The scientific study of the interrelations between living organisms and their environment, including both the physical and biotic factors, and emphasising both inter-species and intra-species relations; more specifically and more narrowly, the scientific study of the distribution and abundance of living organisms (ie exactly where they occur, and precisely how many there are), and any regular or irregular variations in distribution and abundance, followed by explanation of these phenomena in terms of the physical and biotic factors at work in the environment.