Consider a natural ecosystem, a forest(Figure 1.I) l'he physical component of the forest consists of the atmosphere, climate, soil, and water. The many dif- ferent plants and animals that inhabit the forest make up the biotic component. Each organism not only responds to the physical environment but also modifies it, becoming part of the environment itself. The trees in the canopy of a forest intercept the sunlight and use this energy to fuel the process of photosynthesis. In doing so, they modify the environment of plants below them, reducing the sunlight and lowering air temperature. Birds foraging on insects in the litter layer of the forest reduce insect numbers, modifying the environ- ment for other organisms that depend on this shared food resource. Thus in ecosystems the living and the physical environment interact in ways that become complex, as succeeding chapters will show.