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Mount St. Helens

Mount St. Helens National Monument

Mount St. Helens
Pacific Northwest Research Station
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Portland, OR 97204

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(FAQ 6) How were small and midsize mammals affected by the eruption?


The deer mouse uses a wide variety of habitats and eats many foods, including insects as well as seeds and berries. Thus this species colonized nearly all of the disturbance zones in the blast area (photo by Charlie Crisafulli).


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.

-In zones that received only 6 inches or less of cool ash and pumice, survival was widespread and the small and midsize mammals present were typical of undisturbed sites beyond the volcanic eruption.

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.

Within the blowdown zone

The down trees, surviving plants, and colonizing vegetation provided a complex ground layer that offered abundant cover and hiding places and also produced diverse food items including seeds, insects, green plants, and roots. This area thus provided all habitat needs for a variety of ground-dwelling rodents such as the Cascade golden-mantled ground squirrel (Spermophilus saturatus), yellow-pine chipmunk (Tamias amoenus), deer mouse (Peromyscus maniculatus), and insectivores such as the montane shrew (Sorex monticolus).

These small mammals were the prey for predators like the coyote (Canis latrans), short-tailed weasel or ermine (Mustela erminea), and longtail weasel (Mustela frenata). Two midsize aquatic predators, the American mink (Mustela vison) and northern river otter (Lutra canadensis), inhabit the blowdown, debris avalanche, and pyroclastic flow zones, where the mink and otters eat crayfish, amphibians, and fish, in addition to other aquatic and terrestrial prey.

Midsize herbivorous mammals, such as the American beaver (Castor canadensis) and common porcupine (Erethizon dorsatum), colonized the blowdown zone once their forage base of bark, leaves, twigs, and buds was established.

Within the pyroclastic flow zone

On the pumice plain in the severely altered pyroclastic flow zone, none of the former forest remained after the 1980 eruption. Vegetation that developed since the 1980 eruption was generally sparse and close to the ground (except for the springs and seeps, discussed below), providing little habitat for most mammals. Here, the dominant mammal was the deer mouse. About 12 years after 1980, the northern pocket gopher (Thomomys talpoides) reached the pumice plain and became established. Gophers move primarily through their tunnel digging, which explains why the species took so long to reach the plain.

Within springs and seeps

A handful of cool springs and seeps emerged on the pumice plain, and small patches of dense, lush willow and herb plant communities developed around these wet spots. Scientists found that a diverse assemblage of 10 small mammal species, including forest insectivores and riparian habitat specialists, colonized these ecological hotspots. In fact, these small mammals, some weighing a mere 0.2 ounce, traversed up to 2 miles or more over barren terrain among these small, isolated patches of suitable habitat, reaching and colonizing all of them. Small mammals were several times more abundant in these oasis-like habitats than in the larger pumice plain area.

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.

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US Forest Service - Pacific Northwest Research Station
Last Modified: Thursday,14May2015 at13:56:27CDT

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