"But it wasn't just one event," Dale said. "You had pyroclastic flow"a mix of hot gas and ash that speeds down a volcano's slope"debris avalanche, and heavy ash deposits. And each of these events had a different pattern of survival and then recovery."
Traditional thinking on ecological successionthe pattern by which life-forms come back over timeheld that first, certain species would recover, and then others would, along a particular order. For instance, grasses might appear, followed by rabbits and other animals that eat grasses.
But the reality at Mount St. Helens has proved far more complex. Ecological succession has proceeded at different speeds in different areas. Succession proved to be highly unpredictable.
"Chance factors, such as what time of day or year the eruption occurs, can strongly influence survival and the course of succession," Dale said.
For example, the eruption occurred in May, when some of the mountain was still covered in snow. The snow proved to have a strong influence on plant survival. Also, many of the salmon that inhabit local rivers were out to sea when the volcano eruptedas they are every May.
Many other animals also did better than expected.
Twelve of the 15 amphibian species found in the Cascade mountains survived in many locations. Today amphibians thrive even in areas where no amphibians survived the eruption. Not long after the blast, frog, toad, salamander, and newt species traveled several miles of inhospitable ground and moved into the affected habitats.
The recovery of Spirit Lake was especially impressive. Located on the northern side of Mount St. Helens, the lake had been displaced by an enormous avalanche of rock and superhot gases, and covered with floating trees that had been ripped from the surrounding forest.
But instead of taking decades to recover, Spirit Lake underwent a complex set of biological, chemical, and physical changes. Within about six years the lake looked almost undisturbed.
Frederick Swanson is a geologist with the U.S. Department of Agriculture's Forest Service in Corvallis, Oregon. He studied how the physical landscape around St. Helens functioned as a stage on which ecological processes played out.
"We found that there was a succession of geological processes," Swanson said.
"Just as we think of ecological successionwhere some plants come into a bare area and prepare the site for the occupancy by other plants and animalsso too in the geophysical world did we find a succession of processes paving the way for each other," he said.
For example, in some areas falling debris removed the top layer of the land, liberating buried plant parts. "We usually think of erosion as bad, but in this particular set of circumstances it actually helped the ecological recovery," Swanson said.
The scientists agree that Mount St. Helens, with its easy accessibility and close proximity to research institutions, has been the perfect laboratory.
"It has been an incredible place to study and learn because of the great diversity of things that happened. And we could come in and observe them immediately after they happened and in some cases as they occurred," Swanson said. "Our understanding of volcanic processes [among other things] has been greatly enhanced."
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