The relationships between vegetation architecture and spider community attributes were examined in a big sage (Artemisia tridentata) community. Spiders were separated into guilds using similarities of species' hunting behavior. Shrub architecture was experimentally manipulated in the field by either clipping 50% of a shrub's foliage to decrease foliage density or tying together a shrub's branches to increase foliage density. Temporal patterns of spider species density, diversity (H') and evenness (J') showed midsummer peaks in both 1974 and 1975. Seasonal spider guild trends reflected the temporal prominence of a member species or genus. These temporally abundant species appeared to play a major functional role in this community. Shrub perturbations resulted in changes in spider species and guild densities. Spider species and guild density in the tied shrubs were significantly higher than that in the clipped or control shrubs sampled. Spider species diversity, density and guild density were also positively correlated with indicators of shrub volume and shrub foliage diversity. This suggests that structurally more complex shrubs (tied) can support greater spider species densities and diversity. Spider guild densities and IV's were significantly altered by changes of shrub architecture. The observed guild distribution were in agreement with known hunting behavior and life history data of the member species. The data suggest that architectural properties of habitat may be an important determinant of predatory invertebrate species diversity and distribution. Guild analysis may be useful in examining the roles of species groups in community studies.

Changes to vegetation architecture within a natural habitat can have profound impacts upon ecological community function, but the relative influence of vegetation architecture itself and potential indirect influences of associated food resources are often difficult to disentangle. I present the results of a three-year study designed to address the community level impacts of changes in big sagebrush (Artemisia tridentata) architecture and associated prey availability upon the ground-dwelling arthropod community. Three experimental sagebrush architecture treatments (low, intermediate, and high foliage density) and two levels of prey insect availability treatment (natural and baited/increased prey availability) were imposed in a sage-steppe ecosystem. The ground-dwelling spider (top predators) and insect (prey) response to all combinations of the six treatments were assessed through abundances in pitfall traps. Chapter 2 demonstrates that changes in a single shrub’s architecture did not markedly impact prey arthropod availability on the ground, though an overall increase in arthropod abundance was detected within baited shrubs. This indicates changes to vegetation architecture do not impact prey insect availability for ground-dwelling spiders. Thus, changes in shrub architecture and not the associated prey base directly alter the ground-dwelling spider community. Chapter 3 shows that architectural manipulation of a single sagebrush directly influences the ability of certain ground-dwelling spider guilds to persist beneath the shrub. Data within Chapters 2 and 3 support the conclusion that vegetation architecture directly affects ground-dwelling spider community structure and composition. Finally, Chapter 4 explores how the relative impacts of vegetation architecture and prey availability upon ecological communities can shift depending upon the spatial context in which the manipulations take place, given the differences in mobility of species within the grounddwelling spider and insect prey communities. Within this study, Diurnal and Nocturnal Wanderers responded to manipulations in prey availability at a larger spatial context (patches of 15 shrubs) while more stationary Trappers and Ambushers did not. Through simultaneous manipulation of vegetation architecture (Artemisia tridentata, big sagebrush) and associated insect prey base in different spatial contexts, this dissertation demonstrates that vegetation architecture directly affects ground-dwelling spider community organization at the species and guild levels of diversity. Shifts in spider (predator) functional diversity can ultimately impact arthropod decomposer and herbivore populations, influencing fundamental ecosystem processes such as decomposition and productivity.