Life can be organized into several different levels of function and complexity. These functional levels are: organisms, species, populations, communities, ecosystems, biome and biosphere.

Organism

Any living being on planet earth can be called an organism. Every organism is a part of ecosystem.Examples: Bacterium, algae, dog, human being, elephant, sapling, tree etc.

Species

Species is a group of similar organism found on the Earth. Every organism is a member of certain species. Each species is given a scientific name. Example: Human beings belongs to the species Homo sapiens, dogs to Canis familiaris neem tree Azadirachta indica. A more exact definition of species is a group of interbreeding organisms that do not ordinarily breed with members of other groups. If a species interbreed freely with other species, it would no longer be a distinctive kind of organism. This definition works well with animals. However, in some plant species fertile crossings can take place among morphologically and physiologically different kinds of vegetation. In this situation, the definition of species given here is not appropriate.

Populations

A population is a group of individual members  of the same species living in a given area  at the same time. Examples: Colony of ants in a garden,  group of whales in ocean, oak trees in a forest.

Even though members of a population use common resources  and regulated by the same natural phenomena still, not all individuals are identical. Populations contain genetic variation within themselves and between other populations. Even fundamental genetic characteristics such as hair color or size may differ slightly from individual to individual. More importantly, not all members of the population are equal in their ability to survive and reproduce.

Communities

Community refers to all the populations of different species ( plants, animal and micro - organisms)    in a specific area or region at a certain time. Its structure involves many types of interactions among species. Some of these involve the acquisition and use of food, space, or other environmental resources. Others involve nutrient cycling through all members of the community and mutual regulation of population sizes. In all of these cases, the structured interactions of populations lead to situations in which individuals are thrown into life or death struggles.

In general, ecologists believe that a community that has a high diversity is more complex and stable than a community that has a low diversity. This theory is founded on the observation that the food webs of communities of high diversity are more interconnected. Greater interconnectivity causes these systems to be more resilient to disturbance. If a species is removed, those species that relied on it for food have the option to switch to many other species that occupy a similar role in that ecosystem. In a low diversity ecosystem, possible substitutes for food may be non-existent or limited in abundance.

Ecosystems

Ecosystems are dynamic entities composed of the biological community and the abiotic environment. In an ecosystem one or more communities of different species interact with each other and with their nonliving environment.  Both abiotic and biotic composition and structure of an ecosystem is determined by the state of a number of interrelated environmental factors. Changes in any of these factors (for example: nutrient availability, temperature, light intensity, grazing intensity, and species population density) will result in dynamic changes to the nature of these systems. For example, a fire in the temperate deciduous forest completely changes the structure of that system. There are no longer any large trees, most of the mosses, herbs, and shrubs that occupy the

forest floor are gone, and the nutrients that were stored in the biomass are quickly released into the soil, atmosphere and hydrologic system. After a short time of recovery, the community that was once large mature trees now becomes a community of grasses, herbaceous species, and tree seedlings.

Biome

A Biome is a terrestrial and regional ecosystem with a distinct climate and a distinct biological community of plants, animals and other organisms. Examples: Forest, grasslands, tundra, desert, mountain, ponds, lakes, rivers etc.

Biosphere

The biosphere is that part of Earth where life exists.

It includes Troposphere, Hydrosphere and Lithosphere. ( for more details about Troposphere, Hydrosphere and Lithosphere refer the world geography book).

Now let's discuss some of the important concepts in some detail along with concepts related to them.

Organisms interact with each other and also with physical conditions that are present in their habitats. The organisms and the physical features of the habitat form an ecological complex i.e. Ecosystem. The term ecosystem was coined by A.G. Tansley in 1935. The function of ecosystem is related to the flow of energy and cycling of materials through structural components of the ecosystem. All ecosystems of the earth are connected to one another, e.g., river ecosystem is connected with ecosystem of oceans.

Components of Ecosystem

Ecosystem has biotic and abiotic component.

Abiotic Components: Abiotic components are basic inorganic and organic compounds such as soil, water, calcium, oxygen, carbonates, and phosphates. It also includes climatic components such as moisture, wind currents and solar radiation etc.

Biotic Components: It includes living elements of the ecosystem and includes producers, consumers decomposers and transformers.

(a)  Producers: The producers are autotrophic like chemosynthetic and photosynthetic bacteria, blue green algae and all other green plants. They use radiant energy of sun in photosynthetic process in which CO2 is assimilated and the light energy is converted into chemical energy and O2 is evolved as a by-product.

