Life History Variation among Experimental Populations of the Mosquitofish, Gambusia affinis

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Abstract
The life history variation of the poeciliid, G. affinis, was examined in a series of experimental ponds. Fish from a single field location near Perth, western Australia, were maintained for a summer breeding season at 5 different food levels. The ponds were used to establish if the life history response of the fish to an environmental continuum was based upon consistent relationships among the traits and close covariation of the life history as a whole. Principal component analysis was used to define the covariation of the traits. The fish initially responded to the increasing food by increasing somatic and reproductive growth. Somatic growth reached a maximum at intermediate food levels although fecundity and brood mass still increased at all food levels. Embryo mass varied little. The principal component analysis identified 3 independent sources of variation. The 1st component, which was interpreted as defining the covariation of the traits in response to the food continuum, was only closely associated with fecundity and brood mass. The results indicate that the covariation of the traits is not defined by consistent relationships and that the response of the fish to the food is not adequately described in terms of a simple life history tactic. This is partly because there are differences between the traits in their ability to vary. Explanations of the life history variation must take into account that, even given relatively simple environmental variation, there may be more than 1 dimension to the life history response and that these cannot be defined by exclusive sets of traits.