An ecological community is all the populations of different species that live and interact in the same area. A community can include plants, animals, fungi, bacteria, and other organisms. What makes it a community is not simply that these species occur in the same place, but that they interact with one another.
For example, a forest community may contain trees, shrubs, grasses, insects, birds, mammals, fungi, and microorganisms. These organisms compete for resources, eat one another, pollinate plants, spread seeds, decompose dead material, and affect the conditions in which other species live.
What makes up an ecological community?
The basic units of an ecological community are populations. A population is a group of individuals of the same species living in a particular area.
A community contains multiple populations of different species. In a pond, for example, populations of algae, aquatic plants, insects, fish, frogs, fungi, and microorganisms may all form part of the same community.
The organisms do not need to interact equally or directly with every other species. One species may affect another indirectly. For instance, a predator can reduce the abundance of a herbivore, which may allow certain plants to become more abundant. In this way, species can influence one another through a network of relationships.
How species interact within a community
Interactions among species are a central feature of ecological communities. Some interactions benefit one or both species, while others involve competition or harm.
Competition occurs when organisms use the same limited resource, such as food, water, light, nutrients, or space. Plants growing close together, for example, may compete for sunlight and soil nutrients.
Predation occurs when one organism captures and consumes another. Herbivory, in which animals eat plants or algae, is also an important form of consumption.
Mutualism is an interaction in which both species benefit. Pollination provides a familiar example: an animal may obtain food from a flower while transferring pollen between flowers.
Parasitism benefits one organism while harming its host. Parasites can therefore influence the abundance, health, and distribution of other populations within a community.
Species also interact with organisms that do not directly consume or compete with them. Fungi and microorganisms, for example, play important roles in breaking down organic material and recycling nutrients.
Communities include more than the most visible organisms
An ecological community is not limited to the plants and animals that are easy to see. Microorganisms can make up a major part of a community and perform essential ecological functions.
Bacteria and fungi, for example, help decompose dead organisms and organic material. Their activity releases nutrients that can become available to plants and other organisms. Microorganisms can also form close relationships with plants and animals.
This means that understanding a community often requires looking beyond its largest or most familiar species. Small organisms can have effects that influence the entire community.
How a community differs from an ecosystem
An ecological community consists of the living organisms and their interactions. An ecosystem includes the community plus the nonliving physical environment.
The nonliving components of an ecosystem include factors such as temperature, water, sunlight, soil, minerals, and other physical and chemical conditions.
For example, the organisms living in a lake constitute a biological community. The lake’s water, sediments, nutrients, temperature, light conditions, and other physical factors are also part of the lake ecosystem.
The distinction is useful because organisms are affected by their physical surroundings, but the terms describe different levels of organization.
Communities have structure
Ecologists study community structure by examining which species are present, how abundant they are, and how they interact.
Two communities can contain similar types of organisms but have very different structures. One forest, for example, might be dominated by a few tree species, while another might contain many tree species in relatively different proportions.
Species richness refers to the number of different species in a community. Species abundance describes how many individuals of each species are present. Together, these characteristics help describe the community’s composition.
Ecologists may also consider characteristics such as the physical distribution of organisms, the strength of interactions among species, and the roles different species play.
Why ecological communities change
Communities are not fixed collections of species. Their composition and structure can change over time.
Seasonal changes can alter which organisms are active or abundant. Drought, storms, fire, disease, and changes in temperature can affect populations. Predators may become more or less common, new species may arrive, and existing species may decline or disappear.
Human activities can also alter communities by changing habitats, introducing non-native species, removing organisms, polluting environments, or changing resource availability.
When environmental conditions change, species do not all respond in the same way. Some may tolerate the new conditions, while others may decline. As populations change, their interactions with other species can change as well, producing further effects throughout the community.
Ecological communities are connected networks
The most useful way to understand a community is as a network of interacting populations rather than a simple collection of species.
A change in one population can affect others through predation, competition, mutualism, or changes in resources. For example, a decline in a plant population can affect the herbivores that feed on it, which can then affect predators that depend on those herbivores.
These connections mean that the effects of an environmental change can spread beyond the species directly affected. The structure of those interactions helps determine how a community responds to disturbance and how it changes over time.
An ecological community, therefore, is a living network: different species share a place, interact with one another, and influence the abundance and distribution of organisms around them.
