Feeding Territoriality in Aquatic Insects: Cost-Benefit Models and Experimental Tests

Abstract

Two points are evident from a review of the literature describing feeding territoriality in aquatic insects. First, feeding territoriality is much more common in this group than was previously recognized. Second, most of the examples involve species that have small foraging areas and harvest rapidly renewing resources, such as filterable particles and attached microalgae. To interpret these patterns, I discuss how the net benefits of territorial defense vary as a function of several components of food availability. I present the results of recent laboratory and field experiments testing cost-benefit models that predict patterns of territory size and defense frequency. Feeding-territory size in grazing caddisfly larvae increases with the resident's body size and is inversely related to food abundance, which agrees with the predictions of several models of optimal territory size. Two kinds of stream insects that rely on food items delivered by water currents ( i.e. , surface-feeding water striders and filter-feeding larval black flies) respond to increases in food abundance by reducing their allocation of time to territorial defense.In black flies, complex interactions between competitor density and food abundance also influence the amount of time spent defending a territory. I consider several connections between territorial behavior and interspecific competition. The distribution and abundance of both territorial species and their competitors may depend in part upon how the costs and benefits of feeding territoriality vary along resource abundance gradients. The tendency of some territorial grazers to settle preferentially near conspecifics may occur because animals living in groups exclude interspecific competitors more efficiently than isolated individuals, which suggests some simple tests of optimal group size models. I conclude by summarizing the strengths and weaknesses of these study systems, both as a source of new and broader theories of feeding territoriality, and as a testing ground for those theories. An important strength of these consumer-resource systems is the ability to conduct realistic experimental studies examining the causes and consequences of territoriality. One important weakness is the lack of information on the time-energy budgets of these insects. The acquisition of such information deserves a high priority, since it will permit more rigorous tests of cost-benefit models that evaluate the adaptive significance of territorial behavior.