Community (Ecology): Definition, Structure, Theory & Examples

Community ecology is the study and theory of how populations of organisms interact with each other and react to their non-living surroundings. As a subset of the general study of ecology, this field of specialization explores the organization and functioning of biological communities.

Community ecologists protect the environment and save species from extinction by assessing and monitoring environmental conditions such as global warming.

One of the earliest formal definitions of community ecology was suggested by Cornell professor Robert Whittaker in 1975. Whittaker characterized community ecology as an assemblage of living organisms that interact and form a community with a unique structure and species composition. Knowing how a community functions is vital to promoting and preserving biodiversity.

Community ecology examines how coexisting organisms interact and compete in a particular niche or geographical location such as a woodland, prairie or a lake. Community ecology encompasses all populations of all species that live together in the same area.

Community ecologists study ecological interactions and consider such things as how to intervene when a rising deer population is destroying the understory layer of a woodland.

Community ecology encompasses many types of ecological interactions that continue to change over time. A forest community includes the plant community, all trees, birds, squirrels, deer, foxes, fungi, fish in a forest stream, insects and all other species living there or migrating seasonally.

Similarly, a coral reef community includes a vast number of different species of corals, fish and algae. Abundance and distribution are strong forces that shape the biological community.

Community ecology focuses on how interactions between different species affect health, growth, dispersion and abundance of the ecological system. At the community level, species are often interdependent. Several short food chains are common in most biological communities. Food chains often overlap and form food webs of producers and consumers.

American, European and British scientists have long held many differing theories on the definition of community ecology, which was first called plant sociology. In the 20th century, opinions differed as to whether ecological niches were self-organized organismic communities or random assemblages of species that thrived because of their particular traits.

By the 21st century, theories broadened to include such ideas as the metacommunity theory that focuses on community structures and the evolutionary theory that incorporates principles of evolutionary biology into community ecology.

Currently held community ecology theory is based on the supposition that ecological communities are the result of different types of assembly processes. Assembly processes involve adaptation, speciation in evolutionary biology, competition, colonization, altitude, climate, habitat disturbances and ecological drift.

The theory of community ecology expands upon niche theory, which has to do with an organism having a specific place and role in an ecosystem.

Species richness refers to the richness, or number, of species found. For example, an annual bird count might yield a species richness of 63 different species of birds spotted in a nature center. One pileated woodpecker is counted the same as 50 chickadees in determining species richness of the area.

Species richness does not factor in the total number of individuals found within each species. The number and type of species present in a community gradually increases toward the equator. Species richness decreases towards the polar region. Fewer plant and animal species are adapted to cold biomes.

**Species diversity** looks at overall biodiversity. Species diversity measures species richness as well as the relative number of species present. High species diversity characterizes stable ecological communities. Sudden or significant changes in a community such as an influx of predators can disrupt the predator-prey ecological balance and reduce species diversity.

Community ecologists study the interaction between structure and organisms. Structure describes characteristics of ecological niches, species richness and species composition. Species interact with each other and with their environment in many different ways, such as competing for finite resources or working together to trap game. Population dynamics play a pivotal role in communities.

The **energy pyramid** shows how energy is made and transferred by organisms that comprise the food chain. Heterotrophic producers of usable food energy from the sun form the broad base of the pyramid.

Primary consumers such as herbivores cannot make food to fuel their cells and must eat producers to live. Secondary consumers are carnivores that eat primary consumers. Tertiary consumers devour secondary consumers, but the apex predator at the top of the pyramid has no natural enemies.

A food chain represents the flow of food energy in a community. For instance, phytoplankton are eaten by fish that may be caught and cooked by a human. Only 10 percent of the energy consumed is transferred at each trophic level, which is why the energy pyramid is not inverted. Decomposers play a role by breaking down dead organisms to release nutrients back into the environment.

In biology, interspecific interactions refer to the ways in which species interact in their community. The effect of such interactions on different species may be positive, negative or neutral for one or both. Many types of interactions occur in an ecological community and influence population dynamics.

These are a few examples of those types of interactions:

• **Mutualism**: both species benefit from interaction, such as
bacteria in the gut that speed digestion (+/+).

• **Commensalism**: one species benefits without affecting the
other, such as a spider spinning a web on a plant (+/0).

• **Parasitism:** one species benefits, but the other
is harmed, such as pathogenic microbes (+/-).

• **Predation**: one species preys on the other for survival
(+/-).

• **Competition**: two species fight over limited resources (-/-).

Even small changes in nature can have big effects on community ecology. For instance, structure is influenced by factors such as slight temperature changes, disturbances to habitat, pollution, weather events and species interaction.

Relative abundance of food is a stabilizing factor in communities. Normally, there is a check-balance system of food and consumption.

Foundation species, like coral in a coral reef community, play a pivotal role in community ecology and shaping structure. Coral reefs are commonly called "rainforests of the sea" because they provide food, shelter, breeding areas and protection for up to 25 percent of all marine life, according to the Smithsonian Museum of Natural History. Threats to coral reefs include climate change, pollution, overfishing and invasive species.

Keystone species like wolves profoundly affect community structure relative to the abundance of the other species. If removed, the loss of key predators dramatically changes the entire community. Predators keep other populations in check that would otherwise overgraze and threaten plant species resulting in a loss of food and habitat. Overpopulation can also lead to starvation and disease.

Invasive species are invaders that are not native to the habitat and disrupt the community. Many types of invasive species like the Zebra Mussel, destroy native species. Invasive species grow rapidly and reduce biodiversity, which weakens the overall animal and plant community within that niche.

**Ecological succession is a series of changes over time to community structure that affect community dynamics and encourage the assemblance of plants and animals. Primary succession** starts with the introduction of organisms and species, usually on newly exposed rock. Pioneer species like lichens on rock come first.

Secondary succession happens when orderly recolonization occurs in an area that was previously inhabited before a disruption. For instance, after a wildfire decimates an area, bacteria modify the soil, plants sprout from roots and seeds, bushes and shrubs establish, followed by tree seedlings. Vegetation provides a vertical and horizontal structure that attracts birds and animals to the biological community.