Making it to school: As prey individuals form groups the change in patch densities creates uncommon predator-prey functional responses

Background/Question/Methods

As prey aggregate together into groups, they change the distribution and density of prey patches within a particular habitat. This often reduces encounter rates for predators who must now travel over longer distances between patches. However, it generally isn’t recognized that while these patches form, prey individuals that cannot quickly join a group will ultimately face the same challenge. Further, at that time they will often be most vulnerable to a predatory strike. As there are many species that disperse and re-aggregate on a daily basis, this scenario may represent an important predator-prey dynamic. If so, we ask a classic question of how changes in prey density might influence this scenario. We hypothesize that when initial densities of prey are high, it should be easier for individuals to group with one another than it would be at low initial prey densities where individuals are already quite spread out. To explore this hypothesis, we study a schooling fish population using 1) an Ideal Gas model of predator-prey encounter rates, and 2) field estimates of prey population densities and their predator’s foraging success over a 20 year period.

Results/Conclusions

We demonstrate with the Ideal Gas encounter model that the encounter rates between prey individuals when forming schools theoretically can lead to dome-shaped (or Type IV) predator functional responses. Further, we demonstrate empirically that a higher proportion of prey do not join schools when overall prey densities are low, and the counter-intuitive result that predator consumption rates decline as prey abundance increase. These dynamics reflect the influence of density on the probability that an individual prey will find a school. The predator functional response could therefore be influenced more by the indirect effect of density on school formation than by the direct effect of density on the maximum potential encounter rate. This counter-intuitive result challenges the few theoretical functional responses available for schooling fish (and prey patches in general) which are assumed to be represented by a saturating and monotonically increasing relationship between prey density and consumption rates (or Type II).