Shifts in foraging tactics of larval damselflies: effects of prey density

I studied search activity and attack tactics of larval damselflies, Lestes sponsa, feeding on cladocerans, Daphnia magna, at nine densities ranging from 2 to 640 per 1.5 1. Search activity increased from low to intermediate densities (40 Daphnia) and declined again at higher prey densities, as has been predicted by theories on optimal tactics for energy maximising foragers. Similarly, frequency of pursuit attacks first increased and then decreased as prey density increased. Frequency of ambush attacks increased with a decelerating rate with prey availability. Thus the proportion of pursuit attacks decreased linearly as prey density increased. Profitability of pursuing increased faster than ambush profitability at low prey densities and remained higher up to 40 prey. At higher prey densities the profitabilities reversed. Relative profitability of pursuits proportioned to prey density peaked with 5 Daphnia. At other prey densities than 5, frequency of pursuit attacks decreased towards the end of the trials. This change in foraging mode accounted for accelerating proportional prey mortality to an intermediate D. magna density and thus the potential for a dampening effect on the prey population. Adjusting foraging tactics in response to prey availability may be adaptive for the predator, and may also contribute to density dependence in the predator-prey relationship through effects on functional response.