AbstractThe article describes the first step in the formalization of the theory of animal ecology developed by H.G. Andrewartha, L.C. Birch, T.O. Browning, D.A. Maelzer and their students. The classification of the environment given by H.G. Andrewartha inIntroduction to the Study of Animal Populations(1971) is formalized, with the exclusion of the component ‘weather’. In addition, the notion due to D. A. Maelzer that certain objects ‘only modified the value of some resource or other component of environment’ is also formalized and extended. The class of malentities is enlarged to include the case of an animal eating a poisonous animal. This enlarged class includes some animals formally classified as ‘aggressors’. Symbiosis is als