The diversity of life on Earth is due to the innate ability of plant and animal species to evolve, adapt, and develop within a variety of environmental niches and habitats. each species similarly occupies a distinct position in the food chain, as the Sun's energy is captured by plants, passed on to herbivores and then to carnivores, and is eventually returned to the soil and fed back into the cycle. So on our planet there are countless environments that exist within a number of much larger ecological areas or biomes. And while species evolve and adapt to the demands of their habitats, climatic and geological conditions can change dramatically, subjecting environments and their ecologies to a dynamic swirl of physical forces. The intervention of humans, able to effect large-scale changes on entire continents, has accelerated a process that has existed naturally for millennia. But in so doing, human activity has revealed the fragility of the system, threatening to destroy entire ecologies and render many species extinct. This volume looks at the interactions between the environment and biology. It examines the biomes and the physical factors that affect life on Earth, including the major chemical cycles--water, nitrogen, and carbon--that sustain it. The work also covers the interrelationships of the many life forms in food webs and chains, and it culminates in a reasoned discussion of the human impact on the Earth's ecology. At a time when environmental issues are widely discussed, this enlightening account represents an invaluable resource for the home, school, and library.