Size Efficiency Reconsidered: A General Foraging Model for Free-Swimming Aquatic Animals

Publisher Website
Google Scholar
Abstract
A foraging model for aquatic animals is developed based on specific power gained from feeding, minus power spent foraging, minus standard metabolism. Power gained from feeding is derived from a model of encounter in 3 dimensions, and power spent in foraging is based on drag laws of fluid dynamics. The model illustrates the transition from cruising to ambush predation as prey swimming speed increases. The transition is abrupt for protozoa swimming at low Reynolds number, but it is more gradual for fish swimming at high Reynolds number. Consideration of fluid mechanics leads to the conclusion that filter feeding with reciprocating appendages is suboptimal for large animals (e.g., vertebrates) that operate a high Reynolds number. Optimal size of a foraging animal depends on the environmental density of the animal''s food, on its swimming efficiency, and on its metabolic efficiency. At high food densities small animals have the greatest potential growth rate, but at low food densities small animals will starve while larger animals are still able to grow. Increased swimming efficiency or metabolic efficiency will increase the optimal size at low density. Under exploitative competition for food, larger animals will exclude smaller animals under all food conditions. Since predation seems to be the dominant structuring agent in open-water communities, competitive interactions rarely proceed to exclusion. Thus, the distribution of body sizes of groups of aquatic organisms is expected to reflect the general abundance of food, expressed as the productivity of the habitat. The model may then give an explanation for observations that small animals are dominant in eutrophic environments, while larger animals are dominant in oligotrophic environments.