Any ecosystem needs energy to sustain it, and the organisms within it. This energy comes in massive quantities from the Sun, although only a tiny fraction of the solar energy hitting the Earth goes towards sustaining life. The flow of energy through an ecosystem is complex, and vital to sustaining all life within it. This program explores energy flow through ecosystems, and how organisms at various levels in the food chain obtain their energy, use it, lose it, and store it. Number, biomass and energy pyramids are introduced, with a focus on the decreasing amounts of energy available to organisms at higher trophic levels. The concepts of niche, niche overlap, competition and limitations on organisms, are also introduced.