This chapter focuses on the ecosystem concept itself, on ecosystem form and function, and on productivity and equilibrium in ecosystems. It aims to reflect on the problems and prospects inherent in ecosystem analysis, and on the value of the approach to environmental management. The hierarchical structure of food webs is not confined to organic components of the environment, nor is it confined to living plants and animals. The value of the pyramid of numbers is that it allows comparisons to be drawn between the structure and food web diversity of different ecosystems. Despite the problems of establishing meaningful estimates or measurements of energy flows in ecosystems a number of parameters describing energy relations within ecosystems have been suggested and used. The effects of deforestation in the experimental forest on hydrology, nutrient export and ground net production are compared with a forested reference ecosystem in the Hubbard Brook forest.