What Do Herbivore Exclusion Experiments Tell Us? An Investigation Using Black Swans (Cygnus atratus) and Filamentous Algae in a Shallow Lake

Abstract

One common approach to the problems of quantifying herbivory is to determine how much plant material herbivores eat. Another is to exclude the herbivores experimentally and compare the performances of the grazed and ungrazed plant communities. Both approaches may give biased estimates of the interaction strength or effect of grazers on plant biomass increase or productivity (Mitchell and Wass, 1996a). Simple consideration of the fraction of annual plant productivity consumed ignores the effect of the time at which the material is consumed. Because grazing removes not only biomass but also the future productive potential of that biomass, consumption of small amounts of plant tissue early in a plant growth cycle has a greater effect than similar amounts of consumption later (e.g., Kiørboe, 1980). It also neglects the indirect feedback effects of herbivores, such as nutrient recycling, damage to the plants, or relief of density suppression of growth (e.g., Lodge, 1991).