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What have scientists learned since Mount St. Helens erupted?

09.12.2004


When Mount St. Helen’s blew its top in 1980, Charlie Crisafulli was 22 years old and just beginning his career as a research ecologist. One of his first assignments: travel to Mount St. Helens 2 months after the historic eruption and study the aftermath.

Crisafulli and his colleagues traveled by helicopter into the volcanic disturbance zones to gather ecological data. Crisafulli, a scientist at the USDA Forest Service’s Pacific Northwest Research Station, spent the next 25 years analyzing data that enabled him to produce long-term data sets to use to study the ecological patterns and processes of species survival and colonization. He observed the development of ecological relationships across the volcanic landscape. Fast forward to October 2, 2004.

“ We flew a boat in by helicopter to Spirit Lake just 2 days before the October 4, 2004, eruptions to characterize the chemical, physical, and biological conditions of the lake,” says Crisafulli. “We surveyed more than 100 ponds for amphibians and aquatic invertebrates, and live-small mammals at 14 sites right in front of the crater. Unlike in 1980, when we had very little pre-eruption data, we now have a broad network of established plots and lots of data on hundreds of aquatic and terrestrial species—from microscopic-aquatic plants, to insects to mammals. This information is important baseline data for assessing future disturbance from the volcano.”



He says that arriving on the scene within hours to days after a disturbance event is paramount for understanding both initial disturbance severity and long-term biological responses. Of particular importance is documenting the types, amounts, and distribution of surviving organisms.

Scientists learned, explains Crisafulli, that traditional theories of disturbance and the succession processes were often inadequate for explaining ecological responses to the 1980 eruption of Mount St. Helens. Some of the findings include:

  • Disturbance gradients ranged from areas where all life perished, to zones with nearly complete survival. The eruption involved several volcanic disturbances—debris avalanche, lateral blast, mudflows, pyroclastic flow, and tephra-fall—that interacted with a very diverse pre-eruption landscape to create a complex mosaic of disturbance across several hundred square miles.

  • Chance events, such as timing of a disturbance, greatly determine the extent of environmental change. The 1980 eruption occurred on a spring morning. Had it happened in late summer and at night, more plants and animals would have perished. “Events such as the season, and even the time of day, strongly influence survivorship and recovery,” Crisafulli says. “If it had been late summer, there would not have been late-lying patches of snow to protect subalpine vegetation, or ice covering lakes, which shielded aquatic organisms from lethal volcanic forces. If it had been at night, the nocturnal animals would have been exposed to lethal volcanic forces. Instead, they had returned to the safety of their subterranean burrows.”

  • Species life history characteristics allowed some species populations to avoid the volcanic impacts by being away from the area. For instance, some Pacific salmon and steelhead trout were at sea when the eruption occurred. When they returned to Mount St. Helens to spawn in the years after the eruption, stream conditions had improved. Many migrant songbirds were still on their winter grounds in Mexico and Central America when the eruption occurred, and had not yet returned to their summer nesting grounds at Mount St. Helens.

  • Biological legacies—living and dead—accelerated recovery. Surviving plants, fungi, and animals were very important because they challenged the theory of the importance of dispersal from distant source populations that can prolong or limit colonization. These legacies serve many ecological functions such as foraging and nesting substrate and prey and predators, and facilitated the colonization of invading species. When survivors were abundant, complex communities developed rapidly.

  • Lakes, streams, and forests responded at different rates after the 1980 eruption. Of key importance was the extent to which ecological systems became nutrient enriched or impoverished. Lakes became grossly enriched and responded rapidly. In sharp contrast, terrestrial systems experienced diminished production as vegetation was killed and the landscape was covered with nutrient-poor volcanic material. Within 6 years of the eruption, most lakes had returned to conditions typical of the undisturbed Cascade lakes. Whereas, by 2004, terrestrial systems had increased productivity, but remained far below that of a mature forest.

“ Even in areas where all life had perished, small-oasis habitats developed within a few years of the 1980 eruption,” says Crisafulli. “Plants became established around small springs that promoted the colonization of many small mammals, birds, amphibians, and insects. Although embedded within a vast, barren terrain, oases habitats sustained many colonizing organisms, eliminating the need for dispersal corridors between source populations and newly created habitat patches. These oases habitats were of tremendous importance to the overall biodiversity of the larger landscape, they contributed much of the biodiversity.

“ St. Helens has had a very active eruptive past, and if history provides any indication of future eruptive potential, we may well expect Mount St. Helens to cause additional disturbances to the surrounding landscape during this next century,” Crisafulli explains. “Since 1980, the volcano has made a great template to study disturbance. More than 100 physical and life scientists have studied the 1980 eruption and subsequent ecological responses. And these studies continue to expand our knowledge and long-term database.”

Crisafulli and his colleagues have a book due to be published by Springer Verlag in May 2005. Ecological Responses to the 1980 Eruption of Mount St. Helens is a synthesize of findings from the past two decades. A writers’ workshop, “The meaning of Mount St. Helens,” is also scheduled to be held in Portland, Ore., in 2005.

Sherri Richardson Dodge | EurekAlert!
Further information:
http://www.fs.fed.us/pnw/news/2004/12/sthelen.shtml
http://www.fs.fed.us

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