An ecosystem is a system of living (biotic) and nonliving (abiotic) components that interact with each other and are located in a specific place. Biotic components are plants, animals, bacteria, and all other living organisms; the biotic components of an ecosystem are also known as a biotic community. Abiotic components are nonliving, such as water, energy, temperature, nutrients, and minerals. Terrestrial ecosystems are classified based on dominant vegetation, and aquatic ecosystems are based on water body type.

Species is typically defined as a group of individuals, having similar structure and function, capable of interbreeding. A population is a group of individuals of the same species that live in the same area at a given time. A biotic community is made from populations of different species that reside in a common environment.

Biotic and abiotic components are linked through nutrient cycles (e.g., water cycle) and energy flow. Energy flows through an ecosystem starting as sunlight and is transferred from one organism to another as they eat each other. A producer, such as a plant, is an organism able to create its own food. A consumer, such as an animal, is an organism that eats other organisms to satisfy its energy needs. The trophic level of an organism is the position it occupies in the food chain. Autotrophs are primary producers such as plants, which produce their own food through photosynthesis, using sunlight, nutrients, and minerals. Heterotrophs are primary, secondary, and tertiary consumers, which eat other organisms such as autotrophs or other heterotrophs to satisfy energetic needs.

Primary consumers are herbivores that eat mainly autotrophs and include animals such as deer and squirrels. Secondary consumers are carnivores that feed on primary consumers and include animals such as birds. (Secondary consumers, including birds, can also be omnivores.) Tertiary consumers, such as bobcats, are organisms that feed mainly on secondary consumers. Omnivores, such as raccoons, consume both primary producers (plants) and primary consumers (animals). Decomposers feed from the remains of dead organisms and include vultures, bacteria, and fungi.

The way energy flows in an ecosystem can be easily illustrated by a food chain, a linear sequence of organisms that eat each other. The sequence starts with a primary producer creating its own food and ends with a consumer not being eaten by another organism (See Fig. 8-1). Although the food chain is a good representation, energy flows in an ecosystem are more accurately represented by food webs.

Figure 8-1. Food chain.
Credit: LadyofHats. Wikimedia Commons.

A food web, illustrated by all the arrows in Figure 8-2, is a series of many overlapping food chains in an ecosystem. A food web shows the many paths in which plants and animals are connected. A food chain is a subdivision of a food web, which implies linearity. It follows just one path as an animal finds food. A food web, which is nonlinear, is an accurate representation of the energy flow in an ecosystem.

Food web

Figure 8-2. Food web.
Credit: Delwin Benson, Colorado State University.

Figure 8-2 illustrates predators, which are heterotrophs that obtain their energy from species in lower trophic levels. Predators include animals such as cougars, bald eagle, and black bears. Prey species are those consumed by predators. Generally, a predator-prey relationship and interaction exist inside ecosystems. The density of a predator population will almost always be lower than that of its prey. Predators will feed on a variety of prey, if possible. Prey reproduction will be greater than that of predators; usually predators will be larger in size.