Ecosystems

Ecosystems
Transfer of energy and materials among organisms within ecosystems is the ultimate level of organization in nature. Energy and materials are required to construct and to maintain life, and their incorporation into biological systems is called productivity. Productivity is divided into component trophic levels based on how organisms obtain energy and materials. Trophic levels are linked together into food webs (Figure 40-13), which are pathways for the transfer of energy and materials among organisms within the ecosystem.

Primary producers are organisms that begin productivity by fixing and storing energy from outside the ecosystem. Primary producers usually are green plants that capture solar energy through photosynthesis (but see an exception in the box on this page). Powered by solar energy, plants assimilate and organize minerals, water, and carbon dioxide into living tissue. All other organisms survive by consuming this tissue, or by consuming organisms that consumed this tissue. Consumers include herbivores, which eat plants directly, and carnivores, which eat other animals. The most important consumers are decomposers, mainly bacteria and fungi that break dead organic matter into its mineral components, returning it to a soluble form that can be used by plants to restart the cycle. Although important chemicals such as nitrogen and carbon are reused endlessly through biological cycling, all energy ultimately is lost from the ecosystem as heat and cannot be recycled. Thus, no ecosystems, including the biosphere of earth, are truly closed.