Community (ecology)

In ecology, a community is a group of different populations living in the same area at the same time. Scientists sometimes call this a biocoenosis or a life assemblage. Community ecology, also known as synecology, is the study of how these species interact. Researchers look at the distribution, structure, and abundance of these coexisting populations. They also study how species' traits, or phenotypes, influence their interactions. To understand a community, scientists must also consider abiotic factors. These are non-living elements like annual temperature or soil pH. For instance, precipitation levels determine why desert plant communities differ from tropical rainforests.

Every species within a community occupies a specific niche. A niche is the role a species plays and how it interacts with its environment. Species can coexist through a process called niche partitioning. This happens when species use different resources to avoid direct competition. For example, two species might hunt at different times of day. This reduces interspecific competition, which is the struggle between different species. Instead, they experience intraspecific competition, which is the struggle between members of the same species. The number of available niches determines the total biodiversity of the community. If two species share the exact same niche, one will eventually outcompete the other.

Energy moves through a community via different trophic levels. At the base are autotrophs, or primary producers, which create energy through photosynthesis or chemosynthesis. Next are herbivores, known as primary consumers, which eat the producers. These are followed by carnivores and omnivores, which act as secondary or tertiary consumers. At the very top sits the apex predator, an animal not consumed by any other species in that community.

Energy is passed up these levels, but some is lost at each step due to ecological inefficiencies. Decomposers, such as bacteria and fungi, play a vital role by recycling nutrients back to the producers. They feed on dead organisms from all trophic levels to keep the cycle moving.

Some organisms belong to a guild. A guild is a group of species that use the same resources in a similar way. While many guild members are closely related by common descent, this is not always the case. For example, all herbivores form a basic guild. A more specific guild might include all vertebrates that forage for ground-dwelling arthropods. Even flowering plants can form a guild if they share the same pollinator. Because they share resources, species within a guild often experience intense competition.

Certain species have a much larger impact on their community than others. Keystone species have a disproportionate influence relative to their abundance. They are often apex predators, and their removal can cause a top-down trophic cascade.

In Yellowstone National Park, the loss of wolves due to overhunting caused the elk population to increase. This led to overgrazing, which harmed many other organisms. Reintroducing wolves helped restore the community to optimal functioning. Other influential species include foundation species, which create physical changes to the habitat. Red mangroves, for example, provide nursery grounds for young fish. Whitebark pines provide shade and food for grizzly bears.

Ecological engineers also play a major role by modifying the physical environment. Beavers are classic examples of these engineers. By building dams, they change the flow of water in a community. This creates new habitats and increases biodiversity in the riparian zone. These physical changes can help other organisms, like frogs, move more easily between habitats. Such species actively maintain or create aspects of the community that others rely on.

Scientists use different theories to explain how these communities are structured. Frederic Clements developed the holistic theory, viewing the community as a single unit or "superorganism." He believed species were interdependent and that community formation was non-random. In contrast, Henry Gleason proposed the individualistic theory. He argued that species associate somewhat randomly based on environmental gradients. Finally, Stephen P. Hubbell introduced the neutral theory. This theory suggests that species in a community are functionally equivalent. In this view, population changes are driven by stochastic, or random, processes like births and deaths.