Journal of Ecology 2006 94, 1249–1260 © 2006 The Authors Journal compilation © 2006 British Ecological Society Blackwell Publishing Ltd Interactions between insect herbivores and a plant architectural dimorphism JENNIFER A. RUDGERS* and KENNETH D. WHITNEY* Center for Population Biology, University of California, Davis, California, 95616, USA Summary 1 Plants possess many traits that influence their resistance to insect herbivores and thus can shape the structure of herbivore assemblages. While plant chemistry and structural defences have received the most attention, plant architecture may also be important. We examined patterns and consequences of insect herbivory for two distinct architectural morphs (prostrate or erect) of the perennial shrub, Baccharis pilularis (Asteraceae, coyote bush) in coastal California, USA. 2 For B. pilularis, differences in plant height, branch architecture and leaf size persisted in a common garden, consistent with previous evidence suggesting a genetic basis for dimorphism in plant architecture. 3 In both naturally occurring plants and in a common garden experiment, the composition of the herbivore assemblage varied strongly with plant architectural morph. Prostrate plants attracted higher densities of a gall-forming midge (Rhopalomyia californica, Diptera), whereas erect plants supported more moth galls (Gnorimoschema baccharisella, Lepidoptera) and experienced greater folivory. Furthermore, architectural traits were correlated with herbivory levels both across and within architectural morphs. 4 Prostrate plants had greater reproductive output (flowers or seeds) than erect plants in the presence of herbivores. However, under experimental reductions of herbivory spanning 40 months, the morphs had similar reproductive output, demonstrating that herbivores have the potential to act as agents of selection on the dimorphism in plant architecture. 5 These results confirm that intraspecific variation in plant traits can shape the composition of herbivore assemblages. Furthermore, in this system, herbivores alter the performance of the architectural morphs, suggesting a dynamic system of feedbacks between the population genetic and community levels. Key-words: Baccharis pilularis, community genetics, genetic polymorphism, intraspecific genetic diversity, natural selection, parasitoid, tri-trophic interaction Journal of Ecology (2006) 94, 1249–1260 doi: 10.1111/j.1365-2745.2006.01161.x Introduction A recent area of emphasis within ecology attempts to link genetic variation at the population level with com- munity dynamics (Neuhauser et al. 2003; Whitham et al. 2003). In this ‘community genetics’ framework, a number of studies have shown that genetic variation in plants can have strong structuring effects on higher trophic levels and entire communities. For example, different genotypic classes of hybridizing cottonwoods (Populus fremontii × P. angustifolia) support distinct arthropod assemblages (Wimp et al. 2005). Furthermore, the genetic effects reverberate to higher trophic levels, e.g. insectivorous birds (Bailey et al. 2006). However, the majority of this work has been carried out in inter- specific hybrid systems (see also Eucalyptus, Dungey et al. 2000; Salix, Hochwender & Fritz 2004), where we might expect the merging of diverged genomes to produce particularly large phenotypic effects; in turn, large community-level effects could arise. A fair question is whether, and by what mechanisms, intraspecific genetic Correspondence: Jennifer A. Rudgers (tel. +1 713 348 6276; fax +1 713 348 5232; e-mail jrudgers@rice.edu). *Present address: Department of Ecology and Evolutionary Biology, Rice University, Houston, Texas 77005, USA.