Introduction

Much of the emphasis in studying mutualisms has been placed on defining the strength of these associations and the conditions that cause their collapse (Bronstein 1994, 1998). Yet, very few studies of aphid–ant mutualisms have linked the importance of host plant traits with the establishment and persistence of these mutualisms. Aphid performance, as well as the quality and quantity of their honeydew, may be affected by differences in host plant genetics or through environmentally induced effects on host plant quality. Differences among host plants that influence the attractiveness of aphids to tending ants can therefore alter the nature and strength of this association. The importance of host plants in determining the establishment and persistence of aphid–ant mutualisms could have consequences for biodiversity if these aphid–ant mutualisms play an important role in the structure and diversity of ecological communities.

The idea that mutualisms are important components of ecological communities arose 130 years ago (van Beneden 1875, French paper cited in Boucher 1985). However, much of the theoretical and empirical work in ecology for the past 70 years has supported the view that antagonistic interactions among species are more important than positive interactions in determining community organization. Yet, empirical data on an array of different mutualisms has shown that they can be important to community structure and diversity.