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 is 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 behavioral level are as follows:
Predation (+/- Interaction): A member of a species - predator eats a member of another species - prey. It is beneficial for the predator only.
Mutualism (+/+ Interaction): It involves a long-term interaction between two species where both species get the benefit.
Parasitism (+/- Interaction): A long-term association between two species that is beneficial for one and harmful for another.
Competition(-/- Interaction): Species compete for limited resources. Competition affects both participants negatively. It involves a mutually negative interaction among species of organisms with limited resources.
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 the number 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 create 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 ochraceous also known as purple sea star is an 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 trophical 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 are 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.
Threats to biological communities
Due to the increasing population and the rising demand for humans, the earth’s carrying capacity is eventually decreasing with time. This has created threats to the survival of biological communities.
These threats can be categorized as follows:
Human-induced threats and loss of habitat
Due to the increasing human population and resign consumerism culture demands for natural resources has also been increasing. To satisfy this demand, industries and various organizations have been destroying the natural habitat of several communities. For instance, for medicinal purposes, the horns of rhinos are in great demand and thus they are poached on a very large scale.
Deforestation and man-animal conflict
The process of clearing the existing forest is known as deforestation and this results in indirect loss of habitat for several natural species. This has also resulted in an increasing number of man-animal conflicts in recent times. For instance, the number of leopards coming towards human settlements has increased in the past few years.
Desertification
The process by which fertile land becomes a desert is known as desertification. It usually happens due to deforestation, drought, etc. The newly formed desserts after the process of desertification are not able to hold a large variety of species thus reducing species richness and affecting the structure of the community directly.
Climate change
This is the single biggest change affecting the whole of biodiversity and also biological communities. Temperatures have been rising constantly and have impacted climate over the years. This has led several animal and plants species towards extinction. This affects the biological communities directly.
FAQs on Community in Biology
1. What is a biological community? Provide an example.
A biological community is an interacting group of various species in a common location. It consists of multiple populations of different species living and interacting together. For example, a forest community includes populations of trees, shrubs, insects, birds, mammals, fungi, and bacteria, all interacting within the same habitat.
2. What are the main characteristics of a biological community?
A biological community is defined by several key characteristics, including:
- Species Diversity: The variety of different species present.
- Species Richness: The total number of different species in the community.
- Trophic Structure: The feeding relationships between organisms, forming food chains and food webs.
- Dominance: The degree to which one or several species are more numerous or have more biomass than others.
- Dynamic Nature: Communities change over time through processes like ecological succession.
3. How is a biological community different from a population and an ecosystem?
A population is a group of individuals of the same species living in a specific area. A biological community is a step up, comprising all the different populations of all species (plants, animals, microbes) that live and interact in that area. An ecosystem is even broader, including the entire biological community (all the living, or biotic, factors) plus the non-living (abiotic) components of the environment, such as sunlight, water, soil, and temperature.
4. What are the different types of species interactions within a community?
The interactions between species, known as interspecific interactions, are fundamental to a community's structure. Key types include:
- Competition (-/-): Both species are harmed as they compete for the same limited resources.
- Predation (+/-): One species (the predator) benefits by killing and eating the other (the prey).
- Parasitism (+/-): One species (the parasite) benefits by living on or in another species (the host), which is harmed.
- Mutualism (+/+): Both interacting species benefit from the relationship.
- Commensalism (+/0): One species benefits while the other is unaffected.
5. What are foundation and keystone species? Explain with examples.
Foundation species are organisms that create or significantly modify a habitat, forming the 'foundation' of the community. Their presence allows many other species to exist. A classic example is coral, which builds reefs that provide a complex habitat for countless marine organisms. Keystone species have a disproportionately large effect on their community relative to their abundance. Their removal can cause the community structure to collapse. For example, the sea otter is a keystone species because it preys on sea urchins, which would otherwise overgraze and destroy kelp forests.
6. How do ecologists measure the structure of a community?
Ecologists measure community structure primarily through two key metrics:
- Species Richness: This is a simple count of the number of different species present in the community. A community with 100 species is richer than one with 50 species.
- Species Diversity: This is a more complex measure that considers both the number of species (richness) and their relative abundance (how evenly distributed the individuals are among the different species). A community where species have similar population sizes is considered more diverse than one where a single species dominates.
7. Why is species diversity considered crucial for the stability of a community?
High species diversity is crucial for community stability because it enhances resilience and productivity. A more diverse community has a greater variety of species performing similar roles. If an environmental disturbance, like a drought, harms one species, another, more tolerant species can take its place, ensuring the ecosystem function (like nutrient cycling) continues. This functional redundancy prevents the collapse of the entire community, making it more stable and able to resist and recover from disturbances.
8. How can natural disturbances like forest fires actually benefit a community?
While seemingly destructive, natural disturbances like forest fires can be beneficial by promoting ecological renewal and increasing biodiversity. This concept is part of the Intermediate Disturbance Hypothesis. Fires can:
- Clear out dense, dominant vegetation, allowing sunlight to reach the forest floor and promoting the growth of new, different plant species.
- Release essential nutrients back into the soil from burned organic matter.
- Prevent a single dominant species from outcompeting all others, thus creating a more varied (heterogeneous) habitat that supports a wider range of species over the long term.
9. What is 'ecological succession' and how does it change a community over time?
Ecological succession is the predictable and orderly process of change in the species structure of a biological community over time. It demonstrates that communities are not static. For example, after a volcanic eruption creates new, barren land (primary succession), pioneer species like lichens colonise it. They break down rock and create soil, paving the way for grasses, then shrubs, and finally a stable forest community. This gradual replacement of one community by another illustrates the dynamic nature of ecological systems.
10. In what ways do human activities threaten the integrity of biological communities?
Human activities pose significant threats to biological communities by disrupting their structure and function. Key threats include:
- Habitat Destruction: Clearing forests for agriculture or urban development (deforestation) directly eliminates the homes of countless species, fragmenting communities.
- Pollution: Contaminants in air, water, and soil can harm or kill sensitive species, altering community composition.
- Introduction of Invasive Species: Non-native species introduced by humans can outcompete, prey on, or spread diseases to native organisms, destabilising the community.
- Climate Change: Altered temperature and rainfall patterns stress species and can lead to the loss of entire communities, like coral reefs through bleaching.
