Searching for prey: the effects of group size and number

If larger groups are discovered (and thus attacked) by predators more often, the benefits of group living can be reduced or even cancelled. Thus, the relationship between group size and detection rate is critical in assessing risk to prey individuals. Three-spined sticklebacks, Gasterosteus aculeatus L., predating Daphnia magna were used to assess this relationship, with time taken to approach prey as a proxy for detection rate. In a field experiment, a group of 100 Daphnia was approached and attacked more rapidly than a group of two. In addition, the time to approach prey was found to be inversely proportional to group size (range 2–50 Daphnia) under laboratory conditions. Multiple groups of the same size (2 or 30) were then presented to determine the effect of group number on approach time. Time to approach the prey was significantly reduced compared to that when these group sizes were presented alone. This effect of group number suggests that the asymptote observed in the relationship between group size and approach time was limited by encounter rate with the prey group. There was no significant interaction between group size and number, suggesting that prey encounter (number of groups) and prey detection (group size) acted independently in reducing approach time. In a final experiment, it was demonstrated that the visual angle produced by the prey group could account for the greater detection of larger groups.