Spider Community Composition and Structure In A Shrub-Steppe Ecosystem: The Effects of Prey Availability and Shrub Architecture

Habitat structure is of great importance for the distribution and abundance of various organisms. Spiders are especially sensitive to structural features of their environment. Although spiders are influenced by habitat structure, it remains unclear whether spiders respond to architecture or to differences in prey availability associated with different architectures. Here, we investigated the effects of shrub architecture and prey availability on a spider community in a shrub-steppe environment in northern Utah, USA. Big sagebrush shrubs were randomly assigned to six experimental treatments: two levels of prey attractant (shrubs were either baited or not baited) and three levels of foliage density (low, natural/control, or high). We found that spider abundance and species richness were affected by both prey availability and shrub architecture, while variation in spider species diversity (Shannon-Wiener index) was governed by changes in shrub architecture alone. Spider species and family compositions were also associated with changes in shrub architecture, although guild composition was not. We discuss the implications and limitations of these findings and present suggestions for future research. 14 Ecologists have long been interested in patterns of community structure and the mechanisms that generate these patterns (Hutchinson 1959; Putman 1994). Community structure is the result of interactions among many factors, making it difficult to assess the relative contribution and importance of any one factor (Uetz 1991). Clearly, if we are to understand and manage communities, there is a need to disentangle the different ecological factors that shape their composition. Habitat structure, defined as the physical composition and arrangement of objects in space and time, is one of several factors considered important in influencing the distribution and abundance of animals (McCoy & Bell 1991). Structurally complex habitats provide animals with a wider array of microhabitats (Brandt & Lubin 1998), more diverse ways of exploiting food resources (Brandt & Lubin 1998; Tews et al. 2004), amelioration of climatic extremes (Larmuth 1979), protection from predators (Gunnarsson 1996; Langellotto & Denno 2004) and, for some predators, more effective ways to locate and capture prey (Langellotto & Denno 2004). Habitat structure influences a variety of organisms, including birds (MacArthur & MacArthur 1961; Vander Wall & MacMahon 1984), lizards (Pianka 1966), some rodents (Rosenzweig & Winakur 1969; Parmenter & MacMahon 1983), and various invertebrates (Murdoch et al. 1972; Southwood et al. 1979; Lawton 1983; Parmenter et al. 1989), including spiders (Uetz 1991; Wise 1993). Spiders are influenced by several structural attributes of the environment, including vegetation density, height, and orientation (Hatley & MacMahon 1980; Brown 1981; Abraham 1983; Rypstra & Carter 1995; Brierton et al. 2003), as well as 15 interactions among variables such as branch height and orientation (Heikkinen & MacMahon 2004). Spiders may even distinguish between different branch types with some spiders being more common on reproductive than on vegetative branches (de Souza & Martins 2004; de Souza & Modena 2004). Although spider communities differ with changes in habitat architecture, it remains unclear whether spiders are responding to architecture per se or to differences in prey availability caused by different architectures. While some studies suggest that prey availability is important in understanding patterns of spider community structure (Riechert 1974; Spiller 1992; Bogya et al. 2000; Horvath et al. 2005), others emphasize that prey availability is of lesser importance and that spider communities are shaped primarily by habitat structure (Rypstra 1983; Greenstone 1984; Bradley 1993; Halaj et al. 1998, 2000; Nyffeler & Sunderland 2003; Langellotto & Denno 2004; Chan et al. 2009). These findings highlight the need to further evaluate the processes responsible for structuring spider communities. Our goal for this study was to investigate the relative importance of prey availability and shrub architecture in determining the composition of a well-studied spider community in a shrub-steppe environment in northern Utah, USA. Spiders are model organisms for addressing ecological studies. They are ubiquitous, locally abundant, taxonomically diverse, and amenable to experimental manipulations (Hatley & MacMahon 1980; Uetz 1992; Wise 1993; Foelix 2011). Spiders are especially wellsuited for investigating the effect of shrub architecture on community organization because, as carnivores, they are not directly reliant on a particular plant species as a food 16 source (Colebourn 1974; Hatley & MacMahon 1980) and, for web-builders, the building of a web often requires specific substrates for attachment (Turnbull 1973; Riechert & Gillespie 1986; Uetz 1991).