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

James W. Haefner

Committee

Christopher A. Call

Committee

Frank J. Messina

Committee

Edward W. Evans

Abstract

The primary goal of this study was to document spatial variation in the interactions between a granivore, the western harvester ant (Pogonomyrmex occidentalis), and its seed resources in the semiarid sagebrush-steppe of southwestern Wyoming. A major focus of this research was the effect of foraging trail use by P. occidentalis on spatial patterns of seed predation. Three seed removal experiments, soil seed sampling and an experiment designed to test the role of visual landmarks on foraging trail formation were the major elements of this investigation. In the first two seed experiments, seed removal rates, from both high- and low-density seed patches, were higher near foraging trails than away from them. However, there was considerable variation in seed removal patterns among individual harvester ant colonies. In the third seed experiment, seed predation was neither significantly affected by the distance of seeds from a nest nor by the microenvironrnent (undershrub or intershrub space) in which the seeds resided. Rates of seed removal did not vary at the largest spatial scale considered (subplot). However, rates of seed predation within subplots varied between years. The most important factor affecting seed removal rates was the location of seeds within a subplot. Soil seed sampling within the foraging areas of harvester ant colonies over 2 yr revealed several significant sources of variation in seed densities. Seed densities varied at the smallest spatial scale sampled, microhabitat (undershrub vs. intershrub space), and at the largest spatial scale sampled, colony foraging area. Seed abundance also varied temporally (among months). Harvester ants affected patterns of seed density in two ways. First, seed numbers were higher in interspace areas away from foraging trails than in interspace areas near them, indicating that trails restrict colony search effort. Second, seed numbers were higher near harvester ant nests than in surrounding areas. This difference was attributable to higher seed densities of the alien annual grass, Bromus tectorum, and suggests that harvester ants indirectly facilitate B. tectorum near their nests. In the final experiment, the addition of visual landmarks (shrub mimics) near nests did not substantially alter patterns of colony trail placement. These results suggest that the use of chemical or other orientation cues may have prevented ants from responding to shrub mimics. I conclude that seed removal by harvester ants can have important implications for some components of the shrub-steppe plant community. In particular, high spatial variation in rates of seed removal is a potentially important, though unexplored, mechanism by which harvester ants could promote coexistence among plant species.

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