Ecology is the study of how organisms interact with their environment. The environment consists of biotic and abiotic components that are interrelated in an ecosystem. All ecosystems must have a constant input ofenergy from the sun. Producer organisms are capable of trapping the sun's energy and converting it into biomass. Herbivores feed on producers and are in turn eaten by carnivores, which may be eaten by other carnivores. Each level in the food chain is known as a trophic level. Other kinds of organisms involved in food chains are omnivores, which eat both plant and animal food, and decomposers, which break down dead organic matter and waste products. All ecosystems have a large producer base with successively smaller amounts of energy at the herbivore,primary carnivore, and secondary carnivore trophic levels. This is because each time energy passes from one trophic level to the next, about 90 percent of the energy is lost from the ecosystem. A community consists of the interacting populations of organisms in an area. The organisms are interrelated in many ways in food chains that interlock to create foodwebs. Because of this interlocking, changes in one part of the community can have effects elsewhere.

Major land-based regional ecosystems are known as biomes. The temperate deciduous forest, northern coniferous forest, temperate rainforest,tropical rainforest, desert, savanna, and tundra are examples of biomes. Aquatic communities can be divided into marine, freshwater, and estuarine communities.

Each organism in a community occupies a specific space known as its habitat and has a specific functional role to play known as its niche. An organism's habitat is usually described in terms of some conspicuous element of its surroundings. The niche is difficult to describe because it involves so many interactions with the physical environment and other living things.

Interactions between organisms fit into several categories. Predation is one organism benefiting (predator) at the expense of the organism killed and eaten (prey). Parasitism is one organism benefiting (parasite) by living in or on another organism (host) and deriving nourishment from it. Commensal relationships exist when one organism is helped but the other is not affected. Mutualistic relationships benefit both organisms. Competition causes harm to both of the organisms involved, although one may be harmed more than the other and may become extinct, evolve into a different niche, or be forced to migrate.

Many atoms are cycled through ecosystems. The carbon atoms of living things are trapped by photosynthesis, passed from organism to organism as food, and released to the atmosphere by respiration. Nitrogen originates in the atmosphere, is trapped by nitrogen-fixing bacteria, passes through a series of organisms, and is ultimately released to the atmosphere by denitrifying bacteria.

A population is a group of organisms of the same species in a particular place at a particular time. Populations differ from one another in gene frequency, age distribution, sex ratio, and population density. A typical population growth curve consists of a lag phase in which population rises very slowly, followed by an exponential growth phase in which the population increases at an accelerating rate, followed by a leveling off of the population in a stable equilibrium phase as the carrying capacity of the environment is reached.

Humans as a species have the same limits and influences that other organisms do. However, humans can reason and predict, thus offering the possibility of population control through conscious populationlimitation.