Ecosystems (which is short for ecological systems) are functional units that result from the interactions of a-biotic, biotic, and cultural (anthropogenic) components. Like all systems they are a combination of interacting and interrelated components that form a unitary whole. All ecosystems are "open" systems in the sense that energy and matter are transferred in and out of them. The Earth itself can be considered as a single ecosystem which constantly converts solar energy into myriad of organic products, and which has increased in biological complexity over time.

Natural ecosystems are made up of a-biotic components (air, water, rocks, energy) and biotic components (plants, animals, and microorganisms).

Ecosystems are can also be seen to be made up of many smaller ecosystems interlocked through cycles of energy and chemical elements. The flow of energy and matter through ecosystems, therefore, is regulated by the complex interactions of the energy, water, carbon, oxygen, nitrogen, phosphorus, sulfur, and other cycles that are essential to the functioning of the biosphere.

Changes in any of these factors (for example: nutrient availability, temperature, light intensity, grazing intensity, and species population density or composition) will result in dynamic changes of the nature of these systems.

For example, a fire in the temperate deciduous forest can completely change the structure of that system. There are no longer any large trees, most of the mosses, herbs, and shrubs that occupy the forest floor are gone, and the nutrients that were stored in the biomass are quickly released into the soil, atmosphere and hydrologic system. After a short time of recovery, the community that was once large mature trees has been transformed into a community of grasses, herbaceous species, and tree seedlings.