| Summary: | This thesis addresses two questions of fundamental importance to evolutionary ecology - how genetic variation in traits that affect interactions between and within species are maintained within populations, and how such variation .affects patterns of intra- and interspecific interactions in the field. In chapter one, I,review the evidence concerning these issues. I also introduce the pea aphid Acyrthosiphon pisum Harris (Homoptera: Aphididae) - natural enemy model system used in the following experimental chapters. In chapter two, I describe a field experiment that illustrates how pea aphid clonal variation can affect natural enemy recruitment and community structure. The major predator in this system is the hoverfly Episyrphus balteatus De Geer (Diptera: Syrphidae). I measured the costs of parasitoid attack in E. balteatus (chapter three), finding that parasitized E. balteatus consumed fewer aphids, took longer to pupate and experienced higher pupal mortality, and flies that survived attack had reduced life spans. Chapter four describes a series of experiments that test whether a trade-off between competitive ability and dispersal ability could maintain clonal variatiqn in these traits among clones of the pea aphid. Significant clonal variation was detected for interspecific competitive ability, lifetime fecundity and dispersal ability. Lifetime fecundity was found to correlate positively. with the production of winged offspring. Chapter five describes an experiment in which we tested whether the variation in interspecific competitive ability detected in the laboratory (chapter four) could alter the outcome of interspecific competition in the field. In the laboratory the pea aphid populations were always larger than populations of a single clone of the vetch aphid M~,.oura viciae Buckton (Homoptera: Aphididae), but in the field the outcome of interspecific competition depended on pea aphid genotype. Chapter six describes a field experiment that measured the effects of competition and predation on the population dynamics of two clones of the pea aphid. Both clones did equally well in monoclonal colonies, while in mixed colonies one clone was competitively superior. The strength of predation depended on aphid density. One clone did better when sharing a patcli, but not a plant, with the second clone compared with when sharing a patch with a second colony of the same clone (apparent mutualism). In chapte'r seven, I review the principal results of these experiments and discuss how these new insights fit into the wider evolutionary and applied ecological literature.
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