The document provides an overview of key concepts in ecology, including interactions within populations, communities and ecosystems; nutrient cycling and energy flow; and the effects of natural and human impacts. It defines important terms like biomes, species, populations, communities, ecosystems, niches, food chains, trophic levels and food webs. It also describes population, community and ecosystem levels of organization, and examines habitat versus niche, limiting factors, feeding relationships including producers, consumers, decomposers, and symbiotic relationships like commensalism, parasitism and mutualism. Finally, it discusses trophic levels and pyramids, nutrient cycles of water, carbon and nitrogen, and how toxins can biologically magnify up food chains