Ecosystems are complex networks of living organisms interacting with each other and their physical environment. They consist of **producers** (plants and algae) that generate energy through photosynthesis, **consumers** (herbivores, carnivores) that obtain energy by consuming other organisms, and **decomposers** (bacteria, fungi) that break down dead matter, returning nutrients to the soil. 

Food chains illustrate the flow of energy through an ecosystem, starting with **primary producers** and moving through various **trophic levels**: primary consumers (herbivores), secondary consumers (carnivores), and tertiary consumers (top predators). Each link in the food chain represents a transfer of energy, demonstrating the interdependence of organisms and the importance of maintaining balance within ecosystems for their sustainability and health.