An Investigation of The Healthy Herd Hypothesis In The Trinidadian Guppy

Abstract

Predators can influence host–parasite dynamics through density mediated effects, behavioral modification, and selective removal of infected individuals, as explained by the Healthy Herd Hypothesis (HHH). Despite its conceptual prominence, empirical support for the HHH remains taxonomically and methodologically narrow, with limited tests in vertebrate freshwater systems. This thesis integrates a systematic review of the literature on predator–parasite–prey interactions with a replicated mesocosm experiment using Trinidadian guppies (Poecilia reticulata) and the nematode Camallanus cotti. The review identifies substantial biases toward invertebrate systems, directly transmitted parasites, and field studies lacking mechanistic resolution, that is to say tests of what parasites changed in their hosts. In the mesocosm experiment, predator presence reduced overall guppy survival particularly of parasitized individuals, with these having 25.3% lower survival than unparasitized fish when predators were present. Parasite presence was significantly lower in predator treatments, supporting selective predation. These findings provide rare experimental evidence for the HHH in a vertebrate system and clarify the ecological conditions under which predators reduce parasite prevalence through targeted predation.