The territorial defense hypothesis and the ecology of insular vertebrates

Abstract

Insular lizards, birds, and mammals in high-density populations often exhibit reduced situation-specific aggression toward conspecifics. This aggressive behavior can be expressed in the form of (1) reduced territory sizes, (2) increased territory overlap with neighbors, (3) acceptance of subordinates on the territory, (4) reduced aggressiveness to certain classes of conspecifics, or (5) abandonment of territorial defense. These behavioral traits can be explained by two nonexclusive hypotheses. The resource hypothesis suggests that territorial behavior is primarily adjusted to resource densities, and that resources are more abundant on islands than on the mainland (e.g., because of a lack of competing species). The defense hypothesis suggests that, in addition to any effects of resources, the costs of defense against both territorial intruders and contenders for vacant territories are higher on islands. Recent theoretical and empirical studies indicate that these behavioral changes can occur as a result of elevated defense costs, independent of resource densities. Reduced predation, more benign climates, and an absence of habitat sinks on islands would all tend to increase the density of potential intruders and contenders, and hence the costs of defense for owners of insular territories. The two hypotheses differ in their predictions about the rates of biomass production (growth or reproduction) for holders of insular territories. Reproductive and growth data from insular-mainland pairs indicate the importance of elevated defense costs, and also suggest that many insular vertebrates reallocate their breeding resources so as to produce young that are more competitive. The suite of ecological and behavioral traits exhibited by insular territorial vertebrates can best be explained by three factors operating in concert: higher available resource densities, higher defense costs, and (sometimes) a reallocation of resources to produce young that are more competitive.