Ecology is the study of interactions between organisms and their environment. There are biotic (living) and abiotic (non-living) factors in an ecosystem. Ecosystems have multiple levels of organization including species, populations, communities, and the biosphere. For an ecosystem to sustain life it needs an energy source, organisms to convert energy, and nutrient cycling between organisms and the environment. Energy flows through food chains and webs from producers like plants to consumers. Key nutrient cycles in ecosystems include nitrogen, water, and carbon which allow organisms to obtain essential elements. Within a community, species interact through relationships like predation, competition, and symbiosis.