Switching and Sigmoid Functional Response Curves by Damselfly Naiads with Alternative Prey Available

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Abstract
The influence of alternate prey availability on the functional response of final instar damselfly naiads to densities of standard-sized cladocerans (motile Daphnia and sessile Simocephalus) was studied. Experiments were conducted at relatively low prey densities (3-63/1) in large containers (6-45 l) with extra surface area added. Hungry and satiated predators were exposed to prey for 12 h at 25.degree. C in total darkness. Damselfly naiads exhibited Type II functional responses when only 1 type of prey was available. Disc equations showed that handling time was not affected by prey type or predator hunger, but the attack rate coefficient was increased by hunger and was greater for Daphnia than Simocephalus. Damselfly naiads exhibited Type II functional responses when alternate prey were available at absolute densities equal to those of the principal prey. Predator selectivity for Daphnia increased with hunger and prey density. Damselfly naiads exhibited positively accelerated (Type III) functional responses to both Daphnia and Simocephalus when alternate prey were available at absolute densities complementary to those of the principal prey. Predator selectivity was frequency-dependent (they exhibited switching behavior). Switching behavior may be caused by the damselfly naiads'' use of 2 alternate searching modes (ambush and walking) that alter their encounter frequencies with motile Daphnia and sessile Simocephalus.