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The environment refers to the things and conditions around the organisms which directly or indirectly influence the life and development of the organisms and their populations. An organism is always in the state of perfect balance with the environment. The environment literally means the surroundings.

The concept of ecosystem was first put forth by A.G. Tansley (1935). Ecosystem is the major ecological unit. It has both structure and functions. The structure is related to species diversity. The more complex is the structure the greater is the diversity of the species in the ecosystem. The functions of ecosystem are related to the flow of energy and cycling of materials through structural components of the ecosystem.

According to Woodbury (1954), ecosystem is a complex in which habitat, plants and animals are considered as one interesting unit, the materials and energy of one passing in and out of the others.
According to E.P. Odum, the ecosystem is the basic functional unit of organisms and their environment interacting with each other and with their own components. An ecosystem may be conceived and studied in the habitats of various sizes, e.g., one square metre of grassland, a pool, a large lake, a large tract of forest, balanced aquarium, a certain area of river and ocean.

Living organisms cannot live isolated from their non-living environment be­cause the latter provides materials and energy for the survival of the former i.e. there is interaction between a biotic community and its environment to produce a stable system; a natural self-sufficient unit which is known as an ecosystem.
All the ecosystems of the earth are connected to one another, e.g., river ecosystem is connected with the ecosystem of ocean, and a small ecosystem of dead logs is a part of large ecosystem of a forest.

An ecosystem can be classified as below
a. TERRESTRIAL ECOSYSTEM, Examples are Forests, Grasslands and Deserts
b. AQUATIC ECOSYSTEM ,Examples are Fresh Waters and Marine Waters

Structure of Ecosystem: The structure of an ecosystem is basically a description of the organisms and physical features of environment including the amount and distribution of nutrients in a particular habitat. It also provides information regarding the range of climatic conditions prevailing in the area.

From the structure point of view, all ecosystems consist of the following basic components:
1. Abiotic components
2. Biotic components

1. Abiotic Components: Abiotic component of ecosystem includes basic inorganic elements and compounds, such as soil, water, oxygen, calcium carbonates, phosphates and a variety of organic compounds (by-products of organic activities or death).
It also includes such physical factors and ingredients as moisture, wind currents and solar radiation. Radiant energy of sun is the only significant energy source for any ecosystem. The amount of non-living components, such as carbon, phosphorus, nitrogen, etc. that are present at any given time is known as standing state or standing quantity.
Abiotic components are mainly of two types:
(a) Climatic Factors:
Which include rain, temperature, light, wind, humidity etc.
(b) Edaphic Factors:
Which include soil, pH, topography minerals etc.?
The functions of important factors in abiotic components are given below:
Soils are much more complex than simple sediments. They contain a mixture of weathered rock fragments, highly altered soil mineral particles, organic mat­ter, and living organisms. Soils provide nutrients, water, a home, and a struc­tural growing medium for organisms. The vegetation found growing on top of a soil is closely linked to this component of an ecosystem through nutrient cycling.
The atmosphere provides organisms found within ecosystems with carbon di­oxide for photosynthesis and oxygen for respiration. The processes of evapora­tion, transpiration and precipitation cycle water between the atmosphere and the Earth’s surface.

Solar radiation is used in ecosystems to heat the atmosphere and to evapo­rate and transpire water into the atmosphere. Sunlight is also necessary for photosynthesis. Photosynthesis provides the energy for plant growth and me­tabolism, and the organic food for other forms of life.
Most living tissue is composed of a very high percentage of water, up to and even exceeding 90%. The protoplasm of a very few cells can survive if their water content drops below 10%, and most are killed if it is less than 30-50%.
Water is the medium by which mineral nutrients enter and are trans-located in plants. It is also necessary for the maintenance of leaf turgidity and is required for photosynthetic chemical reactions. Plants and animals receive their water from the Earth’s surface and soil. The original source of this water is precipita­tion from the atmosphere.

2. Biotic Components:
The biotic components include all living organisms present in the environmental system.
From nutrition point of view, the biotic components can be grouped into two basic components:
(i) Autotrophic components, and
(ii) Heterotrophic components
The autotrophic components include all green plants which fix the radiant energy of sun and manufacture food from inorganic substances.
The heterotrophic components include non-green plants and all animals which take food from autotrophs. So biotic components of an ecosystem can be described under the following three heads:
1. Producers (Autotrophic components),
2. Consumers, and
3. Decomposers or reducers and transformers
The amount of biomass at any time in an ecosystem is known as standing crop which is usually expressed as fresh weight, dry weight or as free energy in terms of calories/metre.
Producers (Autotrophic elements):
The producers are the autotrophic elements—chiefly green plants. They use radiant energy of sun in photosynthetic process whereby carbon dioxide is assimilated and the light energy is converted into chemical energy. The chemical energy is actually locked up in the energy rich carbon compounds. Oxygen is evolved as by-product in the photosynthesis.
This is used in respiration by all living things. Algae and other hydrophytes of a pond, grasses of the field, trees of the forests are examples of producers.
Chemosynthetic bacteria and carotenoid bearing purple bacteria that also assimilate CO2 with the energy of sunlight but only in the presence of organic compounds also belong to this category.
Those living members of ecosystem which consume the food synthesized by producers are called consumers. Under this category are included all kinds of animals that are found in an ecosystem.There are different classes or categories of consumers, such as:
(a) Consumers of the first order or primary consumers,
(b) Consumers of the second order or secondary consumers,
(c) Consumers of the third order or tertiary consumers, and
(d) Parasites, scavengers and saprobes.

