Branch of biology which focuses on the relationships among various species and among the species and their environments. The basic units for study are the species (all the organisms which are capable of interbreeding), population (all of the members of a species occupying a certain geographical area) and community (number of populations interacting within a certain area). The emphasis in environmental science, however, is on the processes of energy flows and nutrient cycles within these communities and ecosystems and involves such ideas as species abundance, biodiversity, complexity, trophic (feeding status) levels, ecological niches, and structure of communities.