Date of Award:

5-1-1994

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

Edward W. Evans

Committee

Fred D. Provenza

Abstract

This study examined the extent to which northern pocket gophers (Thomomys talpoides: Geomyidae) influence an alpine tundra ecosystem on Niwot Ridge, Colorado. Experiments and observations were designed to determine (1) whether gopher density and associated disturbance was proportional throughout vegetation types on the site; and (2) whether the plants that colonize old gopher mounds are determined by the immediately adjacent plants, or by plant species that characterize disturbed alpine tundra. To quantify pocket gopher disturbance on the site, three 1-ha study plots were established. On each plot, the percentage of each major vegetation type was determined, all active and old (created prior to 1990) mounds were marked and measured for volume, and all gophers were live-trapped and banded. Each mound was matched with the gopher that produced it, and the vegetation type in which each occurred was determined. To investigate differences in plant communities on inactive mounds and on adjacent, undisturbed areas, paired plots were used to measure species composition and cover. Two age groups of mounds (one-year-old and "old") were sampled. Lastly, the soil seed banks of new disturbances and of the same mounds one year later were compared. Pocket gophers disturb vegetation disproportionately to each type's occurrence on Niwot Ridge. Most disturbance occurred in the vegetation types snowbed and gopher garden. Plant community differences between gopher disturbed and undisturbed areas exist for all types (dry meadow, gopher garden, moist meadow, snowbed) on old mounds, but are much more distinct in two types, dry meadow and gopher garden. On one-year-old mounds only these two types exhibit clear differences. Few seeds were recovered from the soil samples; differences in species composition over time were evident for only the two most common species, and for all species together. The vegetation types gophers disturb depend on which portion of the ridge the animals inhabit; disturbance is concentrated in snowbeds on the lower sites (plot 1), and is exclusive! y in gopher garden on the drier ridgetop (plot 2). Both mound location (ridgetop or lower areas) and the surrounding vegetation type appear to influence which plant species colonize inactive mounds.

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