Hyperabundance of a subordinate herbivorous gastropod causes localised overgrazing within kelp beds.

Abstract

Herbivory is a fundamental biological process that can determine the structure and functioning of ecosystems. On temperate reefs worldwide, herbivory by grazing macro-invertebrates can cause extensive overgrazing of kelp beds when grazers reach hyperabundance. Rising to local dominance, typically it is the grazing activity of a single species amongst others that is responsible for overgrazed 'barrens'. However, the potential for subordinate grazing species to impact kelp beds is less understood and often overlooked. Here we observe apparent 'barren' patches within kelp beds associated with local hyperabundance of a subordinate grazer, the turbinid periwinkle Lunella undulata, in eastern Tasmania. We demonstrate capacity for this periwinkle to overgraze by deploying a caging experiment on a 'peri-barren' patch, involving inclusion and exclusion treatments, plus cage and background control plots. Fleshy macroalgae proliferated in cages excluding periwinkles, reaching mean percentage cover of 47 ± 8% SE by end of the 117-day experiment, which was 5-fold, 6-fold, and 10-fold higher than in partially caged plots, open plots, and periwinkle inclusion cages, respectively. Indicating cascading effects of harvesting, legal-sized periwinkles were commercially harvested from the peri-barren during the study, leading to algal recovery across all treatments except for periwinkle inclusion cages where overgrazing persisted. Kelp bed recovery ensued as periwinkle biomass declined, while cover of fleshy algae increased over ~ 16 months. Reflecting the ongoing trophic and climate cascades in eastern Tasmania, we hypothesise that increased harvesting of the now dominant grazer, the range-extending urchin Centrostephanus rodgersii, has enabled localised competitive release of the subordinate periwinkle grazer.