Mount St. Helens Research
On May 18, 1980, after weeks of tremors, Mount St. Helens
erupted spectacularly and profoundly changed a vast area
surrounding the volcano. In the 30 years since the catastrophic
eruption, scientists with the Land and Watershed Management
Program, along with their colleagues, have used the volcano
as a living laboratory for ecological research.
|Links: US Forest Service: Mount St. Helens|
The May 18, 1980, eruption of Mount St. Helens dramatically transformed forests, meadows, lakes, and streams within a vast portion of the Cascade Range in southern Washington. Within days, scientists were on the scene and have remained, documenting the process of ecosystem reassembly. The eruption created exemplary opportunities to learn how plans and animals initially respond to large, intense disturbance and the longer term process of succession.
Geomorphic Change and Vegetation Development
Secondary disturbances such as small landslides and lateral shifting of river channels created a complex landscape mosaic in the decades after the initial 1980 eruption. The debris avalanche that flowed down the north fork of the Toutle River dammed tributary streams, leading to the formation of two new lakes and about 150 ponds. The terrestrial communities in the blast area still have not reached the same levels of productivity that they had before the eruption; it will likely take centuries for forest structure and plant and animal communities that resemble the pre-eruption forest to develop within the blast area.
Figure 1. South Fork Toutle River in the Herrington Flats area in 1981. Photo inset shows the lower section of Herrington Creek where the stream cut a new channel through the mudflow terrace.
Willow Stem-Boring Beetle
The willow stem-boring beetle was one of few nonnative colonizing species that appeared on the landscape after the eruption and, today, continues to influence succession. First documented in 1989, it lays eggs on willow stems, which its developing larvae weaken or kill through their boring action. Once the beetle exploits most of the willow stems in a particular patch, it moves onto the next suitable patch and reinitiates the process. The willows in the exploited patch will rebound until their shoots attain a certain size and become vulnerable to the beetles again, at which point the process begins anew. The net effect is a chronic reset in the developing woody plant cover and the associated animal and plant communities.
Figure 2. The willow stem-boring beetle annually kills a large percentage of willows on the Pumice Plain and is thus influencing development of other plant and animal communities.
Amphibian and Fish Responses
Scientists arriving shortly after the 1980 eruption were surprised to find that most frogs, toads, salamanders, and newts had actually survived in many locations throughout the blast area. The scientists determined that all of the surviving species were associated with water for some portion of their life history, whereas those that lived their entire lives on land perished. Amphibian survival depended strongly on the habitat. The eruption devastated some water bodies with fish and hardly changed others; thus, fish survival and recovery had very different patterns in the various bodies of water across the disturbance zones. Overall, populations have rebounded remarkably in the 30 years since the eruption, owing mainly to the amount of sunlight reaching streams, which enriched the food supply. Fish are now thriving in many bodies of water.
Figure 3. Pacific tree frogs began colonizing newly formed ponds 1 year after the eruption.
Changes in Spirit Lake
As the closest and largest lake adjacent to the volcano, Spirit Lake underwent extensive changes during the 1980 eruption. It was transformed from a relatively pristine cold-water mountain lake to a larger, shallower lake containing a warm microbial broth in which no air-breathing organisms survived, including fish. The lake gradually returned to conditions that supported flora and fauna; today, it is inhabited by an extremely large rainbow trout population that has exceptional growth rates and unusual life histories.
Figure 4. Fish biologists weigh and measure this healthy rainbow trout, caught from Spirit Lake. It took fish over a decade after the eruption to return to Spirit Lake, but fish are now thriving in the lake.
Small Mammal Responses
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.
Figure 5. For about 12 years after the eruption, the deer mouse was the dominant mammal on the pumice plain. The deer mouse also survived in the blowdown zone.
Bird survival during the 1980 eruption depended on the distance of birds from the volcano and disturbance zone. All birds died throughout the entire blast area and in areas crushed by the debris avalanche. In contrast, many birds outside the blast area but in the path of mudflows likely fled to safety, and birds in tephra-fall areas were temporarily displaced. The power of flight gives birds tremendous ability to move freely, and scientists observed some birds flying into the blast area within days after the eruption. After the eruption, the pattern of bird colonization was strongly influenced by habitat structure and complexity, which differed substantially across the disturbance zones.
Figure 6. The song sparrow nested in shrubs in the blowdown and scorch zones. Four speckled eggs are incubating in the song sparrow nest shown here.