Communities, Practice Problems and FAQs

We like to watch the behaviours of different types of animals. Like, how they live in their habitat, how they eat, how they interact with other individuals of their species etc. So, what is the easiest way to know more about wild animals? Yes, it is a zoological park. If we go there, we can see the animals which are caged, or sometimes in an open space. But, all the animals will be in separate cages, so that everyone can easily identify them. Inside the zoo a herd of deer will be in one cage, a streak of tiger in another cage, a troupe of monkeys in another cage and so on. Hence we can call each of them a population; a population of deer, a population of lions and a population of monkeys. So in zoological parks we cannot see the interaction between different species of animals. 

                                 Fig: Animals caged in a zoological park

We have to go to a biological reserve to see the natural habitat of animals and their interaction between other organisms around them. So in a biosphere reserve we can see the tigers and deer together in the same place and we can see the monkeys climbing through the trees. Biosphere reserves are also a way to conserve the animals and plants. Hence there will be more species together in the same place. So here we cannot call them a population, because there are more species in a particular area. Then what will we call them? We can call them a community!! It will be the collection of different species of animals and plants. We can specifically call them a biotic community. Hence it is also clear that communities play an important role in the maintenance of the ecosystem. Now we will discuss more about the biotic communities in this article.

Table of contents:

• Communities
• Types of communities
• Characteristics of communities
• Population and community
• Practice Problems
• FAQs

Communities 

A group of organisms of different species living together in a given area is called a biotic community. A biotic community is also called biota or biocenosis. All the organisms in a biotic community interact directly or indirectly with each other within the habitat. The biotic interactions can be herbivory, parasitism, prey-predator interactions, competition, mutualism etc. Along with the abiotic factors (wind, rain, temperature etc), the biotic factors make up an ecosystem. 

                                              Fig: Community 

Components of community 

The biotic community has three major components. They are the plant community, animal community and microbial community. 

Plant community 

Plant communities are groups or associations of plants that coexist in a certain habitat. Plants from two or more different species make up a plant community. The term vegetation refers to the association or collection of plant communities in any region. Frequency, diversity, cover, biomass, leaf size, abundance, dominance etc., are the major quantitative characters of a plant community.

                                                Fig: Plant community 

Stratification in tropical forest

If we observe the tropical forests there are four vertical strata of the plant community. The vertical distribution of various species in different layers in a forest is called stratification. They are the canopy or crown layer, shrub layer, herb layer and moss layer or ground layer.

Canopy or crown layer

It is the topmost layer of the plant community. Hence it is a dominant layer. This layer includes the canopy of the largest trees. They may be tall up to 30 - 50 m in height.

Shrub layer

This is the second layer. This layer is seen below the canopy. It is represented by the shrubby plants. 

Herb layer 

This is the third layer and it represents the herbaceous pants. 

Moss layer or ground layer

The fourth and last layer represents the mosses which grow on the ground surface is called moss layer. Ferns also can be seen in this layer. 

                           Fig: Stratification in the forest ecosystem

Animal community

Animal community is the aggregation of animal species which are held together in a continuous or discontinuous geographic area. But they will be tied to the same vegetation or the physical environment. There are many benefits for animals living in a community. One is the protection from predators, because a large group of animals will be less attacked by predators. Another benefit is the presence of more females to protect the young ones. It will be easy for animals to look for food sources if they are living in a community. They have higher chances of survival when they cooperate rather than when they live alone. The different species of fishes and zooplanktons living in the sea is an example of the animal community. 

                       Fig: Community of different animal populations in sea

Microbial community

The assemblage of co-occurring and potentially interacting microbes are called a microbial community. They are present in a defined habitat in space and time. Even though they are microscopic, they play a crucial role in the biological dynamics of the biosphere. All the major nutrient cycles of the Earth depend on microbial populations. They also play an important role in the desert ecosystem where plants are less or even totally absent. We can consider the different types of microbes present inside the human intestine as a microbial community. There you can see different species of microbes like Lactobacillus, Clostridium perfringens, Staphylococcus, Streptococcus etc. 

