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PARTS Of THIS PAGE POSTED 2 JAN. 1997; UPDATED OCT. 2004 |
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6. Ecology after the eruption  |
Lessons from the
wasteland Trees were flattened by the blast wave at Elk Rock, 11 miles from the crater at Mount St. Helens. The standing trees at the top were shielded by the peak. Photo: USGS Much life got extinguished, but amazingly, many areas were not dead. Then, within weeks, an exotic mammal started meandering through the devastation: A small army of biologists swarmed over what was left of the mountain, cataloging every leaf, spider, bird and gopher. Some of these organisms had survived underground, others had drifted in on the wind. For ecologists, St. Helens represented a unique opportunity, says Virginia Dale, an ecologist at Oak Ridge National Laboratory. "The eruption was quite devastating to human life and property, but it was an opportunity for ecological science to start a study on succession that was unparalleled." Dale, a member of that first group of biologists to visit the mountain, and co-editor of a forthcoming book on the ecological recovery at St. Helens, focuses on the ecology of disturbances. (Disturbances may include mudslides, logging, fire, and invasive species).
If you were in our shoes, trying to describe the recovery at St. Helens, you might get your wheels stuck in the assumption that the recovery was one process. But Dale insists that the key lesson from St. Helens is that the volcano created many disturbances, not one. Those ash deposits, the pyroclastic flows, and the huge landslide all affected life in different ways, and life responded differently. Adding to the confusion, some disturbances were repeated. In the debris avalanche deposits, Dale says, "there were lots of subsequent events, some due to small eruptions that caused glacial melting and mudflows, some due to heavy rains." Rapid regeneration
Overall, Dale says, the recovery has been "highly variable but rapid in some disturbance types," and quite different from what ecologists would have predicted. Early studies of regeneration, Dale says, tracked farm fields that were left alone for a few decades. Often, weeds and/or seeds were already present -- and the fields themselves were considerably smaller than the giant disaster scenario in Southwest Washington. "This was such a huge area that was impacted, and it's not under old field conditions at all," she says. The degree of diversity seen at St. Helens was the "most surprising" finding, says Dale. "Textbooks describe ecological succession as one thing." At St. Helens, "There are no cookie-cutter answers, but there are basic principles. Few organisms survived in the most heavily affected disturbance types, but those survivors in less severely impacted areas are the keys to the future. Many organisms that lived though the 1980 eruption escaped the brunt of that event by being underground. The establishment of new organisms is greatly influenced by the type of disturbance as well as by the presence of survivors (if any) and other legacies of the pre-eruptive conditions. Once organisms establish, they improve conditions for the next wave of migrants. They provide shade and enrich the soils. Even so, environmental conditions and species are quite different than before the eruption." The Krack of Krakatau
In a awesome proof of the power that's usually concealed by Earth's quiescent surface, Krakatau (sometimes spelled Krakatoa) obliterated two-thirds of the 11-kilometer-long island of Krakatau and deposited 30 to 60 meters of red-hot ash. The explosion lofted 20 cubic kilometers of rock, ash and smoke into the sky. Shifts in the seafloor triggered tsunamis that killed 36,000 and destroyed 160 villages. Remnants of the giant waves reached the English Channel! Geologically, Krakatau and the many active volcanoes on Sumatra and Java mark where the Indo-Australian plate dives under the Pacific plate. In the geologic basement, the scenario is much like what's found under Mount St. Helens. The Krakatau explosion was soon recognized as a giant natural experiment in colonization -- the movement of species to new habitat, and Krakatau blossomed into a biologists' bonanza (see "Krakatau: the Destruction..." in the bibliography). In Krakatau, transportation was the first hurdle: How do species reach and occupy on a sterile island? (This problem also arises on the volcanic islands of Hawaii, which formed thousands of kilometers from terrestrial life). Natural transport authority Air: by flying, as a bird or insect, or by passive transportation, as a light orchid seed or fern spore. Sea: by swimming or floating on a log. Animal: (The hitchhikers' express). By traveling inside or attached to animals that swim or fly, a trick used by plant seeds and animal larvae. Many plants and animals have evolved to benefit from these techniques. Some mollusks grab moving objects with a sticky goo. Seeds may have adhesives or barbs to grab animals, or have wings that allow them to be lifted by light breezes. Fruit-eating animals can carry seeds in their gut and deposit them on an islands or sterile patches of land. If the colonization lasts long enough, the rate of local extinction will start to equal the rate of immigration, and the number of species will stabilize.
Reseeding the remains In 1997, professor emeritus Ian Thornton of Latrobe University in Australia told us that studies of colonization of Krakatau offered "An optimistic lesson: That tropical rainforest ecosystems are capable of recovery from extreme, traumatic damage, if left alone and given time. Within a century the remnant of Krakatau, Rakata, on which not a blade of grass was visible for a year, is now clothed in tropical forest from the shore to its 800-meter peak. On the three islands devastated in 1883 there are now over 400 species of vascular plants, thousands of species of arthropods including 54 species of butterflies, over 30 species of birds,18 species of land mollusks, 17 species of bats and 9 reptiles. And these components of the system have had to cross 44 kilometers of sea water to even reach the islands." Thornton added that "the assembly process is fairly deterministic in the early stages. That is to say that if we had several Krakatau situations, we would be able to predict which species would become established in the growing ecosystems with a fair degree of success." To understand this determinism, return to our discussion of dispersal, and notice that some plants and animals are suited to colonization. "There is a core of about 60 species of pioneer plants, mostly sea-dispersed (a few wind-dispersed), which may be expected to become established early," Thornton wrote us. Later in the process, he added, animals take a greater role in plant dispersal, and chance begins to play a "much greater role." What's the youngest volcano on Earth? |
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Conifers
growing on a once-sterile debris field at Mount St. Helens show
the status of natural ecological succession in 2004. That stuff
under foot is ground-up mountain, delivered by the giant landslide
24 years ago. Courtesy Virginia Dale
In 1960, the young volcano Anak Krakatau had a minimum diameter
of about one mile and a height of 545 feet. The 2,000-foot-diameter
crater on the south side contained the growing cinder cone seen
just below the ash column. Courtesy 