A biotic community, also known as a biota or ’biocoenosis’, is the group of organisms that live together and interact with each other within an environment or habitat. Together, the biotic community and the physical landscape or abiotic factors make up an ecosystem.
Communities consist of a group of different species, which partake in direct and indirect biotic interactions, such as predator-prey interactions, herbivory, parasitism, competition and mutualisms. Alternatively, the interrelationships may take a more diffuse route, such as an organism that creates certain necessary climatic conditions, or one that acts as a substrate for another organism.
Types of Community
There are two main types of community.
A major community is the smallest ecological unit which is able to sustain itself and is self-regulating. These communities are usually relatively independent of other communities, for example a pond, a forest, a grassland or lake. Long lasting and mature major communities contain only those organisms, which are successfully adapted to the environment and to the other species within the community.
A major community is an assemblage of a faunal community or ’zoonenosis’, a floral community or ’phytocenosis’, and a microbial community or ’microbiocenosis’.
Minor communities, or merocenoses, which make up major communities, are smaller ecological units that are not individually self-sustaining and rely on interactions with other communities. An example of a minor community is the collection of organisms, which lives within a piece of deadwood on the forest floor.
Characteristics of a Community
The features within communities are highly variable, and there are a number of characteristics that can be used as descriptors to distinguish them.
Each organism within a community can be categorized within a specific trophic level, which relates to the way which it obtains nutrition. These trophic levels can be divided into three main groups: i) primary producers (also known as autotrophs) manufacture their own food using energy from the sun to perform photosynthesis. Primary producers are usually green plants and algae. ii) Consumers, or heterotrophs must obtain their nutrition from other organisms. Primary consumers, or herbivores eat the plant material while secondary and tertiary consumers, carnivores or omnivores, eat the primary consumers. iii) Decomposers (which are also heterotrophs) consume dead plant and animal material, recycling the nutrients back in to the earth.
Communities can be described by the way that the energy is transferred through these trophic levels. For example, in a grassland community, the grass (primary producer) is consumed by a mouse (primary consumer), which is consumed by a snake (secondary consumer), and subsequently an eagle (tertiary consumer). The dead body of the eagle may be consumed by fungus (decomposer).
Each interaction, from the sun’s energy to the decomposers, makes up a link in the food chain. It is usually the case however, that a number of primary producers are eaten by several different primary consumers, which are subsequently eaten by various secondary consumers. This lack of specialization results in various interconnected links within a food chain and so the nutritional relationships take the form of a food web instead.
There is usually one or two species at each trophic level, which exert a more dominant influence over the function and structure of the community than others. This may be due to their physical size, population numbers, or activities that have an impact upon other organisms or the environment. These so called ’ecological dominants’, can have a major effect on the nature of the community.
Plants usually dominate land communities, and so the name of the community is often based on the ecologically dominant vegetation, for example Douglas-fir Woodland or Rocky Mountain Maple Forest. The ecological dominants may be responsible for modifying the abiotic conditions of a habitat, although rare species might be equally as important for the correct functioning of the community. For instance, in a forest, a dominant tree species may control amount of light available to other plants, the temperature in the lower canopy, and the nutrients that are available to other organisms, whilst their reproduction may depend on pollination by a rare insect
Communities are not just a random mixture of plants, animals and microbes; each of the organisms within a community has a fundamental dependence on at least one other, although most organisms will engage in multiple interactions.
There are three main forms of interdependence.
Nutritional interdependence describes the transfer of energy and nutrients through feeding. Certain organisms may be more reliant on the presence of others to fulfill their nutritional requirements, for example insects that can feed only from one species of plant.
Reproductive independence can take several forms. A common example is that of pollination, which is present within most communities. Whilst for the pollinator the interaction provides a food source of nectar, for the plant, the interaction is essential to its reproductive success. Certain species may only be able to reproduce on a particular plant or substrate and are therefore dependent on the presence of this within the community. Other reproductive independences involve parasitic interactions, for example cuckoos, which lay their own eggs in the nests of other birds.
Protective interdependence is the third main interaction. Most organisms require a level of shelter, and may rely on other organisms within the community for this. For example, insects living on a tree are dependent on the leaves and branches to shelter them from predation by birds.
