Assemblages of spiders on shrubs in semiarid environments: Experimental models of shrub architecture

Assemblages of spiders on big sagebrush are dominated by salticids. The pattern could result from many salticids in the colonist pool and random distribution as spiders disperse or a selection process by which spiders are choosing to remain on big sagebrush after initial colonization. Choices may be based on insect prey on the shrubs or related to the plant's physical attributes.; Habitat structure has been associated with the distribution of many animals, and plant architecture influences the distribution of spiders. Models of big sagebrush were constructed to test plant architecture variables that spiders may use as cues in deciding to stay. The models varied in branch orientation (horizontal or vertical) and height above the ground (0, 10, or 40 cm). A third variable was guild membership, based on hunting strategy (jumpers, pursuers, ambushers, or trappers), which was used to characterize spider assemblages. Five replicates of the six treatments were randomly placed in a 60-m by 50-m grid among big sagebrush in a shrub-steppe habitat and sampled at 3-week intervals from July to October in 1997 and 1998. A loglinear model was developed with spider abundance on each model type as the dependent variable and guild, height, and orientation as independent variables.; The random distribution hypothesis was tested by measuring initial colonization and comparing it to the distribution of spider guilds after 3 weeks. Different models had distinct assemblages of spiders indicating that selection processes were at work. Spiders may select big sagebrush because it is a source of readily available prey. Spider peak abundance occurred after the peak in the insect population on big sagebrush. Apparently spiders were not choosing to remain on big sagebrush because of prey.; Differences in the assemblages of spiders on the models were the result of architecture. Jumpers selected horizontal, 10-cm models and pursuers selected vertical, 0-cm models. Cob-weaving trappers were most abundant on horizontal, 0-cm models and orb-weaving spiders on vertical, 40-cm models. This study demonstrated that the interaction of two architectural variables influenced spider distribution.