Ecology And Environment

The Surprising Biological Succession Process after the Mt Saint Helens Eruption

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"The Surprising Biological Succession Process after the Mt Saint Helens Eruption"
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The eruption of Mt. Saint Helens on May 18, 1980 caused a disturbance to the surrounding landscape that graded from complete sterilization to remnant late successional legacies. The resulting biological succession that almost immediately started to unfold became a fascinating lesson in just how much scientists had to learn about the surprises still innate in our complex world. We always thought that it all began with the fungus, the lichen, the mosses and ferns, all the way up to a believed logical conclusion; on Mt. Saint Helens the stage was set in such a multitude of ways as to orchestrate a new and vibrant symphony. Neither the tolerance, inhibition, nor facilitation models of succession could each, by themselves, explain what has been observed over the last 27 years. Rather, the regeneration of Mt. Saint Helens was a complex blend of facilitation and inhibition each taking their turn and playing out in the life cycles of the colonizers. In the blast zone the land was scoured by the 1300 F. temperatures of a pyroclastic wave that eradicated everything in its path and spared no terrestrial or aquatic habitat. Considering the conditions in the time of its origins, it is not surprising that ancient microorganisms such as Archaebacter survived to clog the desiccated lakes that remained, feeding on the toxic soup that bubbled in the hot muddy water and consuming what precious oxygen could be found. This mindless inhibition was not long lasting however as heavy precipitation and the debris of fallen trees and vegetative matter cleaned, oxygenated and fertilized the remaining fragmented lakes, making way for the next step in evolution, phytoplankton. Following the first stages of this aquatic rebound we saw algae, aquatic insects and amphibians return; this process was observed in both lakes and adjacent streams, albeit at different rates. On land in the days after the blast the pumice field was being bombarded by insects, most notably the ballooning spiders and a multitude of winged and walking insects, such as the carabid beetles; these beetles scavenged on the carcasses of less hardy insect that managed to arrive in this blasted landscape. The insects fell at a rate of about 2 million per day and this rich carpet of organic material began to infuse much needed nutrients back into the tephra thus facilitating the establishment of seeds also traveling on the winds. There was one very important element still in poor supply in the enriching ground: Nitrogen (N2). This allowed a very integral pioneer, the Lupine, to capitalize on its specialized ability to draw down nitrogen from the atmosphere with the assistance of symbiotic bacteria and integrate it into its structures. Other herbs such as Fireweed took root in patches but the Lupine is a very hostile plant to other species and restricted their dispersal across the landscape. Dispersing prodigious amounts of seeds and a short lifespan give the Lupine a different role in death as all of that rich decaying matter, suffused with nitrogen, could now facilitate the growth of other newcomers. Mammals also had an important role to play, using their very motility to recreate their formerly complex web of herbivory and predation, in particular, one small animal's role in creating dispersal routes which promoted the recolonization of the disturbed aquatic habitats. The pocket gopher created vast networks of tunnels which not only allowed it to survive much of the blast effects but also churned up the substrate, improved its water retention, and created routes for lizards and amphibians such as the pacific tree frog to colonize all of the lakes within 5 years. Herds of Roosevelt Elk (Cervus elaphus) were seen very shortly after the eruption which also assisted in churning up the ash cover and exposing favorable sites for plant growth as well as providing fertilizer in their droppings. In the following years the Elk returned in greater numbers as the willow-herb community and Lupine patches continued to grow. A predictable response to this increase in the population size of large grazing mammals ushered in the return of those species that prey on them such as bears, wolves, and even wolverines. Bird colonization in the pumice field yielded a surprising blend of species such as Red-winged Blackbirds and water Pipits. These species usually colonized different habitats than would be expected on the mountain but the blowdown of the trees and the pyroclastic flow created a mixture of habitats that would normally be found elsewhere. In the late seral stages on the pumice field we see the return of some species of woody vegetation such as the Silver Pine, 5 species of conifers as well as sword ferns and lady ferns alongside the communities of wind blown herbs and willows, although it is still relatively sparse. The lakes and streams have made varying degrees of recovery with respect to biodiversity but these aquatic biomes have proven to be very resilient, even the amphibians, which were previously believed to be one of the most susceptible animals to environmental disturbance. As you travel farther from the blast zone we see, of course, gradients of recovery that are inverse to the intensity of the effects of the disturbance felt. So, in the present, the surrounding area slowly encroaches and will someday bring back into itself, the devastated core. Until then, the dynamic stage of Mt. Saint Helens has taught us, surprised us and continues to enthrall and fascinate us as we watch one of the best examples of the process of healing that nature has ever provided us.

More about this author: Cory Blanchard

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