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).