Although top-down forces are undeniably important in many communities, determination of the factors underlying variation in the strength of top-down effects remains an ongoing issue in ecology. To test the relative influences of intrinsic factors that dictate top-down effects vs. extrinsic factors that modify top-down effects, we carried out field experiments and measurements at sites spanning ~250 km over more than a decade to evaluate variation of predation by sea stars in space and time. Intrinsic factors are defined as behavioral, body size, and physiological determinants of functional response, per capita and per population effects, while extrinsic factors are defined as environmental stress, species interactions, inputs of prey propagules, prey size structure and diversity, secondary production of prey (growth), and energy inputs from bottom-up sources (nutrients and primary productivity). We hypothesized that predation-rate experiments controlling prey availability and size can provide insight into the intrinsic component of predation, and that predation-effect experiments can provide insight into the net result of factors that lead to prey increase (recruitment, growth) and decrease (predation). We used predator exclusion and prey loss rate experiments, measurements of prey inputs and growth, and surveys of predator size structure and density to test our hypotheses.
Results/Conclusions
Both predation rate (loss of transplanted mussels) and predation effect on community structure varied across the study region. Highest rates occurred at sites with higher rates of primary production of phytoplankton (food for sessile prey), rates of propagule inputs, rates of growth of mussels, highest predator densities, and fastest predator growth rates. However, contrary to results in the 1990s, which showed higher predator recruitment rates to the sites with the higher rates noted above, in the 2000s, predator recruitment was sharply reduced at high predation sites, and more variable among low predation sites. Although we don’t know the mechanisms underlying these latter changes, they coincide with climatic shifts. We conclude that extrinsic forces are primary drivers of top-down effects of the sea star predator, with the major effect being to determine size and/or abundance through high prey inputs and rapid prey growth in more productive environments. These results are consistent with the idea that top-down and bottom-up effects are intimately linked, and if viewed at the proper scale, likely underlie these dynamics in many if not most meta-ecosystems.