Associate Professor University of California, Santa Barbara, United States
How strongly predators interact with prey can have widespread implications for the structure and function of ecosystems. However, the strength of predator-prey interactions can vary tremendously over narrow spatial and temporal scales, making it difficult to predict when and where predators will have strong trophic impacts. Across taxa and species, previous work has demonstrated strong patterns relating body size to the strength of interactions, suggesting that it may be possible to predict how strongly a given predator and its prey interact using general size-scaling relationships. Here, we test how accurately general size-scaling relationships predict interactions between an economically and ecologically important predator-prey pair—the California spiny lobster (Panulirus interruptus) and its purple urchin (Strongylocentrotus purpuratus) prey. Specifically, we conducted a mesocosm foraging experiment where we estimated the body-size dependence of the lobster functional response by manipulating lobster size, urchin size, and urchin density. Using our experimental results, we quantified variation in plausible lobster-urchin interactions based on 10-years of spatially explicit observational data. Finally, we disentangled the effects of body size and density as drivers of variation in interaction strength and compared our experimental estimates to predictions based on general size-scaling relationships.
Our experiment revealed that consumption rates increased with urchin density and lobster size and decreased with urchin size. Using a Bayesian hierarchical model, we demonstrate that these patterns were primarily due to differences in lobster handling time: handling time decreased as a power-law function of lobster size and increased as a power-law function of urchin size. Based on long-term data, our size-dependent function response revealed that historic interactions may have varied considerably between sites and years. Body size accounted for to up to 91% of this variation compared to differences in density. However, even the closest prediction of size-scaling from the literature underestimated how strongly lobster and urchin interacted by a factor of 4. Together, our results demonstrate the extent to which individual variation in body size may drive differences in interaction strength across narrow spatial and temporal scales and highlight the importance of accounting for individual body size—not only biomass or density—when estimating the strength of predator-prey interactions. Furthermore, our analysis suggests that general size-scaling relationships may misestimate interactions between specific predator-prey pairs, which has important implications on the development of management models that seek to conserve and sustainably harvest interacting species.