                                Fig: Microbial community inside the intestine 

Types of community 

There are two types of community based on the size. They are the major community and the minor community. 

Major community 

The large, self regulating and self sustaining community is called a major community. It is an independent unit comprising a number of minor commu­nities. It is an assemblage of three minor communities like a faunal community (animals), a floral community (plants), and a microbial community.

                  Fig: Components of major community

These groups of communities are typically quite autonomous from other groups of communities. Examples of a major community include a lake, forest, grassland, or pond. This community can be long lasting and it will become a mature community after a period of time. Such communities will only contain organisms which are successfully adapted to the particular environment. They will also coexist with the other species within the community. For example, the organisms present in a forest ecosystem.

                               Fig: Communities in a forest ecosystem

Minor community 

The smallest communities which are not self-sustaining are called minor communities. These communities will depend on other communities for their existence. If we consider the major community forest, it will contain many minor communities like the plant community (the plant population of the forest), the animal community (the animal population of the forest) and the microbial com­munity (bacteria and fungal population). 

                                           Fig: Minor communities 

Example for minor community

For example, the collection of organisms, which lives within a piece of deadwood on the forest floor can be taken as an example of a minor community. Here the animal community will include termites, bugs etc. The plant community may include mosses and the microbial community will include some saprotrophs like fungi and bacteria. 

     Fig: Minor community on a log of wood

Characteristics of community

Each biotic community is made up of a wide variety of organisms from various kingdoms of living things. Communities also differ substantially in terms of the variety of species and population density. There are several characteristics that can be used as descriptors to distinguish communities despite their very diverse features. Some of the characteristics of the community are species diversity, dominance, succession, trophic organisation and interdependence. Let’s discuss more about this. 

Species diversity

The number of different species of organisms present in an ecosystem and their relative abundance is defined as the species diversity.

                         Fig: Species diversity in ocean ecosystem

Constituents of species diversity

There are two major attributes of species diversity. They are species richness and species evenness. 

Species richness

The number of species of organisms present in an ecosystem refers to species richness. Due to the favourable habitat for a large number of species, tropical locations have higher species richness.

Species evenness 

The relative abundance of each species of organisms refers to the species evenness. A species is said to have high evenness if the number of individuals is generally constant throughout communities, and low evenness if the number varies from community to community. For greater species diversity there should be high evenness. 

Dominance 

At each trophic level, there are typically one or two species that have a more dominant impact on the structure and function of the community than others. This might be as a result of their size, population, or behaviours that have an effect on other living things or the environment. They are hence called ecological dominants. The co-action between two or more species results in the dominance of one species in the community. For example, the grass species are dominating over other herbs in a grassland ecosystem. 

                                              Fig: Grassland ecosystem 

Succession

Communities can also be distinguished by the stage of their succession. Ecological succession is the gradual and predictable transition in the species composition of a given area. There are two types of succession. They are the primary succession and secondary succession.

Primary succession

The biotic succession that occurs on a substratum devoid of earlier life like in a bare rock or uninhabited area is called primary succession. It can occur after a severe ecological disturbance like a volcanic eruption. For example, primary succession on bare rock.

                                          Fig: Primary succession on a bare rock

Secondary succession 

Secondary succession takes place in an area which has become bare due to destruction of previously existing biotic communities by fire, landslide, drought, earthquake etc. It can occur in a deforested area or an abandoned farmland. Secondary succession occurs much more rapidly than primary succession, because the soil has enough nutrients, which are needed for the favourable growth. It also has life forms like seeds and roots. 

                               Fig: Secondary succession in a deforested area

Types of communities in succession 

There are three major communities which are used to describe the plants occurring in different stages of a succession. They are the pioneer community, seral communities and the climax community.

Pioneer community

The first plants whose seeds or spores migrate from surrounding areas and germinate successfully on a bare area are called pioneer species. They are the first to make up the community. For example phytoplanktons are the pioneer species in hydrarch succession (succession in shallow water). 

