Consumption rates of a key marine herbivore: a review of the extrinsic and intrinsic control of feeding in the green sea urchin

Abstract

Herbivory fundamentally shapes ecosystem functioning by influencing the abundance and distribution of plants. Whereas a great deal of attention has been given to factors that influence rates of primary productivity (e.g., light, temperature and nutrients), considerably less attention has been given to factors governing herbivory rates. In the marine environment, urchins are a dominant herbivore on rocky shores, and their grazing often leads to overconsumption and the formation of “barren grounds” with few fleshy macroalgae. However, despite decades of laboratory experiments and field observations, there is still a high degree of uncertainty with regard to the factors that control sea urchin consumption rates. To synthesize current knowledge on this subject, we compiled and analyzed data from the peer-reviewed literature on consumption rates of the green urchin, Strongylocentrotus droebachiensis, to examine the effects of intrinsic (e.g., size) and extrinsic (e.g., temperature and algal type) factors. Generally, urchins consumed 1.3–4.1% of their body weight per day, though the range was much greater (0.1–18%) when all measurements were included. Not surprisingly, larger urchins ate more than smaller urchins, but this difference was almost entirely attributed to differences in body mass and when expressed as mass-specific rates, small urchins consumed food at the same rate as large urchins. A simple measure of total urchin biomass thus appears sufficient for estimating potential herbivory at a given location. More surprising, temperature had no discernible effect on feeding rates despite our expectations and assertions in the literature. Although consumption rates of different macroalgae varied, urchins consumed all 42 taxa presented to them. Generally kelps (excluding Agarum sp.) and green algae were eaten the fastest. However, species that were typically favored were occasionally ignored, and chemically defended species that were generally ignored (e.g., Agarum sp.) were occasionally eaten at very high rates, making predictions of consumption rates difficult. Our review provides estimates of the maximum amount of algae an urchin can consume and when coupled with the potential productivity of an area, may help identify ecological tipping points between productive kelp beds and urchin barrens.