Community (ecology)
In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, or life assemblage. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".
Community ecology or synecology is the study of the interactions between species in communities on many spatial and temporal scales, including the distribution, structure, abundance, demography, and interactions of coexisting populations. The primary focus of community ecology is on the interactions between populations as determined by specific genotypic and phenotypic characteristics. It is important to understand the origin, maintenance, and consequences of species diversity when evaluating community ecology.
Community ecology also takes into account abiotic factors that influence species distributions or interactions. For example, the plant communities inhabiting deserts are very different from those found in tropical rainforests due to differences in annual precipitation. Humans can also affect community structure through habitat disturbance, such as the introduction of invasive species.
On a deeper level the meaning and value of the community concept in ecology is up for debate. Communities have traditionally been understood on a fine scale in terms of local processes constructing an assemblage of species, such as the way climate change is likely to affect the make-up of grass communities. Recently this local community focus has been criticized. Robert Ricklefs, a professor of biology at the University of Missouri and author of Disintegration of the Ecological Community, has argued that it is more useful to think of communities on a regional scale, drawing on evolutionary taxonomy and biogeography, where some species or clades evolve and others go extinct. Today, community ecology focuses on experiments and mathematical models, however, it used to focus primarily on patterns of organisms. For example, taxonomic subdivisions of communities are called populations, while functional partitions are called guilds.
Organization
Niche
Within the community, each species occupies a niche. A species' niche determines how it interacts with the environment around it and its role within the community. By having different niches species are able to coexist. This is known as niche partitioning. For example, the time of day a species hunts or the prey it hunts.Niche partitioning reduces competition between species such that species are able to coexist because they suppress their own growth more than they limit the growth of other species.
The number of niches present in a community determines the number of species present. If two species have the same niche then one species outcompetes the other. The more niches filled, the higher the biodiversity of the community.
Trophic level
A species' trophic level is their position in the food chain or web. At the bottom of the food web are autotrophs, also known as primary producer. Producers provide their own energy through photosynthesis or chemosynthesis, plants are primary producers. The next level is herbivores, these species feed on vegetation for their energy source. Herbivores are consumed by omnivores or carnivores. These species are secondary and tertiary consumers. Additional levels to the trophic scale come when smaller omnivores or carnivores are eaten by larger ones. At the top of the food web is the apex predator, this animal species is not consumed by any other in the community. Herbivores, omnivores and carnivores are all heterotrophs.A basic example of a food chain is; grass → rabbit → fox. Food chains become more complex when more species are present, often being food webs. Energy is passed up through trophic levels. Energy is lost at each level, due to ecological inefficiencies.
The trophic level of an organism can change based on the other species present. For example, tuna can be an apex predator eating the smaller fish, such as mackerel. However, in a community where a shark species is present the shark becomes the apex predator, feeding on the tuna.
Decomposers play a role in the trophic pyramid. They provide energy source and nutrients to the plant species in the community. Decomposers such as fungi and bacteria recycle energy back to the base of the food web by feeding on dead organisms from all trophic levels.
Guild
A guild is a group of species in the community that utilize the same resources in a similar way. Organisms in the same guild experience competition due to their shared resource. Closely related species are often in the same guild, due to traits inherited through common descent from their common ancestor. However, guilds are not exclusively composed of closely related species.Carnivores, omnivores and herbivores are all basic examples of guilds. A more precise guild would be vertebrates that forage for ground dwelling arthropods, this would contain certain birds and mammals. Flowering plants that have the same pollinator also form a guild.
Influential species
Certain species have a greater influence on the community through their direct and indirect interactions with other species. The population of influential species are affected by abiotic and biotic disturbances. These species are important in identifying communities of ecology. The loss of these species results in large changes to the community, often reducing the stability of the community. Climate change and the introduction of invasive species can affect the functioning of key species and thus have knock-on effects on the community processes. Industrialization and the introduction of chemical pollutants into environments have forever altered communities and even entire ecosystems.Foundation species
largely influence the population, dynamics and processes of a community, by creating physical changes to the environment itself. These species can occupy any trophic level, but tend to be producers. Red mangrove is a foundation species in marine communities. The mangrove's root provides nursery grounds for young fish, such as snappers.Whitebark pine is a foundation species. Post fire disturbance the tree provides shade enabling the regrowth of other plant species in the community, This growth prompts the return of invertebrates and microbes needed for decomposition. Whitebark pine seeds provide food for grizzly bears.
Keystone species
have a disproportionate influence on the community than most species. Keystone species tend to be at the higher trophic levels, often being the apex predator. Removal of the keystone species causes top-down trophic cascades. Wolves are keystone species, being an apex predator.In Yellowstone National Park the loss of the wolf population through overhunting resulted in the loss of biodiversity in the community. The wolves had controlled the number of elks in the park, through predation. Without the wolves the elk population drastically increased, resulting in overgrazing. This negatively affected the other organisms in the park; the increased grazing from the elks removed food sources from other animals present. Wolves have since been reintroduced to return the park community to optimal functioning. See Wolf reintroduction and History of wolves in Yellowstone for more details on this case study.
A marine example of a keystone species is Pisaster ochraceus. This starfish controls the abundance of Mytilus californianus, allowing enough resources for the other species in the community.
Ecological engineers
An ecosystem engineer is a species that maintains, modifies and creates aspects of a community. They cause physical changes to the habitat and alter the resources available to the other organisms present.Dam building beavers are ecological engineers. Through the cutting of trees to form dams they alter the flow of water in a community. These changes influence the vegetation on the riparian zone, studies show biodiversity is increased. Burrowing by the beavers creates channels, increasing the connections between habitats. This aids the movement of other organisms in the community such as frogs.
Theories of community structure
Community structure is the composition of the community. It is often measured through biological networks, such as food webs. Food webs are a map showing species networks and the energy that links the species together through trophic interactions.Holistic theory
Holistic theory refers to the idea that a community is defined by the interactions between the organisms in it. All species are interdependent, each playing a vital role in the working of the community. Due to this communities are repeatable and easy to identify, with similar abiotic factors controlling throughout.Frederic Clements developed the holistic concept of community, as if it were a superorganism or discrete unit, with sharp boundaries. Clements proposed this theory after noticing that certain plant species were regularly found together in habitats, he concluded that the species were dependent on each other. Formation of communities is non-random and involves coevolution.
The Holistic theory stems from the greater thinking of Holism—which refers to a system with many parts, all required for the system to function.
Individualistic theory
developed the individualistic concept of community, with the abundance of a population of a species changing gradually along complex environmental gradients. Each species changes independently in relation to other species present along the gradient. Association of species is random and due to coincidence. Varying environmental conditions and each species' probability of arriving and becoming established along the gradient influence the community composition.Individualistic theory proposes that communities can exist as continuous entities, in addition to the discrete groups referred to in the holistic theory.