                                            Fig: Pioneer community

Seral communities

The intermediate communities present between the pioneer community and climax community are called seral or transitional communities. They are the transitional communities with intermediate size. They help to increase the nutrients in the soil through biogeochemical cycling. The entire series of communities of biotic succession from pioneer to climax community is known as sere. For example, submerged stage, floating stage and reed swamp stage are some of the seral stages in hydrarch succession. 

                                    Fig: Seral communities in hydrarch succession

Climax community

The most stable community in the succession is the climax community. This is a self regulating biotic community which is the last stage of the succession established after many years. Most of the species in the climax community are longer-lived and large species. They have high niche specialisation, complex food webs and interdependent relationships. 

                                                Fig: Climax community

Trophic organisation

The living organisms occupy a place in the natural surroundings or communities according to their feeding relationship with other organisms. The specific place occupied by different organisms in the food chain in an ecosystem constitutes trophic levels. There are three main groups in the trophic levels in a community. They are producers, consumers and decomposers. 

Producers

They are also known as autotrophs. They can prepare their own food through photosynthesis using energy from the sun. Examples include green plants and algae.

                                            Fig: Producer

Consumers 

They are also known as heterotrophs. They obtain their energy or nutrition from other organisms. There are three types of consumers. They are the primary, secondary and tertiary consumers. Primary consumers include herbivores like rabbits which feed on grass. Secondary consumers include the carnivores and omnivores. Examples include snakes. The tertiary consumers also include the carnivores and omnivores. Examples include eagles. 

Decomposers

They are also known as heterotrophs. They also obtain nutrition from dead plants and animals. They help in the recycling of nutrients back into the Earth. 

                                                       Fig: Ecological pyramid

Food web 

A network of food chains which are interconnected at various trophic levels to form a number of feeding connections amongst different organisms of a biotic community is called a food web. Now it is clear that communities play a major role in maintaining the stability of the food web. The food web is crucial for the proper dynamics of energy transfer in an ecosystem as well as the feeding relationships between species within a community. It also reveals species interactions and community structure, because every organism in each trophic level is connected to the organisms in the next trophic level.

                                 Fig: Food web in an ecosystem

Interdependence 

Each kind of organism, whether it be a bacterium, an animal, or a plant, that makes up the community is undoubtedly influenced by other species in the community as well as by the inanimate physical and chemical environment in its existence and activities. It is known as interdependence. The three major forms of interdependence are nutritional interdependence, reproductive interdependence and protective interdependence. Let’s see what these are.

Nutritional interdependence

The transfer of nutrients and energy through feeding is referred to as nutritional interdependence. 

If we consider an insect which can feed only from a particular plant species, then we can say that the life of the insect will be more dependent on that plant. The silkworm feeding on mulberry leaves can be taken as an example for nutritional interdependence. 

                              Fig: Silkworm feeding on mulberry leaves

Reproductive interdependence

The dependency of an organism over another organism to reproduce is termed as the reproductive interdependence. The most common example is the relationship between the pollinator and the plants. During pollination, the pollinator interacts with flowers for nectar. But it also carries the pollen grains unknowingly to other flowers. Hence this kind of interaction provides reproductive success to the plant community.

 
                 Fig: Interaction between pollinator and flower

Protective interdependence

Most species need some degree of shelter, and they sometimes depend on other organisms in the community to get it. This is called protective interdependence. The insects living on a tree can be taken as an example. The insects are dependent on the leaves or branches of the tree for their shelter. It also provides them protection from the predator birds. 

                                             Fig: Bugs on the tree

Population and community

A population and a community are necessary to describe how organisms interact in an environment. Hence we can define the population as the group of interbreeding individuals of the same species. They will be isolated from other groups. While the community can be defined as a group or a collection of populations of two or more different species. Both the population and community are two ecological layers that include various groups of species residing in a particular habitat at a particular period. In a specific ecosystem, a community is a collection of populations, whereas an ecosystem is made up of both a community and abiotic factors. There are some similarities and differences between population and community. First we will see what are the similarities between population and community.

