Small Mammal Recolonization on the Mount St. Helens Volcano: 1980-1987

Abstract

-In this paper we summarize small mammal studies on Mount St. Helens since its catastrophic eruption in May 1980. Species surviving the initial eruption (residuals) have been described by Andersen and MacMahon (1985a). Herein we document recolonization of small mammals [Rodentia, Insectivora, Lagomorpha and Carnivora (Mustelidae)] into representative sites of four broadly defined montane habitats (forests, clearcuts, subalpine meadows and riparian ravines) that were subjected to increasing degrees of volcanic disturbance (undisturbed, tephrafall, mud flow, tree blowdown and pyroclastic/ debris flow). We live-trapped small mammals on 19 sites (with extensive population studies on 12 sites) from 1980-1987. By 1987, of a hypothetical list of 32 resident small mammal species, six species were captured or observed in the pyroclastic/debris flow zone, 15 species in the tree blowdown zone, and 22 species in the tephrafall zone. Small mammal communities in forest and clear-cut habitats, and across different disturbance zones, exhibited little species similarity among sites, even after 7 years of posteruption recovery. Small mammal species composition in several subalpine meadows converged through time to that found on a clearcut site in a tephrafall zone. Pre-eruption conditions, such as the patchiness of habitats and small mammal populations, and stochastic events, such as (1) the initial survival of small islands of vegetation and resident mammals; (2) localized habitat alteration by posteruption erosional processes, and (3) an unusually wet summer (1983) followed by 3 yr of summer drought (1984-1986), had a major influence on small mammal recolonization in this complex mosaic of habitats. We argue that observed differences in species recolonization and establishment success were more attributable to the availability of requisite food and shelter resources than to differences in species-specific dispersal capabilities. INTRODUCTION The explosive eruption of Mount St. Helens on 18 May 1980 resulted in a large-scale destruction of montane/subalpine forest and meadow habitats and caused the local extirpation of many small mammal species (Franklin et al., 1985). At least 14 species of small mammals survived the initial eruption and concomitant habitat alterations (Andersen, 1982; Andersen and MacMahon, 1985a). Individuals of many residual species survived the volcanic blast by virtue of their fossorial habits; being in underground burrows at the time of the early morning (0832 PDT) eruption shielded them from the rapidly moving hot gasses and tephra. Although the blast virtually annihilated all exposed plants and arthropods in nearby areas N of the volcano, the existence of underground plant parts and invertebrates, combined with a limited rain of recolonizing arthropods onto the disturbed areas (Edwards et al., 1986), provided an adequate food supply for some of the surviving mammals. Localized 'Present address: Department of Biology, University of New Mexico, Albuquerque 87131 2 Present address: World Wildlife Fund, 1250 24th St. NW, Washington, D.C. 20037. 3 Present address: Mount St. Helens National Volcanic Monument Headquarters, Route 1, Box 369, Amboy, Washington 98601.