Herbivory Has a Major Influence on Structure and Condition of a Great Barrier Reef Subtropical Seagrass Meadow

Grazing by all members of an herbivore community can act to structure the ecosystems they feed on. The outcome of this grazing pressure on the plant community also depends on the interaction between different herbivore groups that are present. We carried out a three-month multi-level field exclusion experiment to understand how different groups of herbivores act both individually and interactively to structure a subtropical seagrass meadow in the Great Barrier Reef. Megaherbivore grazing had the largest impact on this seagrass meadow, significantly reducing aboveground biomass and shoot height, whereas there was no measurable impact of meso- or macroherbivores on seagrass metrics or epiphyte biomass. Megaherbivores here grazed broadly across the meadow instead of targeting grazing in one area. The principal seagrass-herbivore dynamic in this meadow is that megaherbivores are the main group modifying meadow structure, and other grazer groups that are present in lower numbers do not individually or interactively structure the meadow. We demonstrate that herbivory by large grazers can significantly modify seagrass meadow characteristics. This has important implications when designing and interpreting the results of monitoring programs that seek to conserve seagrass meadows, the ecosystem services that they provide and the herbivores that rely on them. Collectively, our results and those of similar previous studies emphasize there is unlikely to be one seagrass and herbivory paradigm. Instead, for individual meadows, their unique species interactions and differences in biotic and abiotic drivers of seagrass change are likely to have a strong influence on the dominant seagrass-herbivore dynamic.