Resource-ratio Theory Applied to Large Herbivores

The theoretical description of exploitation competition, known as resource competition theory (RCT) or resource-ratio theory, has been tested in terrestrial plant communities and microorganisms in laboratory cultures. Applications in animal ecology have been rare, although the theory itself is generic. A major difficulty is that the description of resources in RCT is fundamentally different from that used in classical studies of animal competition. In presenting the first fully specified RCT models for terrestrial animals, we distinguish between positive attributes (mineral elements) and negative attributes (plant defenses) as indicators of quality in animal resources. Using the latter we apply RCT to ungulate communities that exploit just two resources: the cell wall and cell contents of plant material. We show how coexistence in the same habitats depends on the strategy of resource exploitation. Ungulate species that differ in body size adopt a "demand-minimizing" strategy that permits them to coexist on ratios of the two resources by acquiring less of the resource that most limits their competitor. Ungulates that differ in mouth width adopt an "extraction-maximizing" strategy that leads to competitive exclusion because they acquire more of the resource that most limits their competitor. We conclude that differential resource utilization permits grazing herbivores of different body size to coexist on the same grassland habitats, but that the full diversity of grazing communities depends on spatial heterogeneity in plant defenses at the landscape level.