Date of Award:

5-1-1986

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

Keith L. Dixon

Committee

James A. Gessaman

Committee

Ivan G. Palmblad

Committee

Frederic H. Wagner

Abstract

Patterns of survival and recovery of small mammal populations were studied from August 1981 through October 1984, following the 18 May 1980 eruption of Mount St. Helens, Washington. Eight research plots were established, representing a matrix of two initial site conditions (forested and clearcut) by three levels of volcanic disturbance (control, heavy tephra, and blowdown). Other scientists contributed data from the level of heaviest disturbance, the blast zone. Small mammals were trapped, percent cover of vegetation and downed woody debris was measured, and food resource availability was sampled. By defining residuals (or their offspring) as all animals trapped in 1980-1982, the following minimal estimates of residual small mammal species richness were derived: two species in the blast zone (11% survival of all species potentially present), four-six species in blowdown clearcuts (24-35% survival), five in blowdown forests (45% survival), 10 in heavy tephra clearcuts (59% survival) and four in heavy tephra forests (36% survival). Deer mice (Peromyscus maniculatus) and dusky shrews (Sorex monticolus) were the most abundant residuals in the blowdown, although some typically forest-dwelling species such as redbacked voles (Clethrionomys gapperi) and shrew-moles (Neurotrichus gibbsii) survived in localized microenvironments in a few locations. In 1982-1984, blowdown forest and clearcut plots were colonized by early seral species of moderate to high vagility. Recolonizers on heavy tephra plots were difficult to distinguish from low density residuals. Deer mice were trapped in every habitat; population numbers were highest on early seral plots. Deer mouse numbers fluctuated dramatically from year to year, probably because of the influence of winter and spring weather on survival and reproduction. Weather probably also influenced breeding season length, which varied among plots and years. Red-backed voles were trapped in most zones of disturbance; highest numbers were recorded on the control forest and heavy tephra forest plots. Blowdown forest plots were colonized by several typically early seral species, including yellow pine chipmunks (Eutamias amoenus), Cascades golden-mantled ground squirrels (Spermophilus saturatus), long-tailed voles (Microtus longicaudus), and heather voles (Phenacomys intermedius). Although the qualitative effects of the Mount St. Helens eruption small mammal populations were similar to those observed for other volcanoes and other natural and anthropogenic disturbances, quantitative effects were unique.

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