Increasing Trophic Complexity Influences Aphid Attendance by Ants (Hymenoptera: Formicidae) and Predation

Abstract

Mutualisms, interactions between species whereby both benefit, can alter ecosystems by affecting community structure and species' distributions. Aphid tending behavior by ants, a common mutualism, directly affects aphid population growth and distribution. Aphids consume plant phloem and excrete honeydew that is eaten by ants. In return, ants provide protection for aphids. Foeniculum vulgare is an invasive plant in coastal scrub habitats of California that hosts a mutualism between invasive Argentine ants (Linepithema humile) and three aphid species (Cavariella aegipodii, Dysaphis apiifolia, Dysaphis foeniculus). This association can modify insect assemblage and affect host plant distribution by impacting herbivory rates. Host plants may experience a net reduction in herbivory caused by release from insects like the native specialist lepidopteran, Papilio zelicaon. This may facilitate the invasive success of F. vulgare. In this study, I quantified the effects of the ant-aphid association on host plant growth and insect assemblage. I tracked growth of F. vulgare on ant-aphid hosts versus non-hosts and found no effect of aphid presence. I recorded the presence of other fauna and found significant reductions on plants hosting the mutualism. I also quantified an effect of the mutualists on the distribution of P. zelicaon. In a greenhouse experiment, I found that P. zelicaon reduced growth of F. vulgare while aphids did not. I conclude that the mutualism can increase plant fitness and may be an important factor in facilitating/maintaining the invasion of F. vulgare. There may be implications for P. zelicaon given the increasing prevalence of L. humile.