| |
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.
Continued  | |
Page: 1 | 2 | 3
| |
