The effects of group size, shape and composition on ease of detection of cryptic prey

[The role of aggregation in prey defence has been the subject of much debate. For example, the commonly cited assumption of encounter-dilution effects requires that attack rates increase asymptotically with increasing group size. One key parameter that is still poorly understood is how ease of detection of cryptic prey aggregations is affected by group size, shape, density and composition. We have developed a computer based visual test to elucidate these effects using human subjects. We show that ease of detection of cryptic prey increases asymptotically and quickly saturates with group size: this is in accordance with a previously reported laboratory test of birds. We also discovered that horizontally-orientated groups were more easily detected than either circular or vertically arranged ones. No effect of the degree of spatial clustering of prey occurring in groups was found, and nor did heterogeneity or homogeneity in the visual appearance of group members affect ease of detection — although future work on these aspects is warranted., The role of aggregation in prey defence has been the subject of much debate. For example, the commonly cited assumption of encounter-dilution effects requires that attack rates increase asymptotically with increasing group size. One key parameter that is still poorly understood is how ease of detection of cryptic prey aggregations is affected by group size, shape, density and composition. We have developed a computer based visual test to elucidate these effects using human subjects. We show that ease of detection of cryptic prey increases asymptotically and quickly saturates with group size: this is in accordance with a previously reported laboratory test of birds. We also discovered that horizontally-orientated groups were more easily detected than either circular or vertically arranged ones. No effect of the degree of spatial clustering of prey occurring in groups was found, and nor did heterogeneity or homogeneity in the visual appearance of group members affect ease of detection — although future work on these aspects is warranted.]