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 fr om 2 to 640 per 1.5 1. Search activity increased from low to intermediate d ensities (40 Daphnia) and declined again at higher prey densities, as has b een predicted by theories on optimal tactics for energy maximising foragers . Similarly, frequency of pursuit attacks first increased and then decrease d as prey density increased. Frequency of ambush attacks increased with a d ecelerating rate with prey availability. Thus the proportion of pursuit att acks decreased linearly as prey density increased. Profitability of pursuin g increased faster than ambush profitability at low prey densities and rema ined higher up to 40 prey. At higher prey densities the profitabilities rev ersed. Relative profitability of pursuits proportioned to prey density peak ed with 5 Daphnia. At other prey densities than 5, frequency of pursuit att acks 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 p opulation. 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.