The March National Geographic describes research in two Amazon ecosystems in which differences in soil quality seem to drive the setpoint in the struggle between plants and insects. Plants mount all sorts of defenses against herbivorous insects, which are then under selective pressure to evade them; and once a plant's defenses are evaded, that species comes under pressure to develop something new. And this struggle is indeed very old: fossil beds from as long ago as the late Silurian (400 million years ago) already show land plants and the arthropods which eat them already in an ecological assemblage.
What's new about the National Geographic article is that plants seem to devote more resources to fending off insects-- poisons (like caffeine), spiky indigestible leaves, etc.-- when the soils are poor; and more on leaf production and growth when the soil is richer. This tendency is so strong that the Amazon soil districts have distinct sets of plants, even though much the same sets of insects are doing the eating. To me this sounds reminiscent of desert plants, who invest heavily in defense at the expense of slow growth. In a sense, then, the nutritional input determines the equilibrium point in the ecosystem.