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Mount St. Helens
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(FAQ 8) How were amphibians affected by the eruption?
Amphibians were thought to be very sensitive to environmental change and, therefore, scientists expected to find most amphibians in the volcanic disturbance zonesdead after the 1980 eruption. Scientists arriving shortly after the eruption were surprised to find most of the 15 species of frogs, toads, salamanders, and newts had actually survived in many locations throughout the blast area. The scientists determined that all the surviving species were associated with water for some portion of their life history (egg, larval, or adult stages), whereas the species not surviving lived their entire lives on land. The 1980 eruption was on May 18, which is late winter in the Cascade Range. Most lakes around Mount St. Helens were still frozen and much of the high-elevation country was covered with snow, two factors which protected many amphibians. In addition, some amphibians spend most of their life beneath the ground where soil protects them.
However, an important question remained: Would the amphibians continue to survive in this dramatically altered land? Over the next few years, scientists learned that amphibian survival depended strongly on the habitat.
Eruption’s effects on amphibians in lakes and ponds
Amphibians using lakes and ponds were present and/or breeding in most study sites only a year after the eruption. Their eggs and larvae developed completely and they successfully metamorphosed. These species continued to do well in the ensuing years and some actually flourished. Some species, such as the northwestern salamander (Ambystoma gracile), have flourished in the posteruption landscape. Surveys conducted 15 to 20 years after the eruption showed that lake-dwelling amphibian species and the percentage of sites they occupy at Mount St. Helens were comparable to nearby undisturbed areas, such as Mount Rainier National Park in Washington and the Three Sisters Wilderness Area in Oregon.
Eruption’s effects on amphibians in streams
Amphibians associated with streams initially survived the eruption, but they died off rapidly as the streams were clogged with tremendous inputs of volcanic sediment, smothering the amphibians’ food sources. The changed streams also left the amphibians little protection from floods. Within a few years, however, the steep, swift, mountain streams flushed much of the sediment from their channels, and stream amphibians began to recover. With streamside trees and other shade plants gone, sunlight fueled exceptional growth of algae, the primary food of some amphibian larvae, and tailed frogs (Ascaphus truei) multiplied rapidly.
Eruption’s effects on amphibians in seeps
Amphibians associated with seeps survived and persisted after the eruption because their habitats were either protected from the eruption by topography or were minimally impacted because the volcanic sediment deposited on these steep habitats was rapidly removed by gravity and water. These species are among the most sensitive amphibian species in North America to environmental changes such as increased temperatures, making their survival in seeps around Mount St. Helens particularly surprising.
Eruption's effects on amphibians in ponds in the debris avalanche zone
All amphibians perished in the debris avalanche and pyroclastic flow zones. The debris avalanche left a landscape of hummocks, and small ponds formed in the low spots. Because of this new topography, the number of lakes and ponds in the area increased fivefold, from 33 before the eruption to 163 afterward.
Scientists found this event an outstanding opportunity to study the pace and pattern at which amphibians colonize newly created habitat. Amphibians began to colonize the new ponds within 1 year after the eruption. Four species of frogs and toads colonized first, followed by one salamander species and one newt species. Some of these early colonizers traversed impressive distances (sometimes over 2 miles) across barren, ash-covered ground to reach the ponds. By 1990, six amphibian species lived in the ponds, and these species continue to live in the ponds (as of 2005).
Eruption’s effects on non-pond-dwelling amphibians in the debris avalanche and pyroclastic flow zones
Scientists found that even by 2005, a full 25 years after the 1980 eruption, amphibian species associated with streams, seeps, and land had still not colonized the debris avalanche deposit outside the ponds and the pyroclastic flow zone. These species probably remain absent because of dispersal barriers (either distance or harsh terrain) or lack of habitat. These amphibian species may not be able to colonize these zones until forests develop.
US Forest Service - Pacific Northwest Research Station - Mount St. Helens