Mount St. Helens

Key Research Findings

Natural disturbances such as eruptions, floods, fires, and earthquakes are heterogeneous events, meaning that the disturbance creates a complex mosaic of disturbed areas and effects are not evenly distributed. The May 18, 1980, eruption of Mount St. Helens involved several distinct large disturbances—a huge debris avalanche, an explosive blast out the mountain’s north side, mudflows, hurricane-force winds of hot gases, and ejected volcanic rock and ash (tephra). These events interacted with a diverse landscape to create a complex mosaic of disturbance zones covering several hundred square miles. The severity of disturbance ranged from areas where all life perished to zones with nearly complete survival.

Chance events, such as the timing of a disturbance, greatly determine the extent of environmental change. For Mount St. Helens, the season and time of day strongly influenced survival and recovery. The 1980 eruption occurred on a spring morning; plant buds had not yet opened, patches of snow and ice protected some organisms, and nocturnal animals had returned to their underground burrows. If it had happened on a summer night, more plants and animals would have perished.

Life history characteristics of species are an important factor in survival rates. In the case of Mount St. Helens, some Pacific salmon and steelhead trout were at sea when the eruption occurred. When they returned to mountain streams to spawn in the years after the eruption, stream conditions had improved. Many migrant songbirds had not yet returned to their summer nesting grounds at Mount St. Helens when the eruption occurred, so these birds escaped the immediate effects.

The Mount St. Helens eruption had many specific mechanisms that also occur in other types of disturbances: the heat was comparable to wildfires, wind blast comparable to hurricanes, mudflows comparable to rain-caused mudflows, wave surge in Spirit Lake comparable to tsunamis, and so forth. Thus the findings on ecological responses at Mount St. Helens have broad relevance to ecological responses to other types of disturbances.

Living and dead organisms left after the eruption, termed “biological legacies,” accelerated recovery at Mount St. Helens. Surviving plants, fungi, and animals served many ecological functions—plants provided forage and shelter, animals were prey and predators, and so forth. Dead organisms provided significant amounts of nutrients. Biological legacies made it much easier for species to colonize the landscape and in areas with many survivors, complex biological communities developed rapidly. Mount St. Helens showed that even in a radically disturbed environment, organisms can survive and become source populations for colonizing the disturbed area. This finding challenged the theory that colonization comes primarily from outside the disturbed area.

The biological response to the spectacular 1980 eruption was rapid, with the most important factors being the biological legacies, the diverse source populations surrounding the blast area, the presence of unconsolidated volcanic deposits in which animals could burrow and plants could take root, and a moist climate with plenty of rain and snow encouraging plant growth.

At Mount St. Helens, erosion cut through the new volcanic deposits and exposed soil where plants could sprout. Thus in this disturbance, erosion was a positive process for plants, improving habitat.

Lakes, streams, and forests responded at different rates after the 1980 eruption. A key factor for response rate was the extent to which ecosystems became nutrient-enriched or impoverished. Lakes were greatly enriched with nutrients, and life in lakes multiplied rapidly. Within 6 years after the 1980 eruption, most lakes had returned to conditions typical of undisturbed Cascade Range lakes. In sharp contrast, terrestrial ecosystems, covered with nutrient-poor volcanic ash and rock, had greatly diminished biological productivity after the 1980 eruption. Although terrestrial ecosystems increased their biological productivity by 2004, their productivity was still far below that of a mature forest.

Disturbance eliminates or reduces the amount of many habitats, but it can also create new habitats. At Mount St. Helens, about 90 square miles of forest habitat were lost because of the 1980 eruption, but the amount of lake and pond habitat increased fivefold. These new habitats were quickly colonized by a great diversity of aquatic life, such as amphibians, insects, plankton, and plants. Many of these new ponds are among the most productive ecosystems, terrestrial or aquatic, at Mount St. Helens.

In the Mount St. Helens National Volcanic Monument, where natural processes have been allowed to take place since 1980, the biological communities that have developed are highly varied with respect to species diversity, composition, and structure. Chance and contingencies have strongly influenced the rates and patterns at which these communities developed. These naturally recovering herb and shrub communities are very different ecologically from the highly managed stands of young conifers growing on land outside the national monument but within the blast area. The naturally recovering communities may play an important role in the regional biodiversity of the Pacific Northwest.

Human actions taken to protect life, property, or commerce influenced the patterns and rates of ecological response at Mount St. Helens. The most significant actions ecologically were engineering projects to reduce hydrologic and sediment hazards, fish stocking in lakes and streams, salvage logging of blowdown trees, and creation of even-aged, single-species, conifer plantations (the last two actions occurred outside the national monument).

Twenty-five years after the 1980 eruption, the landscape at Mount St. Helens is a patchwork of biological hot spots and cold spots embedded within a larger landscape of intermediate biological diversity. Biological cold spots include areas that are episodically or chronically disturbed by erosion, landslides, or animal burrowing. Biological hot spots are areas of high biodiversity that developed around pockets of survivors or around places such as seeps and springs where moisture was available and plants grew. Although these hot spots, or “oasis” habitats compose less than one percent of the total landscape they contribute much of the total biodiversity. Many animals have colonized these isolated habitat patches. This finding calls into question the necessity of dispersal corridors for connecting source populations with newly created habitat patches.

USDA Forest Service - Pacific Northwest Research Station - Mount St. Helens
Last Modified: Monday, 18 April 2005 at 13:47:12 EDT