Each community is unique, but those occurring in similar environments are generally similar to one another. Adjacent communities often blend into each other. On any site, the organisms interact in various ways. Some help each other while others are harmful. All of the organisms on a site, along with its physical environment, comprise an ecosystem.
All ecosystem functions depend upon energy.
The bodies of all organisms are built from chemical elements called nutrients. Decomposers convert the nutrients back into their non-biological form. Ecosystems are constantly changing, either rapidly or slowly. Those changes can be due to changes in physical factors, to the arrival or local extinction of the component species, to the organisms progressing through their life cycles, to altered relationships between the species, and to the species changing by evolution. More than any other species, humans have changed the earth's ecosystems.