Harvesting in an uncertain world: impact of resource competition on harvesting dynamics

Abstract

Different minimum resource levels ( R∗) of competitors can result in switches in competitive dominance mediated by predation on the competitors. A simple food-web model with two herbivores competing for a food resource but having different R∗ values was explored to determine the impact of the species' different R∗ on harvesting dynamics. When a superior competitor is harvested and the inferior competitor left unharvested, the superior competitor is driven to extinction by the inferior competitor once harvesting raises the shared resource to a level that the inferior competitor can survive. Yields to the predator then drop to zero for a very small change in harvesting effort. It was shown that these dramatic drops in yield will, in most cases, occur before the maximum sustained yield (MSY) inflexion point predicted without the inferior resource competitor. The maximum yield achieved by the superior competitor in the presence of the inferior competitor increased with the difference between the two species R∗ values (i.e. MSY ∝ R∗ s − R∗ i ). Therefore, high yields to a predator require either polyphagy (i.e. eating all competitors) or large differences in the R∗ of competitors. The second case is most likely to occur between species with different energetic requirements and, therefore, with different evolutionary lineages (i.e. bony fish versus invertebrates). However, even in this circumstance drops in yield are likely to occur before a MSY predicted without the inferior competitor. MSY predictions need to consider community dynamics and the minimum resource requirements of competing species.