[Biology Class Notes] on Community in Biology Pdf

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:  

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 that 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 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.

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.

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:

Due to the increasing human population and resign consumerism culture demands for natural resources has also been increasing. To satisfy this demand, ind
ustries 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. 

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. 

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.

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.