Definition

A consumer is a living thing that cannot make its own food, so it eats other living things, and consumers depend on producers even when they never eat a plant directly, since a wolf that never touches grass still eats deer that do. More formally, a consumer (heterotroph) obtains energy by feeding on other organisms and is classified both by diet, herbivore, carnivore, omnivore, or scavenger, and by position in a food chain, primary consumers eating producers, secondary consumers eating primary consumers, and tertiary consumers eating secondary consumers, with only about $10$ percent of the energy at each level passing to the next, which is why top predators are always rarer than the animals they eat. At the trophic-dynamics level, consumers occupy levels above the producers with typically $10$ percent energy transfer between them, constrained by unassimilated material, respiration, and heat loss; many consumers feed at multiple trophic levels, so trophic position is often measured as a fractional value using stable isotope ratios such as $\delta^{15}\text{N}$, and consumer-resource interactions, including predator-prey cycles, are modeled by coupled differential equations such as the Lotka-Volterra system.