Introduction

Organisms experience myriad interactions with both antagonists and mutualists. There is widespread recognition that these multispecies interactions are not independent and that community membership can have important consequences for host fitness as well as patterns of natural selection (Strauss and Irwin 2004; Morris et al. 2007). For example, herbivore feeding can influence plant interactions with other herbivores or mutualists (e.g., pollinators), which can have subsequent effects on host plant fitness (Karban and Baldwin 1997; Mothershead and Marquis 2000; Strauss et al. 2001). Moreover, traits involved in these multispecies interactions can represent an adaptive compromise due to host interactions with antagonists and mutualists (Galen and Cuba 2001). While the effects of antagonist–antagonist and antagonist–mutualist interactions on hosts have received attention from both theoreticians and empiricists, the ecological and evolutionary consequences of host interactions with multiple mutualists have received less study (Hoeksema and Bruna 2000). That the study of multispecies mutualisms has lagged behind other suites of multispecies interactions is not surprising, given that mutualisms in general receive less study than competition and predation (Bronstein 1994). However, because many species interact with multiple mutualists either simultaneously or sequentially throughout their lifetimes (Janzen 1985) and mutualisms can have powerful impacts on host fitness and evolution (Bronstein et al. 2006), it is germane to ask how these multispecies mutualisms affect the ecology and evolution of their shared hosts (also see Stanton 2003).