Predator-prey interactions between mosquitofish (Gambusia affinis holbrooki) and whistling frog (Litoria ewingi) tadpoles : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Palmerston North, New Zealand

Abstract

Aspects of the impact of newly introduced predatory mosquitofish (Gambusia affinis holbrooki) on the resident introduced whistling frog (Litoria ewingi) were examined by means of experiments. Microhabitat preferences of whistling frog tadpoles and mosquitofish were investigated through a series of laboratory trials and the behaviour of both species during attacks by mosquitofish on tadpoles was also recorded. Microhabitat use of these species will overlap in natural situations, but both species changed their microhabitat use with age. In the presence of plants, small tadpoles moved towards the surface, while larger tadpoles prefered to associate with the substrate. The same pattern occurred when an alga chip (food source) was placed on the substrate. Small female mosquitofish were more attracted to plants than larger mosquitofish, and less attracted to tadpoles (potential prey). The extensive recent literature on predation is reviewed with emphasis on existing works on predator-prey interactions involving amphibian larvae. Behaviour of both species during attacks changed with age. Small tadpoles were more likely than large tadpoles to flee from an attack. Larger female mosquitofish were more likely to attack moving tadpoles (in comparison with small female mosquitofish), and to attack the body of tadpoles (as opposed to the tail). There was no difference between the behaviour of small female and male mosquitofish. Most attacks were non-fatal. Inactive prey were more likely to be attacked in this combination of predator and prey, which is in direct contrast with previous studies. When a plant with complex three-dimensional architecture was added attacks still occurred, but overall the frequency of attacks was lower, indicating that the presence of sufficient refuge may lessen the effect of mosquitofish introductions on whistling frog tadpole populations. A field experiment was also conducted but results were not conclusive. L. ewingi may become reduced to breeding in ephemeral waters if G. a. holbrooki invades permanent waters successfully and removes L. ewingi extensively.