HARVESTING OF BENTHIC ALGAE BY TERRITORIAL GRAZERS - THE POTENTIAL FOR PRUDENT PREDATION

For territorial animals harvesting a spatially fixed, renewable resource, natural selection might favor the evolution of prudent harvesting stategies that maximize the resource's sustained yield. We tested whether the algal assemblage maintained within the foraging area of Leucotrichia pictipes (Trichoptera: Hydroptilidae), a territorial stream insect, has a higher productivity (i.e. yield) than alternative assemblages that could be maintained by Leucotrichia. Field and laboratory experiments were used to manipulate and quantify the relationship between chlorophyll a biomass and C-14 productivity for various algal assemblages. We observed an asymptotic relationship between algal productivity and biomass over a sixteen-fold range of algal standing crops, which differs from the hump-shaped relationship expected for populations following logistic growth. Our results demonstrate that primary productivity would increase significantly if Leucotrichia maintained a higher algal biomass within its territory (e.g. by reducing its short-term harvesting rate), which implies that it is not a prodent predator. Because some of this production is consumed by territorial intruders, however, the yield to Leucotrichia may be maximized at a lower algal biomass than that which maximizes total productivity.