A habitat is the environment in which an organism, species, or community lives. Habitats can be classified in a number of ways in order to compare them at different times, across different geographic areas, and in terms of different life history strategies.

Within nearly every type of habitat there exists a species that has adapted to that habitat, from the deep ocean floor to the polar ice caps. These specializations mean that the organism can survive in one type of habitat but not necessarily in another, a concept termed habitat requirement. Organisms that are free to move about can choose which habitat they will live in, and these choices are made based on the costs and benefits of each place.

In order to make comparisons between the many that exist, habitats need to be classified in some manner. First, it is important to understand that what is a...