(b)  Consumers: They are heterotrophic living members of ecosystem, which consume the food synthesized by producers. They are of three types.

(i)   Primary consumers: Those are purely herbivorous animals that are dependent for their food and producers or green plants. The herbivores of ecosystem are known as “key industry animals”.

(ii)  Secondary consumers: These are carnivores and omnivores, which eat primary consumers and producers.

(iii) Tertiary consumers: These top carnivores, which feed upon other carnivores, omnivores and herbivores.

(c)  Decomposers and transformers: They are living components of the ecosystem and are fungi and bacteria. Decomposers attack the dead remains of producers and consumers and degrade the complex organic substances into simpler compounds. These simple organic matters are attacked by another kind of bacteria, transformers which finally change these organic compounds into the inorganic form that are suitable for reuse by producers or green plants.

 (d) Detritivore: These are those organisms which eats detritus, which consist of organic litter, debris and dung. Examples of detritivore are flies, cockroach, crabs, earthworms, wood beetles and carpenter ants.

Food Chain

Organisms of an ecosystem are linked together in food chains. Green plants alone are able to trap in solar energy and convert it into chemical energy. The plants are eaten by consumers, which are being eaten by their predators. Thus, food from one trophic level reaches the other trophic level and in this way a chain is established. This is known as food chain. A food chain many be defined as the transfer of energy and nutrients through succession of organisms through repeated process of eating and being eaten.

Food Web

In ecosystem, one organism does not depend wholly on another. The resources are shared specially at the beginning of the chain. The marsh plants are eaten by variety of insects, birds, mammals and fishes and some of the animals are eaten by several predators. Similarly, in the food chain, grass-mouse-snakes-owls, sometimes mice are not eaten by snakes but directly by owls. This type of inter relationship inter links the individuals of the whole community. In this way, food chains become interlinked. A complex of interrelated food chains makes up food web.

Phenomenon in Food chain or Web

Biomagnification, also known as bioamplification or biological magnification, occurs when the concentration of a substance, such as DDT or mercury, in an organism exceeds the background concentration of the substance in its diet.[1] This increase can occur as a result of: Persistence – where the substance can't be broken down by environmental processes

Food chain energetic – where the substance concentration increases progressively as it moves up a food chain

Low or non-existent rate of internal degradation or excretion of the substance – often due to water-insolubility

Bioaccumulation occurs within a trophic level, and is the increase in concentration of a substance in certain tissues of organisms' bodies due to absorption from food and the environment.

Bio concentration is defined as occurring when uptake from the water is greater than excretion.Thus, bioconcentration and bioaccumulation occur within an organism, and biomagnification occurs across trophic (food chain) levels.

Biodilution is also a process that occurs to all trophic levels in an aquatic environment; it is the opposite of biomagnification, thus a pollutant gets smaller in concentration as it progresses up a food web.

Ecological Pyramid

At each step in the food chain a considerable portion of the potential energy is lost as heat. As a result, organisms in each trophic level pass on lesser energy to the next trophic level than they actually received. Longer the food chain the lesser energy is available for final members. Because of this tapering off of available energy in the food chain, a pyramid is formed that is known as ecological pyramid. The ecological pyramids are of three types:

(i)   Pyramid of number shows the number of organisms of each trophic level. In grassland and pond ecosystem pyramid is upright while in forest ecosystem it may be of different shapes. For example it can be inverted in a case where a single tree can support number of birds and microorganism.

(ii)  Pyramid of biomass: shows the total dry weight and other suitable measures of total amount of living matter. The biomass is least of producers and upright in case of grassland and forest ecosystem.

(iii) Pyramid of energy: Show the rate of energy flow and or productivity at successive trophic levels. The pyramid of energy is always upright.

Biomes

It is a recognizable community unit formed as a result of interaction of regional climates with regional substrata. The members of a biome are the plants and animals inhabiting it. Thus , a biome is a large region characterized by certain forms of life. Most biomes are determined by their characteristic weather pattern or climate. Every biome is home to a number of ecosystems.

Major terrestrial biomes include:

Major aquatic biomes include:

Intertidal (littoral): Shorelines and coasts; subject to periods of wet and dry

Neritic: Shallow waters to the continental shelf

Oceanic (pelagic): Surface layers of the open ocean

Aphotic (abyssal): Deep-water areas where no sunlight penetrates

Freshwater: Ponds, lakes, rivers, etc.; tied closely to surrounding terrestrial biomes.

Flora and Fauna