Search rate, attack probability, and the relationship between prey density and prey encounter rate

Although numerous influential models in ecology assume a directly proportional relationship between prey density and prey encounter rate, a recent test of this assumption found that the actual relationship was nonlinear (rising slower than proportionately). Here, three-spined sticklebacks were used to test 2 recently proposed mechanisms based on predator search behavior that may induce this nonlinearity: the effect of increasing search rate over the course of the search and the effect of spatial correlation in areas searched. The fish explored a novel apparatus of 16 cells (15 x 16 cm) arranged in a ring, with each cell connected to the 2 adjacent cells by small openings. Hence, the arena was effectively one dimensional, simplifying the recording of search paths and removing edge effects. The relationship between prey density and encounter rate was found to be less than directly proportional, confirming the findings of a previous study using great tits searching for caterpillars. In the majority of our trials, the search rate accelerated as more of the arena was searched. Moreover, the greater this acceleration the less prey density and encounter rate were directly proportional. There was no evidence found, however, that spatial autocorrelation had any effect. We also found an interesting effect of prey density on the likelihood that an encounter with a prey would lead to an attack. Once prey were encountered, the probability of attack increased with the number of cells already visited, leading to a higher probability of attacking first-encountered prey at lower prey densities.