This document discusses interactions between living things within ecosystems. It describes the five levels of organization within ecosystems from largest to smallest as biome, ecosystem, community, population, and organism. Patterns exist in populations related to living space and time. Population sizes change over time due to factors like predator-prey relationships, birth and death rates, limiting factors, and carrying capacity. Organisms interact through predation, competition, cooperation, and symbiosis. Ecosystems also change over time through ecological succession as one community replaces another.