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Mount St. Helens
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(FAQ 6) How were small and midsize mammals affected by the eruption?
Before the 1980 eruption, the Mount St. Helens area supported about 35 small to midsize mammal species, not including bats. Although the volcano dramatically altered a vast terrain, scientists found that a surprisingly large number of these mammal species had survived in many locations.
Survival was related to the type and severity of volcanic disturbance and differed considerably across the volcanic disturbance zones.
Summary of small to midsize mammal survival by disturbance zone
- In zones where the forest was entirely blasted or scoured away and zones where the forest was buried, all mammals perished.
-In zones where trees were toppled and volcanic deposits were deep, many small and midsize mammals were killed—but, for most species, at least some individuals survived. In these zones, only a few small and midsize mammal species were destroyed completely.
Key factors that affected survival of small to midsize mammals
-Location. Small mammals that lived underground had higher survival rates than species that lived in tree canopies or on the surface of the ground.
-Timing. Mammals active at night had generally returned to the safety of their daytime burrows by the time the early morning eruption occurred.
-Season. Because the eruption was in May, with conditions still like late winter in the Cascade Range, patches of snow lingered in places, protecting animals beneath it.
-Landscape features. Ridges, rock outcroppings, and cliffs blocked or deflected the powerful volcanic forces in some places, protecting some animals.
Return of small to midsize mammals in first decade after 1980 eruption
Within 10 years of the 1980 eruption, nearly all the mammal species found in the southern Washington Cascade Range had returned to the blast area. The mammal assemblages, or groupings of species, were quite different in the various disturbance zones. Scientists attribute many of these differences to, first, the amounts and types of the pre-eruption forest components that remained after the eruption, such as fallen trees, standing dead trees, and surviving patches of vegetation; and, second, the rate at which new vegetation developed.
Small mammals that remain conspicuously absent today
Two small mammal species remained conspicuously absent even after 25 years: the northern flying squirrel (Glaucomys sabrinus), a forest canopy species, and the southern red-backed vole (Clethrionomys gapperi), a forest understory species. Given that forests have not developed yet in the blast area, it is not surprising that these species have been absent. Scientists expect that, barring any major eruptions from the very active Mount St. Helens, trees will likely continue their spread across the landscape, filling in gaps, growing taller, and developing into a mosaic of forest types. Scientists expect that as the vegetation changes, they will see shifts in the groupings of mammal species found in different zones, and in the relative dominance of species within these groupings.
US Forest Service - Pacific Northwest Research Station - Mount St. Helens