Body Size and Consumer-Resource Dynamics

Abstract

"Plausible" consumer-resource models, based on energetic reasoning and allometric empiricism, are formulated and their dynamics investigated. Most of the parameters in these models are determined by the body sizes and metabolic categories (endotherm, vertebrate ectotherm, invertebrate ectotherm, or plant) of the populations in question. The remaining parameters have clear biological meanings. The intent of these models is to provide maximum realism from minimum data when needed because of constraints of time or of resources for empirical research. The models also demonstrate the influence of physiological power and body size on consumer-resource dynamics. For a given metabolic type, we define ecological scope as the ratio of maximal ingestion rate to respiration rate. Invertebrate ectotherms have greater ecological scope than do vertebrate ectotherms, which have greater scope than do endotherms. Greater ecological scope implies ability to subsist on scarcer resources and a tendency to more stable dynamics. Changes in dynamics associated with increased resource carrying capacity K (the "paradox of enrichment") are investigated. Robust limit cycles (sustained oscillations) require a Type III functional response, but even in this case the cycles quickly "implode" (with respect to changes in K) to extremely small densities if the resource-consumer body mass ratio is too high. Estimates are given for resource-consumer body mass ratios that permit robust limit cycles, and some pertinent data are discussed. An analytic expression for the periods of limit cycles is derived. In terms of body mass, the periods scale as (consumer body mass)1/8 (resource body mass)1/8.