                     Fig: Organism, population and community in an aquatic ecosystem

Similarities between population and community

There are major three similarities between the population and community as follows: 

• They consist of groups of organisms living in a particular ecosystem at a particular time period.
• They comprise biotic factors.
• They are important while describing the ecological relationships between individuals in a particular ecosystem.

Differences between population and community

The following are the major differences between the population and community:

Practice Problems

Q 1. Among the following statements, find out the wrong one.

I) The association or group of plant communities of any region is called vegetation.
II) One benefit for animals living in a community is the protection from predators.
III) The large, self regulating and self sustaining community is called a minor community.
IV) Minor communities will depend on other communities for their existence.

a. I, II, IV
b. I and II
c. III only
d. IV only

Answer: Plant communities are groups or associations of plants that coexist in a certain habitat. Plants from two or more different species make up a plant community. The term vegetation refers to the association or collection of plant communities in any region. Animal community is the aggregation of animal species which are held together in a continuous or discontinuous geographical area. There are many benefits for animals living in a community. One is the protection from predators, because a large group of animals will be less attacked by predators. The large, self regulating and self sustaining community is called a major community. It is an independent unit comprising a number of minor commu­nities. The smallest communities which are not self-sustaining are called minor communities. These communities will depend on other communities for their existence. Hence the correct option is c.

Q 2 . Choose the correct option for the assertion and the reason given below.

Assertion: At each trophic level, there are typically one or two species of ecological dominants.

Reason: Size, population, or behaviours of some organisms have an effect on other living things or the environment.

a. Both the assertion and the reason are true and the reason is the correct explanation of assertion 
b. Both the assertion and the reason are true but the reason is not the correct explanation of assertion
c. Assertion is true but the reason is false
d. Both the assertion and reason are false

Answer: Dominance is one of the characteristics of the biotic community. At each trophic level, there are typically one or two species that have a more dominant impact on the structure and function of the community than others. This might be as a result of their size, population, or behaviours that have an effect on other living things or the environment. They are hence called ecological dominants. The co-action between two or more species results in the dominance of one species in the community. For example, the grass species is dominating over other herbs in a grassland ecosystem. Hence the correct option is a.

Q 3. What is reproductive interdependence?
Answer: The dependency of an organism over another organism to reproduce is termed as the reproductive interdependence. The most common example is the relationship between the pollinator and the plants. During pollination, the pollinator interacts with flowers for nectar. But it also carries the pollen grains unknowingly to other flowers. Hence this kind of interaction provides reproductive success to the plant community. 

Q 4. How species richness is different from species evenness?
Answer: The number of different species of organisms present in an ecosystem and their relative abundance is defined as the species diversity. There are two attributes of species diversity. They are species richness and species evenness. The number of species present in an ecosystem refers to species richness. Due to the presence of favourable habitats for a large number of species, tropical locations have higher species richness. The relative abundance of each species refers to the species evenness. A species is said to have high evenness if the number of individuals is generally constant throughout communities, and low evenness if the number varies from community to community. For greater species diversity there should be high evenness. 

FAQs

Q 1. What is an ecotone?
Answer: The area that acts as a boundary or transition between two ecosystems is called an ecotone. An area of marshland between a river and its riverbank is an example. They have a great environmental importance. In ecotone there will be more communities. 

Q 2. What is an edge effect?
Answer: When there is a change in the population or community in a boundary of two or more habitats then it is known as edge effect. More edge effects are observed in areas with small habitat fragments. The changes in the community at the boundary when a river and forest meet can be explained as an example of edge effect. 

Q 3. What is neutralism?
Answer: If two species are interacting and the interaction has very less effects on either species, then it is known as neutralism. Neutralism is rare due to the interconnectedness between the communities. Neutralism is hard to explain in the ecosystem, because there will be indirect effects of species on each other. 

Q 4. What is limiting similarity?
Answer: Limiting similarity is a concept explained in the community ecology and theoretical ecology. The study of ecological systems using theoretical techniques, such as simple conceptual models, mathematical models, computational simulations, and advanced data analysis, is the theoretical ecology. Limiting similarity proposes that if there is maximum level of niche overlap between two species, then there will be continued coexistence. 

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