ECOLOGICAL SUCCESSION

The environment may be altered in substantial ways through the activities of humans, other living things, or when natural disasters occur, such as climate changes and volcanic eruptions.  Although these changes are sometimes occur very quickly, in most cases species replace others gradually, resulting in long-term changes in ecosystems. These gradual long term changes in altered ecosystems are called ecological successions.  Ecosystems tend to change with time until a stable system is formed. The type of succession which occurs in an ecosystem depends upon climatic and other limitations of a given geographical area. Pioneer organisms are the first organisms to reoccupy an area which has been disturbed by a disruption. Typical pioneers in a succession include grasses in a plowed field or lichens on rocks. These pioneer organisms modify their environment, ultimately creating conditions which are less favorable for themselves, but establishing conditions under which more advanced organisms can live.Over time, the succession occurs in a series of plant stages which leads to a stable final community which is very similar to the plant community which originally existed in the ecosystem. This final stable plant community is called a climax community. This community may reach a point of stability that can last for hundreds or thousands of years. It has been observed that when natural disasters occur, such as a floods or fires, the damaged ecosystem is likely to recover in a series of successional stages that eventually result in a stable system similar to the original one that occupied the area.

Primary Succession: Begins in new habitats, uninfluenced by pre-existing communities

Secondary Succession: Follows disruption of a pre-existing community (succession after forest fire, etc.)

Succession was among the first theories advanced in ecology and the study of succession remains at the core of ecological science.

FOOD CHAIN

Flow of energy in an ecosystem is one way process. The sequence of organism through which the energy flows, is known as food chain. Plants are eaten by insects, insects are eaten by frogs, the frogs are eaten by fish and fish are eaten by humans. The pattern of eating and being eaten forms a linear chain called food chain. Such a food chain can always be traced back to the producers. The primary producers trap radiant energy of the sun and transfers to chemical or potential energy of organic compounds like proteins, fats and carbohydrates. Formation of ATP (Adenosine triphosphate) during photo-synthesis is the first nutritional level. ATP is stored in food matter that is utilized by herbivores, the plant eaters. This process constitutes the second trophic level. When herbivores eat the plant, the organic compounds are re-oxidized and energy is liberated. Some energy is produced as heat which is not useful. Flow of energy is greatly reduced at each successive level of nutrition because of the energy utilization by the organisms and heat loses at each step in transformation of energy. This accounts for the decrease in biomass at each successive level. It should be noted that the number of steps in a food chain are always restricted to maximum four or five steps. Humans are at the end of a number of food chains.
TYPES OF FOOD CHAIN: 
There are three types of food chain in the nature

Grazing food chain:

The primary producers are the living green plants which are grazed on by grazing animals. It is found in aquatic and grassland ecosystem.

E.g. Phytoplankton →Zooplankton→ Fish→ Hawk

Detritus food chain:

This type of food chain starts from dead organic matter and so it is less dependent on solar energy. The dead organic matter is broken down into simple nutrients by microorganisms like fungi and bacteria. This type of food chain is found in forest ecosystem.

Dead organic matter→ Detritivores → Predators

Parasitic food chain: 

In this type of food chain either the producer or the consumer is parasitized and therefore the food passes to the smaller organism. The energy transfer through this kind of food chain is not significant.

Producer→ Herbivores→ Parasite→ Hyperparasites

SIGNIFICANCE OF FOOD CHAIN:

• They help in maintaining the ecological balance
• They help in understanding the feeding relations among organisms
• Energy flow and nutrient cycling take place through them
• helps in understanding interaction between organism and ecosystem
• Help in understanding the mechanism of energy flow and circulation of matter in ecosystem
• Helps to understand the movement of toxic substance and
• The problem associated with biological magnification in the ecosystem