Interactions between community members are not always linear and can involve several highly complex interactions. Many of such interactions may take place only under precise environmental conditions. An example of this is the symbiosis between corals and their the photosynthetic algae which live within their body structures. The interaction supplies the coral with energy and the algae with nutrients; however, the algae only remain within the body under certain temperatures. If the upper limits of the temperature threshold are crossed, the algae are expelled and the coral cannot survive. The complexity of the interactions between species signifies the delicate balance within communities.
Descriptions of the community structure relate to both the species richness, which is the total number of species, and the species diversity, a community complexity measurement which takes in the species richness as well as their relative abundances (i.e. 5 individuals rather than 100 individuals). Communities in which species exhibit higher species richness and evenness (the numbers of individuals in each species present are more equal) are considered to be more diverse.
The structure of a community may be determined by its natural history, i.e. the chance colonization event of a population onto an island, by (non-living) abiotic factors such as the climatic patterns, the geography and the habitat location, or by (living) biotic factors such as the presence of other organisms which exert pressures such as predation or competition.
Communities at tropical latitudes tend to display high species richness and diversity, due to the high productivity of plants, which receive large amounts of solar energy, and have year-round climatic stability. Alternatively, community structure in habitats such as arctic tundra are very different – usually displaying lower species richness as a result of fewer basic resources such as sunlight and nutrients.
As a general rule, communities that have more species diversity are more resilient against ecosystem damage.
Growth Form & Succession
A community can be described by major categories of its growth form. For example mosses, herbaceous plants, shrubs or trees.
Communities may also be characterized by their successional stage. Ecological succession is the progressive and predictable replacement of one type of community by another, over time. Primary succession is the initial colonization of a bare landscape which has not previously been occupied, often following a significant ecological disturbance such as a volcanic eruption. Secondary succession occurs where a community has existed previously but has been removed from a landscape, for example, an area of deforestation or an abandoned cropland. In this case the nutrients within the soil are already present, and conditions for growth are favorable and so secondary succession happens much more rapidly than primary succession.
Pioneer species are the first to make up the community within a bare landscape once their seeds or spores migrate from surrounding areas and successfully germinate. These pioneer communities consist of fast growing, hardy plants with a short lifespan and low biomass, requiring very little nutrients. The roots of pioneer species contain nitrogen-fixing bacteria, which are important for the formation of soil and other organic material.
Seral communities develop in the area after the pioneer community. These transitional communities consist of intermediate sized species such as shrubs and heaths, which have high biomass and high nutritional content. These species further build the soil and nutrients with biogeochemical cycling.
The climax community is the stable, self-regulating biotic community, which establishes after many years. It contains longer-lived and larger species with high niche specialization, complex food webs and mature interdependent relationships. Diversity is highest within climax communities and the community is at equilibrium with the habitat and climate.
Natural climax communities usually exhibit some form of stratification, by which the populations that make up the community are distributed into defined vertical or horizontal strata.
For example, the bottom-up stratification of a forest community could be divided into:
- The subterranean layer
- The forest floor
- The herbaceous vegetation
- The shrub layer
- The canopy layer
Organisms may not occupy only one stratum, moving between the layers often on a diurnal basis. For example, a bird that feeds on the forest floor during the day but roosts within the canopy.
A community may occur along a horizontal stratification where there is transition between successional stages and ecotones.
Communities occur in a range of different sizes, and the boundaries of each are often not well defined. An ecotone is the transitional area between two biomes, where communities meet and may integrate.
Many organisms may be part of several different communities because they have various geographic ranges, and density peaks; if these boundaries are wide, it is known as an open community. A community in which the species all have similar geographic ranges and density peaks, resulting in a discrete unit where the boundaries are well defined, is called a closed community.
Open communities tend to occur where there is a long environmental gradient, such as that of soil moisture content or the altitudinal slope of a mountain. Organisms with different tolerances to the conditions occur at different spatial scales along the gradients.
Closed communities occur where there is a sharp change in the vegetative structure or the physical environment, for example, an area of a beach, which separates the water from the land.
Ecotones are generally very hard to define because within an ecosystem there are usually organisms, which can disperse between both open and closed communities.