Biodiversity is related to indirect interactions among species of large effect

Introduction Because communities are structured by the interactions among species, indirect interactions (i.e., effects of one species on another mediated by a third) are likely to play a major role in determining community composition. Through indirect interactions with plants, herbivores can have large effects on community composition by creating habitats and conditions to which other species respond. For example, beaver herbivory of cottonwoods increases phytochemical defensive compounds in resprout cottonwoods that positively affect the abundance of a leaf-chewing chrysomelid beetle (Martinsen et al. 1998). Herbivores can create these habitats or conditions by modifying plant architecture (Nakamura and Ohgushi 2003), secondary chemistry (Karban and Baldwin 1997), plant species composition (Johnston and Naiman 1990, Chadde and Kay 1991), building of structures (Cappuccino 1993, Jones et al. 1994, Dickson and Whitham 1996, Martinsen et al. 2000, Bailey and Whitham 2003), changes to the spatial distribution of habitat (Chadde and Kay 1991), or some combination of these effects, any of which can influence community composition. When herbivores are dominant species, keystone species (Hunter 1992) and/or ecosystem engineers, they can have strong positive or negative effects on associated species (Jones et al. 1997, Wimp and Whitham 2001, Bailey and Whitham 2002). Hereafter, we refer to such organisms as species of large effect, i.e., species which create ecological conditions to which other species respond resulting in a change in community composition.