"As an illustration of the interdependence that characterizes any natural community, consider what is necessary to support certain levels of animal life on one single acre of grassland. One study indicated that such a community had nearly 6 million grass plants, the primary food producers. These plants supported 700,000 herbivores — most of them insects — which fed off the grass and absorbed its energy. At the next higher level of this food chain were more than 350,000 carnivores — spiders, ants, beetles, and other small creatures — feeding on the herbivores. And at the very top level were three larger carnivores — two moles and one bird — that fed on the lower forms of life and owed their existence to the 6 million grass plants at the base of the pyramid."