Consumer pressure is widely believed to be strongest in the tropics, with a host of macroecological and evolutionary implications. Yet despite a large literature in latitudinal gradients and the documented importance of consumers in structuring communities, empirical tests for global-scale patterns in consumer impact are limited. In marine environments, the long tradition of experimentally excluding herbivores in their natural environments provides a rich database for quantifying consumer impacts on global scales using consistent and comparable methodology. We undertook a comprehensive literature search of marine herbivore exclusion experiments, and present a quantitative synthesis of 615 experiments, using General Linear Models to test the influences of consumer traits, producer traits, and environment on global-scale patterns in the strength of herbivore impacts on benthic producers.
Results/Conclusions
Herbivory exerts strong and pervasive top-down control on marine producers: averaged across the globe, marine herbivores reduced aggregate producer abundance by 68% on average. Impacts varied among habitats, with strongest average effects on rocky intertidal shores and weakest effects in habitats dominated by vascular plants (seagrass beds and salt marshes). Unexpectedly, we found little or no influence of latitude or mean annual temperature. Instead, herbivore impacts differed most consistently among producer taxonomic and morphological groups. Our results show that grazing impacts on plant abundance are better predicted by producer traits than by large-scale variation in habitat or mean temperature, and that there is a previously unrecognized degree of evolutionarily ancient phylogenetic conservatism in producer susceptibility to consumption.