The main levels of organisation of ecology are six and are as follows.

Individual < Population < Community < Ecosystem < Ecotone < Biosphere

1). Individual:
Organism is an individual living being that has the ability to act or function independently. It may be plant, animal, bacterium, fungi, etc.

It is a body made up of organs, organelles, or other parts that work together to carry out on the various processes of life.

2). Population:
Population is a group of organisms usually of the same species, occupying a defined area during a specific time.

Population growth rate is the percentage variation between the number of individuals in a population at two different times. Therefore the population growth rate can be positive or negative.

The main factors that make population increase are birth and immigration. The main factors that make population decrease are death and emigration.

3). Community:
In order to survive, individuals of any one species depend on individuals of different species with which they actively interact in several ways. For eg: Animals require plants for food and trees for shelter.

Plants require animals for pollination, seed dispersal, and soil microorganism to facilitate nutrient supply. Communities in most instances are named after the dominant plant form (species). For example: A grassland community is dominated by grasses, though it may contain herbs, shrubs, and trees, along with associated animals of different species.

A community is not fixed or rigid; communities may be large or small. On the basis of size and degree of relative independence communities may be divided into two types:

Types of Community:

(a) Major Communities:
These are large-sized, well organized and relatively independent. They depend only on the sun’s energy from outside and are independent of the inputs and outputs fromadjacent communities. E.g: tropical ever green forest in the North-East.

(b) Minor Communities:
These are dependent on neighbouring communities and are often called societies. They are secondary aggregations within a major community and are not therefore completely independent units as far as energy and nutrient dynamics are concerned. e.g: A mat of lichen on a cow dung pad.

4). Ecosystem:
An ecosystem is defined as a structural and functional unit of biosphere consisting of community of living beings and the physical environment, both interacting and exchanging materials between them.

It includes plants, trees, animals, fish, birds, micro-organisms, water, soil, and people. Ecosystems vary greatly in size and elements but each is a functioning unit of nature.

Everything that lives in an ecosystem is dependent on the other species and elements that are also part of that ecological community. If one part of an ecosystem is damaged or disappears, it has an impact on everything else.

When an ecosystem is healthy (i.e. sustainable) it means that all the elements live in balance and are capable of reproducing themselves. Ecosystem can be as small as a single tree or as large as entire forest.

Components of Ecosystem:
The components of ecosystem and environment are same.

i). Abiotic Components :

Abiotic components are the inorganic and non-living parts of the world. The abiotic part consists of soil, water, air, and light energy etc. It also involves chemicals like oxygen, nitrogen etc. and physical processes including volcanoes, earthquakes, floods, forest fires, climates, and weather conditions.

Abiotic factors are the most important determinants of where and how well an organism exists in its environment. Although these factors interact with each other, one single factor can limit the range of an organism.

a) Energy:
Energy from the sun is essential for maintenance of life. In the case of plants, the sun directly supplies the necessary energy. Since animals cannot use solar energy directly
they obtain it indirectly by eating plants or animals or both. Energy determines the distribution of organisms in the environment.

b) Rainfall:
Water is essential for all living beings. Majority of biochemical reactions take place in an aqueous medium. Water helps to regulate body temperature. Further, water
bodies form the habitat for many aquatic plants and animals.

c) Temperature:
Temperature is a critical factor of the environment which greatly influences survival of organisms. Organisms can tolerate only a certain range of temperature and humidity.

d) Atmosphere:
The earth’s atmosphere is responsible for creating conditions suitable for the existence of a healthy biosphere on this planet.

e) Substratum:
Land is covered by soil and a wide variety of microbes, protozoa, fungi and small animals (invertebrates) thrive in it. Roots of plants pierce through the soil to absorb water and nutrients. Organisms can be terrestrial or aquatic. Terrestrial animals live on land. Aquatic plants, animals and microbes live in fresh water as well as in the sea. Some microbes live even in hot water vents under the sea.

f) Materials:
Organic compound such as proteins, carbohydrates, lipids, humic substances are formed from inorganic compound on decomposition.

