Mount St. Helens: The perfect laboratory
by Tim Steury | © Washington State University
"No biological legacy."
The phrase John Bishop uses to describe the effect of Mount St. Helens's eruption on the main blast zone, the pumice plain, holds an understated charm. By now, everyone has heard the story of Mount St. Helens-how it blew on a Sunday morning in May 1980, after rumbling for weeks, an earthquake triggering an enormous landslide, hot gas and rock debris blasting across the landscape at 1,100 kilometers an hour, devastating 60 square kilometers and killing 60 people. But it is impossible to accept the immensity of the mountain and the eruption's legacy, unless you are able to stand beneath the enormous crater on the pumice plain-and hear Bishop, an ecologist at Washington State University at Vancouver, talk about lupines.
No biological legacy. Trees, birds, elk, bacteria, spring flowers, humans-all simply vaporized. A whole region was completely sterilized.
But this devastation left a rare and perfect laboratory, a clean slate on which to observe the fundamental process of "primary succession," the reestablishment of life where there was none.
Here on the pumice plain, on a perfect August morning 23 years after the eruption, plumes of dust and ash blow off the volcano's rim, now 1,200 feet lower than it was before the eruption. The students working for Bishop have scattered across the plain, checking experiment sites. Grasshoppers clatter around us, and a raven whoosh-whooshes overhead, toward Spirit Lake to the south. Elk scat is everywhere. The occasional rumble of rockfalls in the crater drifts across the plain. Life has returned to the pumice plain, but the echoes of cataclysmic drama are very much with us.
Imagine how startling it must have been, when in the midst of this devastation, scientists discovered a lone lupine plant barely a year after the eruption. How could it possibly have gotten there? Lupines are not mobile, says Bishop. Birds, which serve as distributors of many plants, don't seem to care for lupine seeds. And lupine seeds are hard and heavy, lacking the adaptations of wind-borne seeds. Normally, lupines spread slowly. The seed heads shatter, the seeds fall to the ground and sprout, and the lupines march incrementally, albeit inexorably, across the landscape. Bishop has observed voles gathering seeds. But no vole journeyed across the barren pumice plain to plant a lupine.
So how else could that original seed have arrived, except by wind? Lupinus lepidus var. lobbii is considerably smaller than most lupines. It is adapted to hot, dry, alpine conditions and grows mainly on the slopes of volcanoes. Its seeds are small, and the wind blows fierce in these mountains. So the seed could have-no, must have-arrived by wind. But the original plant's conception is still no less mysterious, for a very basic reason.
Even though lupines, with the help of rhizobial bacteria that colonize their roots, can pull nitrogen from the air and transform it to the form of nitrogen all plants need to grow, they also need phosphorus. But the plain was sterile, with no nutrients available for plant growth.
Bishop can only speculate. Plants were not the first organisms to repopulate the pumice plain. The first would have been insects. Blown in by the same strong winds that must have carried the first lupine seed, they fell into a barren world with only each other to eat. To imagine the steady deposition of insects is to understand, to some extent, the inexorable force of life. And so eventually, presumably, enough insects arrived, and died, and were recycled through other insects, to build up enough phosphorus to nurture that first lupine.
Once that first lupine got established, says Bishop, it became an ecosystem engineer. Legumes produce more soil nitrogen than they consume, making it available for other nitrogen-dependent plants. As the lupines grew and died, they provided organic matter to start rebuilding the soil. They also attracted insects, which would add, as they died, other nutrients.
In the first 10 years after the eruption, lupine patches were the place to be. Other plants had ventured onto the pumice plain, but they stuck right next to the lupines. Meanwhile, that first lupine had become millions. But its spread was not unchecked. Many herbivorous insects love lupines, especially when they're the only meal on the mountain. Most are moths and their caterpillars: leaf-miners, caudex-borers, cut-worms, each of which attacks a different part of the plant.
And this brings up a basic question of ecology. How are populations regulated? Top- down or bottom-up? The top-down hypothesis suggests that predators control populations. In spite of ravenous herbivores, predators eat enough of them to maintain a nice balance that makes this green world possible.
The bottom-up hypothesis suggests that it is resources that control population. And ultimately, of course, we know that it's the resources that really control things. But, says Bishop, impose predation on a system, and things get complicated very quickly.
What is it that controls primary succession on Mt. St. Helens?
It's this complication that drives Bishop's research. What is it that controls primary succession on Mount St. Helens? Early in his work on the mountain, Bishop noticed that things were not quite as one would expect in the lupine patches.
A number of different herbivores love lupines. But their behavior and demography are, to say the least, odd. If you were a hungry caterpillar, where would you head for lunch? Why, the thickest part of the lupine patch, of course. But such is not the case. In fact, the high-density patches are devoid of insect herbivores. Move out to the lower-density patches, though, to the suburbs of lupineville, and the herbivores are happily profuse.
Bishop and postdoctoral researcher Jenny Apple are testing two opposing explanations. The first is that herbivores do indeed move in, early on, to the dense patches. But so do their predators, the ants and spiders and caterpillar-hunter beetles. As they colonize the thick patches, they suppress the herbivores.
The other explanation might be that the lupines in the high-density patches are poor-quality food sources. Because of their density, they compete for limited resources, providing lower-quality food for the herbivores. Moths simply choose not to lay their eggs where the food quality for their young is poor.
Bishop has shown that the phosphorus of the denser areas is indeed lower. Plants in the outlying areas have more nitrogen and phosphorus available, and Bishop has shown that caterpillars indeed grow faster on those plants.
In fact, food choice and population patterns could be controlled by basic nutrients. A subdiscipline within ecology, ecological (or biological) stoichiometry, is based on our understanding that all life is composed of three basic nutrients: carbon, nitrogen, and phosphorus. Organisms use nitrogen to build protein and nucleic acid, the basic ingredient of DNA. Phosphorus is used primarily for nucleic acid.
"People have long thought that the amount of nitrogen in an environment is what limited plant and insect communities most of the time," says Bishop. "But maybe it's not just nitrogen, but also phosphorus."
If you grind up a plant, says Bishop, and measure the amount of carbon, nitrogen, and phosphorus, and then do the same with an insect, you can compare those amounts of each nutrient and ask whether the carbon-nitrogen ratio in an insect is such that it could get sufficient nitrogen from that plant. Theoretically, an insect could be limited by either nitrogen or phosphorus. Given a nutrient-poor system, it could be that the nitrogen-phosphorus ratio is what actually drives the whole process within an ecosystemÊin this case, the pumice plain. It could be that insects choose to feed on the plants with the correct N-P ratio and ignore those with a poor ratio.
Unlike plants, insects have a relatively fixed N-P ratio. So if they're eating a phosphorus-poor plant, they can't change their own N-P ratio. Instead, they have to eat more, until they get enough P.
By the time we stop for lunch in a patch of willows at the stream coming down off the volcano's glacier, Bishop has gathered a list of research questions that still beg to be addressed. Relatively neat questions about nutrient availability and the effect of lupines on other plants. And much bigger, overwhelming questions. For example, have the lupines and herbivores coevolved since the eruption? In addition to funding from the National Science Foundation, Bishop recently received a grant from Murdock for equipment that will enable him to do more specific genetic analysis. He has lupine seeds from 1985 that he is eager to compare with current lupines to see whether they have adapted to this intense episode of herbivory. The perfect ecological laboratory has much yet to reveal.
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