Resource utilization, competition, and predation: a model and example from coral reef grazers

Abstract

A literature review indicates that coral reef sea urchins out-compete herbivorous fishes (i.e. scarids and acanthurids), and reach maximum biomass levels an order of magnitude higher than herbivorous reef fishes. Additionally, sea urchins have lower respiration and food consumption rates, and at high biomass levels appear to reduce algal resources to levels lower than herbivorous fishes. A simple biomass-based energetic model was developed to help explain these patterns and to determine the energetic properties of organisms which affect their resource competitive ability and ability to sustain themselves under predation. Simulations of a hypothetical herbivore indicate that herbivore steady-state biomass increases with decreasing consumption rates, but a parabolic relationship was found between herbivore biomass and the level to which the herbivore reduces its food resource (i.e. R ∗ ). Simulations of competition between hypothetical herbivores indicate that only the level to which a herbivore reduces its resource ( R ∗ ) was important in determining the superior resource competitor, although biomass dynamics differed depending on feeding, gross production rates, and initial conditions of competitors. High herbivore consumption and gross production increase rates of biomass recruitment. But, unless an herbivore can also keep resources below the level competitors can tolerate, a high rate of gross production does not insure the competitor superior resource competitive ability in the absence of predators. Yet the model also predicts that organisms with high gross production and consumption rates are able to persist better under high levels of harvesting by predators. The model suggests that sea urchins reach high biomass due to low algal consumption rates and out-compete herbivorous fishes, when their predators are reduced (i.e. often attributable to overfishing), by reducing algal biomass to low levels. Sea urchins appear to tolerate low algal biomass due to their low consumption and respiration rates which allows them to persist at low levels of net algal productivity. But, low sea urchin gross production makes them less likely to persist under high levels of harvesting or predation. The model suggests that organisms may frequently need to make energetic trade-offs between traits which will give them superior resource competitive ability and tolerance to predation or disturbance.