Community is composed of all populations living in an area. Communities are not random mixtures of species. They have a unique organization. Most communities have certain important members. These members determine the characteristics of community. For example, a stream community has a large population of rainbow trout. These trouts determine the population of certain invertebrate populations. Trout feed on these invertebrates. Species that determine community characteristics are called dominant species.
Communities contain variety of animals. These animals also characterized • the community. This variety is called community diversity or richness. Different factors promote high diversity. These factors are variety of resources, high productivity, climatic stability, moderate levels of predation, and moderate levels o’ disturbance from outside the community. Pollution often reduces the species diversity of ecosystems.
THE ECOLOGICAL NICHE
MI the attributes of lifestyle of an animal is called ecological niche. These a tributes are:
• Where It looks for food
- What it eats
- Where it makes nest
- What conditions of temperature and moisture it requires.
The ecological niche is an important concept of community structure Sometimes, the niches of two species overlap. It causes competition between these species. The concept of niche helps to understand community structure. The community members complement each other in the use of resource. They try to partition the resources rather and try not to compete for them. The niche concept gives role of
an animal in the environment. •
Communities are born and die. There is a time of continual change between those events. Climatic or geological events cause some changes. Members of the community are responsible for other changes. The dominant members of a community change a community. The process of change of a community in a predictable ways is called succession. Communities can begin in areas where life does not occur. There are following types of communities:
- Pioneer community: The first community establish in an area is called the
pioneer community. Additional nutrients are added to the community by death and decay of older organism.
- Other seral stages: The nutrients accumulate over thousands of years. Therefore, the characteristics of the ecosystem are changed. Each successional stage is called a seral stage. The entire successional sequence is called sere. The dominant life forms of a sere make the area less favorable for themselves. But it makes it favorable for organisms of the next successional stage. Therefore, succession occurs there.
- Climax community: The final community is called climax community. It. can tolerate its own reactions. Therefore, it is different from the earlier seral stages. Accumulation of the products of life and death does not make it unfit for the individuals living there. Climax communities usually have complex structure. It has high species diversity.
TROPHIC STRUCTURE OF ECOSYSTEMS
Communities and their physical environment are called ecosystems. Energy is constantly used in an ecosystem. Once. this energy leaves the ecosystem this energy is never reused. It is the important fact of ecosystems. Energy supports the activities of all organisms in the ecosystem. It enters the ecosystem in the form of sunlight. It is incorporated into the chemical bonds of molecules within living and decaying tissues. At last it is lost as heat. Following factors are involved in the transfer of energy:
1. Food chain: The sequence of organisms through which energy moves in an ecosystem is called food chain. Following is a simple food chain‑Grass-. insects 4. Shrews ÷ owls
Food Web: Complexly interconnected food chains are called food webs. Many kinds of organisms are involved in food web.
Trophic level: The groupings of organisms on the basis of energy are called trophic levels. Food webs can be complex. Therefore, organisms are grouped according to the form of energy. There are following trophic levels:
(a) Producers (autotrophs): They obtain nutrition from inorganic materials. They form the first trophic level of an ecosystem. The Most familiar producers are green plants.
(b) Consumers: Other trophic levels are made , up of consumers (heterotrophs). Consumers eat other organisms to obtain ‘ energy.
Consumers have different kinds: •
Herbivores (primary consumers): They eat producers.
(ii) Carnivores (secondary consumers): They eat herbivores.
(iii) Other carnivores (tertiary consumers): They eat the carnivores that ate the herbivores.
(iv) Scavengers: The consumers that feed on large chunks of dead and decaying organic matter are called scavengers.
(c) Decomposers: They break down dead organisms and feces by digesting
organic matter extracellularly and absorbing the products of digestion.
Fig: Food web