Ecology is the study of how living organisms interact with each other and with their nonliving environment. The nonliving environment is the earth that we are living in. The earth can be viewed as being made up of several layers or concentric spheres. The atmosphere is a thin envelope of air surrounding the planet. The troposphere is an inner layer which extends only about 17km above sea level. It contains most of the planet's air, mostly nitrogen and oxygen. The next layer is called the stratosphere which contains ozone to filter out most of the harmful UV light and protects living organisms on the earth. The hydrosphere is the aqueous envelope of the earth, including the ocean, stream, lakes, and underground waters and the aqueous vapor in the atmosphere. The lithosphere is the outer part of the earth. It consists of the crust and upper mantle. The ecosphere is the portion of the earth in which living organisms exist and interact with one another and with their nonliving environment. It consists of most of the hydrosphere and parts of the lower atmosphere and upper lithosphere. It ranges from the deepest ocean floor to the tops of the highest mountains. This is where life can be found and where interactions occur. Here is a general picture of our earth:

General structure of the earth
[Figure: G. Tyler Miller, JR, Environmental Science P. 72]