Laboratory predation on the Trinidadian guppy: implications for the size-selective predation hypothesis and guppy life history evolution (Poecilia reticulata)

Differences in size-specific predation among populations, attributable to different predator guilds, is believed to be the selective agent responsible for the evolution of disparate life history characteristics in Trinidadian guppies (Poecilia reticulata). Yet, the efficacy of this mechanism is inadequately tested. In this study, populations of different-sized guppies were exposed to individuals of two natural predatory species, the pike cichlid Crenicichla alta and the killifish Rivulus harti, under conditions of varying prey (guppy) density and habitat complexity in the laboratory. Rivulus fed most frequently on newborn and juvenile guppies < 14 mm SL. The mean guppy size consumed by Crenicichla increased with increasing predator length, although some large Crenicichla continued to feed on small guppies. Under test conditions that mimicked typical field conditions of habitat complexity and prey density, Crenicichla was a much more effective guppy predator than Rivulus. High habitat complexity and a shallow water refuge reduced Crenicichla predation rates from 10 to 3 guppies/day, but did not change prey-size selectivity. Rivulus predation rates never exceeded 1 guppy/day, regardless of habitat complexity. These data confirm results from a recent field investigation, but are inconsistent with the prevailing size-selective predation hypothesis regarding differences in life histories among Trinidadian guppy populations. An alternative hypothesis that incorporates differences in predation intensity among populations is supported.