Date of Award:

5-1-1993

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Biology Ecology

Committee Chair(s)

James A. MacMahon

Committee

James A. MacMahon

Committee

Frank J. Messina

Committee

Christopher A. Call

Abstract

Herbivory has been shown to be an important component in influencing vegetation composition and structure. Granivory, a particular form of herbivory, has long been of interest to ecologists. Granivore impact may depend on community type. Comparisons of granivore taxa are few in perennial shrub communities. Several experiments were conducted to assess the influences of rodents and western harvester ants on the soil seed bank in sagebrush-steppe. Rodent and/or ant exclosures were established during July 1990, at 1.3 m (at the edge of the cleared disks maintained by harvester ants) and at 6 m (in the region beyond which obvious vegetation changes do not occur) measured from the apices of five harvester ant mounds. Soil samples were taken from the exclosures in September 1991, and from open access areas monthly (August-September 1990, June-September 1991) at 1.3 m and 6 m to ascertain changes in the soil seed bank. Seed dish experiments were conducted monthly to establish a maximum seed removal rate. Rodent trapping was conducted monthly (June-September 1990-1991) to monitor changes in populations. More seed were recovered from rodent exclosures than ant exclosures. The number of seeds in seed banks changed with time and was different with distance. Rodents removed more seeds more rapidly from seed dishes than ants. Harvester ants removed more seeds from seed dishes at 1.3 m than at 6 m. Seed species composition varied with distance and with exclosure type. Rodent populations declined during the winter and increased from spring through fall. Rodents have a larger impact on the soil seed banks than western harvester ants. Ant impacts on seed banks may be limited as more area is searched for scattered resources as distance from mounds increases. Ant influences may also be limited by the number of foragers/colony and temperature-related foraging restrictions. Other harvester ant activities, such as the maintenance of a denuded disk surrounding ant mounds, soil physical and chemical characteristics modification, fungal spore concentration, and the mosaic created by the foundation and abandonment of mounds, may have more influence on vegetation than seed movement.

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