2. Food Webs: Energy Flows, Matter Cycles

Energy enters most ecosystems through producers: plants, algae, and some bacteria that make food using sunlight. Consumers get energy by eating producers or other consumers. Decomposers, such as fungi and bacteria, break down dead material and return nutrients to the soil or water.

A food chain is one path, like grass -> rabbit -> fox. A food web is more realistic because most organisms eat more than one thing and are eaten by more than one predator. If one species disappears, the effect can travel through the web. Predators may lose food, prey may increase, and competitors may shift.

Energy does not recycle. It flows one way and is lost as heat at each feeding level. A useful GED® rule is the 10 percent rule: only about 10 percent of energy stored at one trophic level is passed to the next. That is why ecosystems usually support many producers, fewer herbivores, and even fewer top predators.

Matter works differently. Carbon, nitrogen, water, and phosphorus cycle through air, water, soil, rocks, and living things. A carbon atom in a plant might later be in an animal, then in soil, then back in the air as CO2 after respiration or decomposition.