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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.
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 One of the many herbivores drawn to lupines, the leaf miner wraps the plant in a web, then "mines" the interior.
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