Date of Award:

5-1-1994

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Department name when degree awarded

Biology Ecology

Committee Chair(s)

James A. MacMahon

Committee

James A. MacMahon

Committee

Kimberly A. Sullivan

Committee

Ivan G. Palmblad

Committee

Thomas C. Edwards Jr.

Committee

Edward W. Evans

Abstract

This dissertation addresses deterministic and stochastic factors that are expected to influence the co-occurrence of spiders (Arachnida: Araneae) in a shrub-steppe environment. First, a field test assessed the role of substrate diameter in spider habitat selection using artificial habitat modules. Second, the effect of alternative spider dispersal modes (passive, long-distance, aerial transport versus active, local ground transport) on assemblage patterns on sagebrush (Artemisia) shrubs was evaluated using an exclosure design. Spider counts from exclosure and reference shrubs were compared at two spatial scales (shrub and plot) and among three potential levels of spider assemblage organization (individuals, species, and guilds). Third, spider assemblages from 1094 sampled shrubs were compared at three levels to both random and probabilistic models of occurrence. Finally, a field test for the existence of priority effects was made using Metaphidippus and Phidippus pioneers on shrubs. No significant contingency was found between substrate diameter and guild identity among habitat modules (P = .7502). At the shrub scale, different combinations of dispersal mechanisms did not affect spider assemblage structure at any organizational level (P > .05). 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 shrubs, yet the two treatments developed similar guild structures. Spider assemblages collected from 1094 sagebrush shrubs were nonrandom at all three levels of organization. At the species level, highly significant differences in assemblage state frequencies were detected on shrubs with two (P = .0005) and three (P = .0001) spiders, and Phidippus was found less often with Metaphidippus and Sassacus than predicted. At the guild level, for shrubs with three spiders, states involving jumpers and pursuers were more common and states involving trappers were less common than predicted (P = .0009). A priority effect was found in which jumper pioneers significantly reduced trapper numbers in subsequent assemblages compared to those from reference shrubs (P = .019). Evidence from these studies suggests that short-term stochastic influences are eclipsed by the operation of deterministic assembly rules based on negative spider-spider interactions including cannibalism, intraguild predation, and interguild predation. Outcomes of these interactions relate to spider body size differences.

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