Multispecies interactions among a plant parasite, a pollinator and a seed predator affect the reproductive output of an invasive plant, Cytisus scoparius JANE N. PRIDER,* JOSÉ M. FACELLIAND JENNIFER R. WATLING Ecology and Evolutionary Biology, School of Earth and Environmental Sciences,The University of Adelaide, Adelaide, South Australia, Australia Abstract Interactions between species pairs are almost always mediated by interactions with other species. The outcomes of these multispecies interactions are often difficult to predict and are rarely studied. In addition to their theoretical importance, multispecies interactions are also important for management situations. Where multiple agents are used to control invasive species, interactions between agents may either enhance or reduce the impacts on the target species, or may simply have additive effects. In this study, conducted in a Mediterranean-type woodland in Australia, we examined how the interaction between an invasive legume, Cytisus scoparius (Legumi- nosae), its pollinator, Apis mellifera and a seed predator, Bruchidius villosus (Bruchidae), are modified by a native hemiparasitic vine, Cassytha pubescens (Loranthaceae). The parasite had a direct negative effect on C. scoparius, reducing flowering by 50% and consequently fruit and seed production. Despite having fewer flowers, infected plants had the same proportion of ‘tripped’ flowers, an indirect measure of pollinator visitation, as uninfected plants; although fruit formed on infected plants it was more likely to abort prematurely. Seed predation by B. villosus was lower on parasite-infected C. scoparius plants than in uninfected plants. Although Ca. pubescens had an antagonistic effect on B. villosus, in consort, the two agents reduced overall seed production by an average of 62%. The acquired parasite Ca. pubescens was more effective in reducing reproductive output than the introduced seed predator, B. villosus, and shows potential as a biocontrol agent for C. scoparius.We documented a subadditive effect of two biological enemies on the invasive species, where the acquired parasite had a stronger effect than the introduced seed predator. Key words: Bruchidius villosus, Cassytha pubescens, enemy release, Scotch broom, tripartite interaction. INTRODUCTION Because of the biological complexity inherent in eco- logical communities, most studies of species interac- tions have focussed on direct pairwise relationships; however, organisms typically interact simultaneously with a number of mutualists and antagonists. The inci- dence or intensity of pairwise interactions between species can be altered by the presence of other organ- isms, resulting in indirect interactions that can have synergistic or non-additive effects on plant fitness (Wootton 1993; Herrera 2000; Bronstein et al. 2003; Strauss & Irwin 2004; Mitchell et al. 2006; Brody et al. 2008). Where species introductions occur, new inter- actions will be established that will impact on both the invading species and the host community. Thus, the ability of an introduced plant to invade a community depends on the network of interactions it establishes with other organisms, both native and introduced, in the new range. These networks include negative inter- actions such as competition, herbivory and parasitism, as well as positive interactions such as mutualistic sym- bioses and pollination (Richardson et al. 2000; Shea & Chesson 2002; Noonburg & Byers 2005; Eppstein & Molofsky 2007). It has been proposed that escaping from natural enemies in the original system also con- tributes to success in the invaded system (Maron & Vila 2001; Keane & Crawley 2002; Parker & Hay 2005). Conversely, species may have evolved tolerance to their native enemies (Callaway & Aschehoug 2000; Bergelson et al. 2001), but may be more susceptible to novel enemies encountered in the invaded system (Maron & Vila 2001; Parker et al. 2006; Verhoeven et al. 2009). Reproductive traits of introduced species are im- portant determinants of their successful spread and eventual dominance in a new range (Rejmánek & Richardson 1996). Biotic interactions that constrain reproductive output can have important effects on the success of invasive species when there is a link between seed production and population size (Turnbull et al. *Corresponding author. Present address: Department of Water, Land and Biodiversity Conservation, PO Box 2834, Ade- laide, SA 5001, Australia. (Email: jane.prider@sa.gov.au) Accepted for publication March 2010. Austral Ecology (2011) 36, 167–175 © 2010 The Authors doi:10.1111/j.1442-9993.2010.02132.x Journal compilation © 2010 Ecological Society of Australia