An Experimental Separation of Interference and Exploitative Competition in Larval Damselfly

Density—dependent reduction in survival, growth, and development rates of larval damselflies can be the result of depletion of food resources or increased interference costs (energy or time) associated with behavioral interactions. While interference has been implicated in several studies, no direct test of this hypothesis by manipulating interaction frequency or intensity has been attempted. To separate these mechanisms, I simultaneously manipulated habitat complexity (number of perches) to alter the frequency of behavioral interactions, larval density, and food supply. Damselflies became more evenly distributed among available perches as the density per perch increased, demonstrating that there were behavioral responses to the manipulation of habitat complexity. Food supply and damselfly density strongly affected survival, timing of emergence, and mass at emergence. However, the proportion of the variation in these performance variables attributable to the habitat complexity manipulation was tiny. In spite of the overt nature of the interactions among individuals, the costs appear to be very low. Future work that implicates behavioral mechanisms in population processes will have to attempt more direct manipulations of the behavior itself to test the hypothesis before concluding that behavior is the cause of an observed pattern.