(a) Primary consumers:
These are purely herbivorous animals that are dependent for their food on producers or green plants. Insects, rodents, rabbit, deer, cow, buffalo, goat are some of the common herbivores in the terrestrial ecosystem, and small crustaceans, molluscs, etc. in the aquatic habitat. Elton (1939) named herbivores of ecosystem as “key industry animals”. The herbivores serve as the chief food source for carnivores.

(b) Secondary consumers:
These are carnivores and omnivores. Carnivores are flesh eating animals. Example small spiders that catch and eat flies omnivores are the animals that are adapted to consume herbivores as well as plants as their food. Large omnivores include bears and humans.
Examples of secondary consumers are sparrow, crow, fox, wolves, dogs, cats, snakes, etc.

(c) Tertiary consumers:
These are the top carnivores which prey upon other carnivores, omnivores and herbivores. Lions, tigers, hawk, vulture, etc. are considered as tertiary or top consumers.
(d) The parasites, scavengers and saprobes :

They are also included in the consumers. The parasitic plants and animals utilize the living tissues of different plants and animals. The scavengers and saprobes(some group of fungi) utilize dead remains of animals and plants as their food.

Decomposers and transformers:
Decomposers and transformers are the living components of the ecosystem and they are fungi and bacteria. Decomposers attack the dead remains of producers and consumers and degrade the complex organic substances into simpler compounds. The simple organic matters are then attacked by another kind of bacteria, the transformers which change these organic compounds into the inorganic forms that are suitable for reuse by producers or green plants. The decomposers and transformers play very important role in maintaining the dynamic nature of ecosystems.

Relationships between members of an ecological community
It is classified within two broad categories, direct effects and indirect effects. The first of these, direct effects, as the name implies, deals with the direct impact of one individual on another when not mediated or transmitted through a third individual. If you have ever watched a cheetah capture a gazelle or a bee pollinate a flower, then you have observed a direct effect in action . There are eight main types of direct effects which are classified by the net effect of the relationship on each individual; positive, negative, or neutral.

Competition: Competition occurs when two organisms compete for the same resource (food, space, mates, etc.). Both individuals are negatively impacted by competition for the resource because either the resource is limited or, if the resource is not limited, they can physically interfere with competitors attempting to obtain the resource.
Predation: Predation takes place when one organism (the predator) consumes another (the prey). Typically, carnivores, such as the cheetah pursuing the gazelle, are excellent examples of predation. In its broadest sense, the term predation includes all consumption of another organism for nutrients, including herbivory and parasitism.
Herbivores: When a plant is eaten by another organism, it is considered herbivory. Unlike predation, in which the whole organism is destroyed, plants often survive grazing by an herbivore. For this reason, herbivores is sometimes referred to as plant parasitism.

Parasitism: A parasite is physiologically dependent upon its host for nutrition. While the host is negatively affected by the loss of nutrients to the parasite, parasitism rarely leads directly to the host's death. Unfortunately, humans are hosts to any number of parasites, including liver flukes, tapeworms, lice, pinworms, giardia, and many others.
Mutualism: In a mutualism, both partners benefit from the relationship. Many coral reefs have "cleaning stations" where some species of fish remove parasites from other fish. The cleaner fish get nutrition from the consumed parasites while the cleaned fish enjoy freedom from their parasites.

Commensalism: Commensalism occurs when one organism is positively affected by the relationship while the other organism is not affected, either negatively or positively, by the interaction. Cattle egrets who feed on the insects stirred up by domesticated cattle are an example of commensalism. Some birds have even taken advantage of the invention of the lawn mower to extend this relationship to the suburban lawn.

Amensalis: Amensalism takes place when one individual is negatively affected by interaction with another individual who is not affected by the relationship. Many molds, including Penicillium, secrete chemicals that kill bacteria in their vicinity.

Neutralism: Interactions between the two individuals are neutral in regards to both species. Some bacteria may exhibit neutralism as it has been reported that some species of Lactobacillus and Streptococcus can coexist without affecting each other positively or negatively. However, it has been suggested that true neutralism is probably rare in nature.

The second major class of interactions, indirect effects, can be defined as the impact of one organism or species on another that is mediated or transmitted by a third. In other words, A (donor) has an effect on B (transmitter), which then affects C (recipient).

Keystone Predation: In keystone predation, the removal of a prey species by the predator indirectly increases the abundance of a competitor of the prey species.
Exploitation Competition: When two predators consume the same prey, the loss of abundance in the prey species can negatively affect the second predator.

Apparent Competition: Apparent competition occurs when an increase in the abundance of one species leads to a decrease in the abundance of a second species due to enhanced predation by a shared predator.
Indirect Mutualism: An indirect mutualisum takes place when positive effects on two consumer species when each negatively impacts a competitor species of the other predator's main prey species.

Indirect Commensalism: Indirect commensalism is similar to indirect mutualism except that one of the predator species will also consume the main prey of the other predator species.
Habit Facilitation: In habitat facilitation, one species indirectly improves the habitat of a third species by its interactions with a second species.

Trophic Cascade: The plant is positively affected by the decrease in herbivores caused by a predator reducing herbivore numbers.

Indirect Defense: Indirect defense occurs when a non-prey species leads to the indirect decrease of a consumer species either by reducing the prey species via competition, which leads to a reduction in the consumer species, or when one prey species leads to an increase in its predator abundance, which then preys more heavily on the third species.