Global patterns in the impact of marine herbivores on benthic primary producers

TLDR

Although consumers are known to shape communities and are assumed to have strongest effects at low latitudes, global‑scale tests of consumer impact magnitude are scarce, yet marine herbivore exclusion experiments provide a consistent methodology to quantify such effects worldwide. The authors synthesize 613 marine herbivore exclusion experiments to test how consumer traits, producer traits, and environmental factors influence the strength of herbivore impacts on benthic producers. The synthesis employs a quantitative meta‑analysis of globally consistent herbivore exclusion experiments. Marine herbivores reduced producer abundance by 68 % on average worldwide, with strongest effects in rocky intertidal habitats and weakest in vascular plant dominated habitats; latitude and mean annual water temperature had little influence, while producer taxonomic and morphological traits, and phylogenetic conservatism, best predicted grazing impacts.

Abstract

Abstract Despite the importance of consumers in structuring communities, and the widespread assumption that consumption is strongest at low latitudes, empirical tests for global scale patterns in the magnitude of consumer impacts are limited. In marine systems, the long tradition of experimentally excluding herbivores in their natural environments allows consumer impacts to be quantified on global scales using consistent methodology. We present a quantitative synthesis of 613 marine herbivore exclusion experiments to test the influence of consumer traits, producer traits and the environment on the strength of herbivore impacts on benthic producers. Across the globe, marine herbivores profoundly reduced producer abundance (by 68% on average), with strongest effects in rocky intertidal habitats and the weakest effects on habitats dominated by vascular plants. Unexpectedly, we found little or no influence of latitude or mean annual water 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 unrecognised degree of phylogenetic conservatism in producer susceptibility to consumption.