The Adaptive Significance of Interspecific Territoriality in the Reef Fish Eupomacentrus Leucostictus

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Abstract
Observations were made on the fishes trespassing in 50 territories of the reef pomacentrid Eupomacentrus leucostictus, and on the responses of the territory—holders to the trespassers. The extent to which each of the tresspasser species elicited agonistic responses from territorial E. leucostictus individuals was quantified: number agonistic acts/total trespass time. Using data on feeding habits, the diet overlap of each trespassing species on E. leucostictus was computed according to the formula &alpha = ϵpihPjh/ϵPih2. Following a log—log transformation to correct for heteroscedasticity, a Pearson product—moment correlation between number gonistic acts/total trespass time revealed a significant positive correlation between these 2 variables. An expression for Potential Competitive Impact that combines trespasser size with diet overlap, α(VB)75, showed a highly significant correlation with number agonistic acts/total trespass time (again using a log—log transformation) and a higher correlation coefficient (r =.613, P<.01). When this correlation was recalculated with the exclusion of the 9 trespassing species known to eat eggs, the correlation coefficient improved to r =.744, P <.01. These 9 egg—eating species have a significantly greater tendency to elicit agonism beyond that expected on the basis of their Potential Competitive Impact than do the other tresspassing species. (t =2.940, P <.02). This pattern of agonism indicates that interspecific territoriality is an adaptive behaviorthat reduces the loss of food in the territory to competitors and protects the nest from egg predators.