INITIAL TESTS FOR PRIORITY EFFECTS AMONG SPIDERS THAT CO-OCCUR ON SAGEBRUSH SHRUBS

Recent work in conservation biology and restoration ecology has highli ghted the need for research on the process of community assembly and t he effect of initial conditions on community development. Theory and l imited experimental work in this area suggest that an initial ''pionee r'' colonist arriving in open habitat can strongly influence this proc ess, resulting in a priority effect. We used a ubiquitous terrestrial animal group, spiders, to test for the existence of priority effects d uring colonization of individual sagebrush shrubs. In 1992, at a site in northern Utah, we applied three treatments to subsets of 60 cleared shrubs that represented available habitat to spiders. Two shrub treat ments received different jumping spider pioneer colonists placed by ha nd (either Metaphidippus aeneolus (Curtis 1892) or Phidippus johnsoni (Peckham & Peckham 1883)), and a third shrub treatment received no pla ced spiders, serving as a reference. After 3-4 days of exposure to the same environmental conditions, including natural colonization by disp ersing spiders, we collected a total of 285 spider assemblages that ha d developed on shrubs. We compared these assemblages by treatment type at both the species and guild levels, defining spider guilds based on differences in morphology and foraging technique (e.g., jumpers, trap pers, ambushers, and pursuers). The total number of spiders per shrub was not significantly different by treatment type (P = 0.279), and ove rall measures of species richness and abundance were similar. At the g uild level of analysis, however, differences were observed. Total coun ts of trappers were 43-50% lower in treatments receiving a placed jump er pioneer. A log-linear model comparing treatments as a whole confirm ed that jumper pioneers significantly reduced trapper numbers in subse quent assemblages compared to those from reference shrubs (P = 0.019), and significantly fewer trappers were collected from shrubs that had Metaphidippus aeneolus as a pioneer (P = 0.034). This evidence of shor t-term priority effects was found despite a conservative aspect of our test, in which the reference shrubs had some likelihood (35%) of rece iving either of these jumper pioneers by chance from natural dispersal . It is not known whether these priority effects persist over longer t ime scales. The observed results are consistent with predictions based on known spider behaviors of cannibalism and interguild predation. Ou tcomes of these spider-spider interactions relate to differences in fo raging technique and body size. We suggest that a guild-level approach and the shrub-spider system we describe have promise for future resea rch on priority effects and animal community assembly.