This AP® Biology content examines how organisms acquire, use, and compete for energy—and how their behavioral and physiological strategies influence survival, reproductive fitness, and population success. Learners will analyze how changes in energy availability impact populations and trophic levels, affecting species interactions, population density, and long-term growth dynamics. They will also investigate how autotrophs and heterotrophs drive the flow of energy through ecosystems, connecting individual-level adaptations to ecosystem-wide patterns. AP® is a registered trademark of the College Board, which has not reviewed this resource.