ORGANIZATION OF SPIDER ASSEMBLAGES ON SHRUBS - AN ASSESSMENT OF THE ROLE OF DISPERSAL MODE IN COLONIZATION

Spiders use two distinct dispersal modes to colonize habitats: long-di stance, passive, aerial dispersal (''ballooning'') and local, active, ground dispersal. Tests were performed at a sagebrush-dominated site i n northern Utah to evaluate the influence of each process on the struc ture of spider assemblages on shrubs. Exclosures were designed that pr evented ground colonization to individual shrubs while allowing aerial colonization to occur. Other (reference) shrubs were left open to bot h dispersal processes. Spider assemblages from each treatment were com pared at two spatial scales (shrub and plot) and across three differen t levels of organization (individuals, species and guilds). At the shr ub scale, no significant differences between treatments were observed at any level. At the plot scale, an average of 74% of the individuals on reference shrubs arrived by aerial means. Ground colonists, though few in number, increased species richness and diversity on reference s hrubs at plot scale, yet the two treatments developed similar guild st ructures. Dispersal mode appears to be less important in structuring s hrub spider assemblages than other factors including spider-spider int eractions at the guild level.