  Theory in ecology consists of the heuristics—or principles—used to construct models. Unlike evolutionary theory, ecology has no generally accepted global principles such as Mendel's (and other) rules of genetic  inheritance.[3]  Contemporary ecology consists of a patchwork of sub-disciplines including population ecology, community ecology, conservation ecology, ecosystem ecology, metapopulation ecology, metacommunity ecology, spatial ecology, landscape ecology, physiological ecology, evolutionary ecology, functional ecology, and behavioral ecology. What is common to all these fields is the view that: (i) different biota interact in ways that can be described with sufficient precision and generality to permit their scientific study; and (ii) ecological interactions set the stage for evolution to occur primarily because they provide the external component of an entity's fitness. The latter aspect makes ecology a central part of  biology.[4]  As van Valen once put it: “evolution is the control of development by  ecology.”[5]  However, the creation of a unified theoretical framework for evolution and ecology remains the task for the future and will be of no further concern in this entry.