Principle: "All individuals are part of ecosystems, which are complex communities of organisms and their physical environments."
Species always occur as part of an ecosystem -- an interdependent community of species and their physical environment. All ecosystems share six key characteristics:
(1) they depend on both living and nonliving parts.
all species that interact in an ecosystem form an ecological community including a host of microbes.
weather and climate, the nature of locate rocks and soils, the temperature and salinity of local bodies of water.
(2) they require energy.
Energy flows through an ecosystem by way of the food web (a pyramid of trophic levels).
(3) they recycle atoms and molecules over and over again.
(4) every organism in an ecosystem occupies an ecological niche.
A niche is a specific strategy for obtaining energy and atoms from the environment. Its like a job description filled by one species at a time.
(5) populations of different species achieve a balance in stable ecosystems.
(6) a change in environment or the introduction or loss of a species can disrupt an ecosystem.
This change can be gradual or it can be sudden and dramatic.
The law of unintended consequences states that any change in one part of a complex system may affect other parts of the system, in ways that are often unpredictable. The law is illustrated by the Peter's Mountain Mallow, an endangered plant that conservationists tried to protect from forest fires, only to realize that fire is necessary for the plant's germination. Another example is the introduction of the predatory Nile perch to Lake Victoria -- a change that drastically altered the Lake's ecosystem.
Human activities change ecosystems, often in unpredictable ways. As the human population grows, we command a greater share of water and nutrients --resources essential to all living things. Habitats are eliminated through deforestation, farming, urbanization and other endeavors. Humans also affect environments by eliminating species. And human activities result in a wide variety of air, water, and soil pollution, which may degrade ecosystems on a regional or global scale. Experimental fieldwork and computer modeling are now being used to gain a better understanding of human actions on local and global ecosystems.