The evolution of species interactions is a fascinating subject, and one of vital
importance if we are to understand how biological communities change over time.
This thesis considers the interaction between aphids (Homoptera) and ants
(Formicidae). Ants tend aphids for sugary honeydew and in return provide a variety of
protective services. A literature review in Chapter 1 introduces the subject and
provides background information. Chapter 2 considers ant- aphid interactions in a
community setting. Specifically, I consider the fitness effects of the ant- aphid
interaction on host plants. Net benefits or costs to plants depend on the densities of
ants and aphids; these densities may themselves change depending on context
dependent factors. Chapters 3 and 4 consider how semiochemicals can allow species
to respectively maintain or avoid synchrony in space and time with mutualists or
antagonists. Chapter 3 shows ladybirds avoid prey patches guarded by ants by
reducing oviposition in response to ant semiochemicals. Chapter 4 shows that aphid
walking dispersal can be limited by ant semiochemicals. This may be adaptive for
aphids to remain in areas of enemy- free space. Alternatively, if levels of kin
competition are high limited dispersal could be costly to aphids. In Chapter 5 I
consider interactions between invasive and native ants. Ecological dominance in ants
may be mediated by the ability to monopolise honeydew- producing resources.
Chapter 6 explores ants’ decisions whether to tend or prey upon aphids. Predation of
aphids depends on colony demand (e.g. through cues from the presence of larvae) as
well as the quality or quantity of supply (e.g. increased predation of unproductive
aphids). Finally, Chapter 7 deals with macroevolutionary patterns in the interaction
between ants and aphids. Specifically, I identify ecological traits that characterise
aphid- tending ants. A final discussion chapter summarises how ant-aphid interactions
fit into existing mutualism theory.