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.
Click here for more photographs of John
Bishop’s research and Mount St. Helens.
For more on Bishop's work on Mount St. Helens, see his WSU
Vancouver Web site. |
