When Should a Predator Sit and Wait Rather than Actively Forage for Prey: Effects of Prey Locomotion and the Presence of a Top Predator

A. Bouskila, E. Summer, E. Nolman and M. Shick

Predators forage for prey either by active search, ambush or a combinationof the two. Each of these foraging modes carries different costs and benefits due to factors such as energetic expenditure, the risk of being detected by their own predators and so on. In order to understand therelative advantages of each foraging mode under various conditions, wehave to consider also the encounter probability between the predator andits prey. Rather than using a simple equation which is often used but cannot be adjusted for different types of locomotion (such as jumping ordirectional movement), we used a mathematical model to simulate the movements of the prey and of two types of predators (widely foraging andambush predator). In general, the ambush predator required more time units to reach an encounter with the prey and the coefficient of variationwas larger than for the active forager. The ambush predator encounters aprey that moves directionally earlier than it encounters a randomly moving prey. An interesting conclusion from these results is that for animals that move directionally, the ambush strategy may not incur the costs inprobability of encounter that is predicted for random moving animals. This conclusion is important because measurements that we made of the locomotion of several species of both prey and predators indicate that these species move directionally.

In order to study the effect of predation risk on the foraging mode, Eric Summer performed experiments with jumping spiders (Plexipus paykully) with or without predation risk from lizards (Laudakia stellio). Experiments were performed in a uniform bowl, with one preyitem (a nymph of a cricket). Under the risk of predation, spiders shiftedtheir foraging towards ambush, and reduced their movements. The spider mayeither minimize movements in order to reduce the chances of being detectedby the lizard, or it may invest more time in vigilance. In addition, Eric found that spiders under the risk of predation require longer time to catch their prey, so there seem to be a trade-off between the rate of prey capture and safety from predation.