Small scale spatial heterogeneity influences predation success in an unexpected way: Model experiments on the functional response of predatory mites (Acarina).

A basic assumption usually made in measuring the functional response of a predator (predation rate) is that it depends on the density of the prey but not on the size, form, or small-scale spatial structure of the studied area. This assumption has been tested by experiments with the predatory mite Phytoseiulus persimilis and the spider mite Tetranychus urticae as prey.In experiments carried out on arenas with different internal spatial structure the predators had a lower predation rate on those arenas with a higher degree of spatial heterogeneity. On arenas with no additional internal structure but of different shape the predation rate was highest on squares with a relatively short border. The differences in predation rate are explained by two effects. The borders influence the otherwise random movements of predator and prey in such a way that both stay slightly more often along the border. The shorter the total length of borders the higher is the probability of predator and prey meeting along a border; therefore predation rates on plain squares are higher than would be expected from completely random movements and distribution. In addition to this edge effect the arrangement of borders forming labyrinths results in a lower predation rate, probably by impeding searching movements. The encounter rate between predator and prey was much higher than the predation rate. Encounter rate and predation rate were not directly proportional.It may be generally concluded that in predatory mites and probably in all other predators too, the functional response not only depends on the density of the prey but also on the size and the small-scale spatial structure of the experimental area or the habitat.