Various Ecological Units: Species, Population, Community, Ecosystem, Biome, Ecosphere, Biosphere

There are several types of inter-related ecological units such as species, population, community, ecosystem, biome, ecosphere, biosphere etc. Many of these concepts suffer from inconsistencies and confusions over terminology.

Organisms and Populations

Theoretically, an organism should be smallest ecological unit. However, since all organisms have finite life spans, reproducing population is considered to be the smallest ecological unit; because it is persistent in time. Population refers to a group of individuals that belong to the same species and that are interbreeding.

Why population and not species is smallest ecological unit?

Species refers to a group of organisms in which two individuals are capable to interbreed and ordinarily don’t breed with other groups. If a species interbreeds freely with other species, it would no longer be a distinctive organism. But biology is a science of exceptions. There are numerous examples where organisms of a species interbreed with individuals of another species. Such interbreeding between species is more common in plants in comparison to animals. Due to these exceptions, what exactly is a species – is very difficult to define.

Since ecology and environment has more to do with interaction among organisms and with their environment; population and not species is considered the smallest ecological unit. Species is in fact the smallest unit of taxonomic classification rather.


A community refers to all the populations in a specific area at certain time. There are two essential things which make a group of populations eligible to be called community. These are interaction and inter-dependence for nutrition, food or other resources. Such interactions may involve life-death struggle among various organisms, as well as nutrient cycles manifested through various kinds of food webs and food chains.

How diversity of populations affects stability in community?

A complex community (i.e. that has a high diversity of populations) is more stable in comparison to community having low diversity. This is because food webs of communities of high diversity are more interconnected, and the greater inter-connectivity makes it more resilient to disturbance. If one species is removed, the other species which depend on it for food have other options to switch.

Producers, consumers and Decomposers

In terms of nutrition, that all organisms within a community are either producers, or consumers or decomposers. The producers or autotrophs are the plants which make their own food from inorganic raw material. This work is accomplished via photosynthesis or chemosynthesis. Consumers or heterotrophs get their nutrition / energy from the things they consume. They cannot produce their own food and have to look outside world for those things to consume. All organisms finally yield to decomposers, which break down organic matter into simple products. Fungi and bacteria are the common decomposers. They serve as the “garbage collectors” or “recyclers” in our environment.


An ecosystem is community plus its physical environment. Thus, ecosystem is a complex community of living plants, animals, and microorganisms (called biotic components of ecosystem) linked by energy and nutrient flows that interact together and their physical environment (called abiotic component of ecosystem) such as soil, air, water, sunlight etc. Kindly note that an ecosystem is the smallest functional ecological unit.

An ecosystem may be natural or manmade (such as aquarium), terrestrial or aquatic, and lentic (stagnant water such as pond) and lotic (running water such as river ) in case of aquatic systems.

Ecosystem Services

Services derived from ecosystems are referred to as Ecosystem Services. There are four categories of Ecosystem Services as follows:

Supporting services

  • Nutrient dispersal and cycling
  • Seed dispersal
  • Primary production

Provisioning services

  • Food (including seafood and game), crops, wild foods, and spices
  • Water
  • Minerals (including diatomite)
  • Pharmaceuticals, biochemical, and industrial products
  • Energy (hydropower, biomass fuels)

Regulating services

  • Carbon sequestration and climate regulation
  • Waste decomposition and detoxification
  • Purification of water and air
  • Crop pollination
  • Pest and disease control

Cultural services

  • Cultural, intellectual and spiritual inspiration
  • Recreational experiences (including ecotourism)
  • Scientific discovery

Ecosystem services may include facilitating the enjoyment of nature, which may generate many forms of income and employment in the tourism sector, often referred to as eco-tourisms, Water retention, thus facilitating a more evenly distributed release of water, Soil protection, open-air laboratory for scientific research, etc.


Biomes are groups of ecosystems that share similar climatic conditions and same kind of abiotic and biotic factors spread over a large area. The biomes are either terrestrial or aquatic. There are several systems of classification of biomes. The main types of biomes include Deserts (Hot, Cold, Semi Arid and Coastal), Aquatic Biomes (marine or freshwater), Forest (Tropical, Temperate, Taiga , Montane etc.), Grassland (Savannah etc.) and Tundra (Arctic Tundra, Alpine Tundra).

Importance of Biomes

Biomes play a crucial role in sustaining life on earth. For example, the Aquatic biome is home to millions of fish species and the source of the water cycle. It also plays a very important role in climate formation. The terrestrial biomes provide foods, enrich the air with oxygen and absorb carbon dioxide and other bad gases from the air. They also help regulate climate and so on.

Biosphere and Ecosphere

All the biomes together make up the biosphere. Biosphere is the entire part of the earth where living things exist. This includes soil, water, light, and air.  The word Biosphere includes sum total of life and life-support systems viz. atmosphere, hydrosphere, lithosphere, and pedosphere.

What is difference between Biosphere and Ecosphere?

Ecosphere is the global ecosystem i.e. the sum total of life on earth together with the global environment and the earth’s total resources  containing five essential elements: energy, air, water, sand, and living things. The term ecosphere was coined to denote sum total of life or living things along with organic and inorganic environment supporting it. In summary, there is no material difference between Biosphere and Ecosphere.

Which is suitable term for largest ecological unit – Biosphere or Ecosphere?

The largest ecological unit depends on how we define it. As per the WWF classification, the largest ecological unit is Biosphere. In other classifications, the term Ecosphere is used as largest ecological unit.


Ecozones are the Biogeographic division of the Earth’s land surface, based on distributional patterns of terrestrial organisms. Since they include only terrestrial part of biosphere, they are called Terrestrial Ecozones also.

Similarities / Differences between Biomes and Ecozones

Both biomes and Ecozones are groups of ecosystems, however, an Ecozone comprises only land parts of Earth surface, while the biomes comprise both aquatic and land parts.

Major Ecozones

There are 8 Ecozones on earth as shown in the following table:

EcozoneArea Km²Included regions
Palearctic54.1Includes the bulk of Eurasia and North Africa, This is largest
Nearctic22.9Includes most of North America
Afrotropic22.1Includes Sub-Saharan Africa
Neotropic19.0Includes South America and the Caribbean
Australasia7.6Includes Australia, New Guinea, and neighbouring islands. The northern boundary of this zone is known as the Wallace line.
Indo-Malaya7.5Includes the Indian subcontinent and Southeast Asia
Oceania1.0Includes Polynesia, Melanesia, Micronesia, New Zealand and some parts of Australia
Antarctic0.3Includes Antarctica.