The document explores the relationships that sustain ecosystems, including feeding interactions such as food chains, food webs, and predator–prey dynamics, as well as the roles of detritivores and decomposers in nutrient recycling. It examines symbiotic relationships—parasitism, commensalism, and mutualism while highlighting energy flow through trophic levels, ecological pyramids, and the non‑cyclic transfer of energy. The text also covers key biogeochemical cycles like water, carbon, and nitrogen, alongside the importance and challenges of recycling manufactured materials. Designed with clarity and practical examples, it provides students with a comprehensive understanding of how organisms interact, recycle energy, and maintain ecological balance.