(1) Ecology is the branch of biology that deals with the relationships between organisms and their environments. (2) Key topics in ecology include energy flow through ecosystems, levels of ecological organization, and recycling of nitrogen. (3) Candidates should be able to define ecological terms like habitat, niche, population, community, ecosystem, and explain concepts such as trophic levels, food chains, food webs, and the role of nitrogen-fixing and nitrifying bacteria in nutrient cycling.