Ecosystems are a key element in the viability of planet Earth as human home. Populations of different plants and animals that live and interact together are called a community. When such a community interacts with the other three components of the physical environment—atmosphere, hydrosphere, and lithosphere—the result is an ecosystem. The cycles of flows and interconnections—physical, chemical, and biological—between the parts of ecosystems form the mosaic of Earth's environments. The geographically informed person needs to understand the spatial distribution, origins, functioning, and maintenance of different ecosystems and to comprehend how humans have intentionally or inadvertently modified these ecosystems.

Ecosystems form distinct regions on Earth’s surface, which vary in size, shape, and complexity. They exist at a variety of scales, from small and very localized areas (e.g., a single stand of oak trees or a clump of xerophytic grasses) to larger areas with precise geographic boundaries (e.g., a pond, desert biome, island, or beach). Larger scale ecosystems can form continent-wide belts, such as the tundra, taiga, and steppe of northern Asia. The largest ecosystem is the planet itself.

All elements of the environment, physical and human, are part of several different but nested ecosystems. Ecosystems, powered by solar energy, are dynamic and ever-changing. Changes in one ecosystem ripple through others with varying degrees of impact. As self-regulating open systems that maintain flows of energy and matter, they naturally move toward maturity, stability, and balance in the absence of major disturbances. In ecological terms, the physical environment can be seen as an interdependent web of production and consumption cycles. The atmosphere keeps plants and animals alive through solar energy, chemical exchanges (e.g., nitrogen-fixing and photosynthesis), and the provision of water. Through evapotranspiration the atmosphere and plants help to purify water. Plants provide the energy to keep animals alive either directly through consumption or indirectly through their death and decay into the soil, where the resultant chemicals are taken up by new plants. Soils keep plants and animals alive and work to cleanse water. The root systems of plants and the mechanical and chemical effects of water percolating through bedrock create new soil layers. Ecosystems therefore help to recycle chemicals needed by living things to survive, redistribute waste products, control many of the pests that cause disease in both humans and plants, and offer a huge pool of resources for humans and other living creatures.

However, the stability and balance of ecosystems can be altered by large-scale natural events such as El Niņo, volcanic eruptions, fire, or drought. But ecosystems are more drastically transformed by human activities. The web of ecological interdependency is fragile. Human intervention can shatter the balance of energy production and consumption. For example, the overgrazing of pasture lands, coupled with a period of drought, can lead to vegetation loss, the exposure of topsoil, layers, and massive soil erosion (as occurred in the 1930s Dust Bowl); tropical forest clear-cutting can lead to soil erosion and ecological breakdown, as is currently occurring in Amazonia; the construction of oil pipelines in tundra environments can threaten the movements of the caribou herds on which indigenous Inuit populations depend.

By knowing how ecosystems operate and change, students are able to understand the basic principles that should guide programs for environmental management. Students can understand the ways in which they are dependent on the living and nonliving systems of Earth for their survival. Knowing about ecosystems will enable them to learn how to make reasoned decisions, anticipate the consequences of their choices, and assume responsibility for the outcomes of their choices about the use of the physical environment. It is important that students become well-informed regarding ecosystem issues so they can evaluate conflicting points of view on the use of natural resources. The degree to which present and future generations understand their critical role in the natural functioning of ecosystems will determine in large measure the quality of human life on Earth.