Inorganic compound such as carbon dioxide, water, sulphur, nitrates, phosphates, and ions of various metals are essential for organisms to survive.

g) Latitude and altitude:
Latitude has a strong influence on an area’s temperature, resulting in change of climates such as polar, tropical, and temperate. These climates determine different natural biomes.

From sea level to highest peaks, wild life is influenced by altitude. As the altitude increases, the air becomes colder and drier, affecting wild life accordingly.

ii). Biotic Components:
Biotic components include living organisms comprising plants, animals and microbes and are classified according to their functional attributes into producers and consumers.

a) Primary producers – Autotrophs (selfnourishing)
• Primary producers are basically green plants (and certain bacteria and algae).
• They synthesise carbohydrate from simple inorganic raw materials like carbon dioxide and water in the presence of sunlight by the process of photosynthesis for themselves, and supply indirectly to other nonproducers.
• In terrestrial ecosystem, producers are basically herbaceous and woody plants, while in aquatic ecosystem producers are various species of microscopic algae.

b) Consumers – Heterotrophs or phagotrophs (other nourishing)
• Consumers are incapable of producing their own food (photosynthesis).
• They depend on organic food derived from plants, animals or both.
• Consumers can be divided into two broad groups namely micro and macro consumers.

Macro consumers
• They feed on plants or animals or both and are categorised on the basis of their food sources.
• Herbivores are primary consumers which feed mainly on plants e.g. cow, rabbit.
• Secondary consumers feed on primary consumers e.g. wolves.
• Carnivores which feed on secondary consumers are called tertiary consumers e.g. lions which can eat wolves.
• Omnivores are organisms which consume both plants and animals e.g. man, monkey.

Micro consumers – Saprotrophs (decomposers or osmotrophs)
• They are bacteria and fungi which obtain energy and nutrients by decomposing dead organic substances (detritus) of plant and animal origin.
• The products of decomposition such as inorganic nutrients which are released in the ecosystem are reused by producers and thus recycled.
• Earthworm and certain soil organisms (such as nematodes, and arthropods) are detritus feeders and help in the decomposition of organic matter and are called detrivores.

Ecosystems are capable of maintaining their state of equilibrium. They can regulate their own species structure and functional processes. This capacity of ecosystem of self regulation is known as homeostasis.

Goods and Services provided by ecosystems include:
• Provision of food, fuel and fibre
• Provision of shelter and building materials
• Purification of air and water
• Detoxification and decomposition of wastes
• Stabilization and moderation of the Earth’s climate
• Moderation of floods, droughts, temperature extremes and the forces of wind.
• Generation and renewal of soil fertility, including nutrient cycling.
• Pollination of plants, including many crops Control of pests and diseases

5). Biomes:

The terrestrial part of the biosphere is divisible into enormous regions called biomes, which are characterized, by climate, vegetation, animal life and general soil type.

No two biomes are alike. The climate determines the boundaries of a biome and abundance of plants and animals found in each one of them. The most important climatic factors are temperature and precipitation.

6). Biosphere:
Biosphere is a part of the earth where life can exist. Biosphere represents a highly integrated and interacting zone comprising of atmosphere (air), hydrosphere (water) and
lithosphere (land).

It is a narrow layer around the surface of the earth. If we visualise the earth to be the size of an apple the biosphere would be as thick as its skin. Life in the biosphere is abundant between 200 metres (660 feet) below the surface of the ocean and about 6,000 metres
(20,000 feet) above sea level.

Biosphere is absent at extremes of the North and South poles, the highest mountains and the deepest oceans, since existing hostile conditions there do not support life. Occasionally spores of fungi and bacteria do occur at great height beyond 8,000 metres, but they are not metabolically active, and hence represent only dormant life.

The energy required for the life within the biosphere comes from the sun. The nutrients necessary for living organisms come from air, water and soil. The same chemicals are recycled over and over again for life to continue.

Living organisms are not uniformly distributed throughout the biosphere. Only a few organisms live in the polar regions, while the tropical rain forests have an exceedingly rich diversity of plants and animals (50% of Global Biodiversity).