Community in Biology

Community Definition and Concept

In biological terms, a community is defined as a population of different species of organisms interacting in a common Environment (Habitat). A group of organisms is a population of species in a specific region at a specific time. Different populations have different characteristics such as natality, age structure, growth dynamics and others. 

A population changes the genetic structure over time and the process is called ecological succession. So, the community is dynamic as it changes over time. Communities may be small with few spe­cies or large with several species populations. 

Their interactions are called interspecific interactions that affect the distribution, abundance i.e. number of each species, and the existence of members of a community. 

For example, a forest is a biological community where various species of animals, plants, bacteria and fungi are inhabitants and constitute a community. 

In a community, most organisms are interdependent for food. This dependency on each other is represented by a food chain where each organism occupies a specific place and known as a trophic level. 

For instance, Predators like eagles feed on herbivores. Scavengers like vultures prey on dead animals for food. 

A community includes populations of different species of organisms. But it does not include two populations of the same species. 


Types of Interactions

Organisms undergo a rich array of interactions. The types of interactions at the reproductive and behavioural level are as follows:  

  1. Predation (+/- Interaction): A member of a species - predator eats a member of another species - prey. It is beneficial for the predator only.

  2. Mutualism (+/+ Interaction): It involves a long-term interaction between two species where both species get the benefit.

  3. Parasitism (+/- Interaction): A long-term association between two species which is beneficial for one and harmful for another. 

  4. Competition(-/- Interaction): Species compete for limited resources. Competition affects both participants negatively. It involves a mutually negative interaction among species of organisms with the limited resource. 


The result of interactions changes as the environment changes. Some of these interactions may increase diversity, while others may decrease it. Diversity is one of the most important characteristics of a community.


Structure of Community

Community structure is the composition of a community that includes the number of species and their relative numbers. The structure of a community is based on its species richness. Communities can be different in terms of types of species and numbers of species they consist of. 

Many factors affect the community’s structure for instance abiotic factors (non-living), interactions, disturbances, and other events.

There are two important measures to describe the composition of a Community:

1. Species Richness: It represents the number of different species in a community. If there are 252525 species in a community, and 250250250 species in another, the second community has high species richness. Rich communities are found in areas that have lots of solar energy, warm temperatures, heavy rainfall, and little seasonal change. 

2. Species Diversity: It is a function of the number of species in the community i.e. species richness and their relative species. Larger numbers of species tend to have higher species diversity. 


Foundation and Keystone Species 

Community structure is affected by some special species. These special species are foundation and keystone species.

  • Foundation Species 

Often, foundation species modify the environment to support the other organisms in the community. For example, Kelp (brown algae) is a foundation species that creates environments for the survival of other organisms in the kelp forest community.

  • Keystone Species 

A keystone species affects community structure disproportionately related to its abundance. Keystone species are more likely to belong to higher trophic levels. The 'Pisaster ochraceus' also known as purple sea star is the example of a keystone species.


Categorization

The communities are categorized in various ways. Categorization can be qualitative or quantitative based. For example, quality based plant communities are dependent on the availability of water, light, etc.  

Communities based on water - hydrophytic - aquatic habitats, mesophytic - moderately and xerophytic.

Communities based on abundant light: heliophytes and sciophytes. 

In a tropical rainforest, we find giant trees as high as 40m and obtain sunlight. They support each other for survival. This is also an example of a community.


Community Dynamics

Communities are dynamic systems as they change over time. The changes are slow and moderate but need to observe at regular intervals for a long period. For example changes in plant communities occur seasonally at every place.

The pro­cess of change in a community and its environ­ment over time is known as “ecological succession”. 


How Disturbances Affect Communities 

There are other phenomena also than species interactions through which a community may change such as dispersal or the movement from one place to another.

Dispersal means a community in an area can influence a community composition at some other place.

For instance, the composition of lizards on islands changes dramatically following hurricanes. Animals killed in floods during hurricanes and float from one island to another during and after the storm.

Many natural phenomena and disturbances, like forest fires, no doubt, are destructive, but they are natural occurrences that bring changes. 

FAQ (Frequently Asked Questions)

Question 1. What is the stability of a Community? Does Diversity increase stability?

Answer: Stability is a measure of a community's ability. It tells the ability to return to a condition that was before disturbance.


When many different species inhabit an area, the resources are optimally used. In doing so, the community is strong because when the abundance of one species decreases, for instance during a drought, the abundance of more drought-tolerant organisms increases.

Question 2: What are the factors that affect the structure of the Community?

Answer: Factors to shape community structure

  • Climate patterns and Geography of the Location - Distance from the equator, variation in temperature, Island or mainland communities etc 
  • Heterogeneity (patchiness) -  Heterogeneity refers to more variation in the environment that allows greater species richness because of various habitats to be occupied. 
  • The Frequency of Disturbances - Disruptive events such as storms, landslides, fire affect the community structure. Communities with a medium level of disturbance would have greater species diversity than communities with frequent or rare disturbances.
  • Type of Interactions between Organisms - Interactions between organisms such as competition, predation, mutualism has the potential to shape a community.