Ecosystems comprise the complex relationships between the organic and inorganic elements of an environment, which include plants, animals, microorganisms and the habitat itself. Each of these elements plays an important role in the health of an ecosystem, and their relationships lie in a delicate balance. A species' specific role in an environment is known as its ecological niche, and its position in a specific food chain is known as its trophic level.
Producers
Photosynthesizing organisms, or producers, convert sunlight into usable energy. This group includes all trees, grasses, bushes and flowering plants. It also includes microorganisms such as algae, and even certain protozoans. Producers play an essential role in maintaining life on earth by converting carbon dioxide into oxygen during photosynthesis, and by serving as a food source for other organisms.
Primary Consumers
Primary consumers are organisms that feed on plants or photosynthesizing microorganisms (i.e., producers). They range from tiny zooplankton to larger creatures like deer, cows and giraffes. Primary consumers serve as an important food source for organisms higher on the food chain.
Secondary, Tertiary and Quaternary Consumers
Secondary, tertiary and quaternary consumers fill out the rest of an ecosystem's food chain. Secondary consumers feed on primary consumers, tertiary consumers feed on secondary consumers and quaternary consumers feed on tertiary consumers. As an example, consider a scenario in which a bear (quaternary consumer) feeds on a fish (tertiary consumer) that feeds on grass shrimp (secondary consumer) that feed on zooplankton (primary consumer) that feed on phytoplankton (producer).
Detritivores
Detritivores feed on detritus, or dead plant and animal matter. These include not only scavenging animals such as crabs or buzzards, but also bacteria and fungi that break down dead plant and animal matter.
Keystone Species
A keystone species plays a critical role in its ecosystem. Removing it from that ecosystem can have drastic effects. For example, a particular species of starfish found in the Pacific Northwest preys on a particular species of mussel. The experimental removal of this starfish from certain tidal pools led to a drastic spike in the mussel population, at the expense of the ecosystem's overall biodiversity. Another example is the removal of natural predators such as wolves, which can sharply increase the population of deer or elk, which can lead to changes in vegetation patterns, which can affect a wide variety of other animals, as well as the speed and extent of natural processes like erosion.
