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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.
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Washington State Magazine Home
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 John Bishop (left) and assistant Matt Ogburn check a light trap for moths that they will take back to the laboratory.  Experiments with lupines and herbivores in Bishop’s Vancouver
laboratory are shedding light on the chemical balance that controls an
ecosystem.
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