The effect of alternate prey presence on the functional response by larval damselflies to changes in cladoceran prey density
Akre, Barbara Gavin
Johnson, Dan M.
Master of Arts
The effect of alternate prey availability on the functional response by naiads of the damsel fly Anomalagrion hastatum to change in the density of a cladoceran prey (Daphnia magna or Simocephalus vetulus) was measured in large-volume simulated weedbed systems with low (natural) prey densities. A study of predator selectivity, a key to functional responses for single prey within a multiple-prey system, was included in the experimental design. The damselflies exhibited hyperbolic (destabilizing) functional responses for single prey, but sigmoid (potentially stabilizing) responses for both species in a complementary density two-prey system. Deviations of two-prey response data from "constant preference" predictions suggested that variable selectivity may have been the source of the observed regulatory potential. Presented with varying relative abundances of the two prey, predators increased their selectivity for the more abundant species; i.e., they switched. Therefore, predator switching may be the source of regulatory potential in the functional response. The naiads' tendency toward switching, and their selectivity for D. magna at equal densities of prey, increased with increasing predator hunger. Increasing total prey density also increased equal density selectivity for D. magna. These changes in predator selectivity are discussed with reference to optimization theory. A behavioral hypothesis explaining switching and the potentially stabilizing functional responses (reward reinforcement of selective search modes) is proposed. Relationships between switching and regulatory potential in functional responses for two competing prey, and for prey within multiple-prey systems, are discussed. Theoretical and documented effects of alternate prey availability lead to the conclusion that relative prey abundance and predator selectivity may be more important determinants of field functional responses than absolute prey density or predator hunger level.