traits, independent of body size, need to be considered. General scaling relationships between body size and predator prey interaction strength need to be generated for different common traits within a community as there is strong potential for other traits over and above body size to considerably influence the stability of food webs.