Feeding under predation hazard: response of the guppy and Hart's rivulus from sites with contrasting predation hazard

Abstract

Populations of guppies, Poecilia reticulata and Hart's rivulus, Rivulus harti, in Trinidad experience different levels of predation hazard from piscivorous fish. Those from the larger rivers (“downstream sites’) experience chronically high predation hazard, while those from headwater streams (“upstream sites”) have few predators. Guppies and Hart's rivulus, collected from downstream and upstream sites, were assayed for their feeding rate in the presence and absence of predators. We defined tenacity as the ratio of the feeding rates in the presence and absence of a predator stimulus. Thus, tenacity expresses the degree to which the forager maintains its feeding rate when a predator stimulus is present. Previous work by Seghers (1973) showed that non-feeding guppies from downstream sites responded more strongly to predators than did guppies from upstream sites. Based on this, we initially hypothesized that fish from downstream sites would show lower tenacities than fish from upstream sites. However, we found the opposite in every case. When confronted with a predator stimulus, guppies and Hart's rivulus from downstream sites fed at consistently greater rates and displayed greater tenacities than did those from upstream sites. These differences were found in experiments using both live and model predators. The results suggest that upstream fish readily trade off feeding for hiding and avoiding predation hazard, a likely response when predators appear infrequently, while downstream fish appear to be selected for boldness and tenacity while foraging under chronically high hazard.