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

Ecology studies the interactions between organisms and their environment across multiple scales. These complex interactions can be organized into several hierarchical ecological units based on common characteristics and functions. These units, from smaller to larger, are organism, population, community, ecosystem, biome and biosphere.

Organism

The most fundamental ecological unit is a single organism. This refers to one individual living thing, such as a tree, squirrel, fish, or bacterium. Organisms inhabit environments suitable for their survival as determined by their biological adaptations. Their behaviors and resource usage directly impact and are impacted by their ecological niches. Some examples of this unit are Bengal Tiger, Bald Eagle, Grizzly bear, Salmon Fish, Blue whale etc.

Populations

Groups of organisms of the same species that live in the same area and interbreed comprise a population. Populations contain members of a single species interacting with their shared habitat and available resources. Their population dynamics are shaped by births, deaths, immigration and emigration. Tracking population trends over time can detect influences like environmental changes, disease, predation, or competition within an ecological community. Some examples of population include:

• Bengal Tiger Population in Bandipur National Park
• Asiatic Lion Population in Gir Forest
• Olive Ridley Sea Turtles in Odisha
• Snow Leopard Population in Ladakh
• Blackbuck Population in Velavadar National Park
• King Cobra Population in Agumbe Rainforests

What is smallest ecological unit?

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.

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.

Communities

A biological community encompasses the various interacting populations of plants, animals, fungi, bacteria and other organisms occupying an area. Community ecology examines the network of complex competitive and beneficial interactions between species populations within an area. These include predator-prey relationships, symbioses, resource partitioning and habitat niches. Some examples of communities are:

• Sundarban Mangrove Forest
• Coral Reef Communities of Lakshadweep
• Himalayan Alpine Meadows

There are two essential things which make a group of populations eligible to be called community. These are interaction and inter-dependence for food or other resources. Interactions may be life-death struggle or mutually beneficial and interdependence can be via 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 within community

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 through 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.

Ecosystem

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. Some examples of ecosystem  are:

• Coral reef ecosystem
• Northern coniferous forest ecosystem
• Desert ecosystem
• Agricultural ecosystem
• Lake ecosystem

Difference between Community and Ecosystem

Biomes

Biomes are groups of ecosystems that share similar climatic conditions and same kind of abiotic and biotic factors spread over a large area. 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 / aquatic 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.

Is International Space Station part of Biosphere?

The biosphere refers to all parts of Earth’s atmosphere, hydrosphere, and lithosphere that harbor life. While the ISS orbits Earth, it is physically detached from our planet’s large-scale biospheric cycles and flows. The ISS is not integrated with broader biospheric components for resources like air, water, and food. These have to be continually resupplied from Earth. So from an ecological perspective, the ISS represents an isolated artificial habitat containing a few humans and microbes; and is not part of Biosphere.

Biomes, ecozones and ecoregions

It’s easy to get confused between biomes, ecozones, and ecoregions as they are similar large-scale ecological classifications. Biomes refer to the major, continental-scale habitat types classified based on vegetation, climatic conditions, and adaptations. Important Biomes are Tropical Rainforest; Savanna; Desert; Chaparral (Mediterranean Biome); Grassland (includes Prairies, Steppes, and Pampas); Temperate Deciduous Forest; Temperate Coniferous Forest; Boreal Forest (Taiga); Tundra (Arctic and Alpine); Freshwater (Lakes, Rivers, and Wetlands) and Marine (Oceans, Coral Reefs, and Seashores).

On the other hand, Ecozones are even broader classifications dividing up the world’s biomes into just a handful of distinct biogeographic zones based on very large geographies with shared macroclimates, soil/terrain attributes, evolutionary histories, and general habitat types. The 8 Ecozones are given in below table:

Ecoregions are more specific subdivisions nested within ecozones that denote areas with similar ecosystems, environmental conditions, and shared species communities. For example, western ghats and eastern Himalayas are two different ecoregions. They have more defined boundaries than biomes but are smaller scales than ecozones. There are over 800 ecoregions within the world’s 14 major terrestrial ecozones that more specifically characterize regions (e.g. Appalachian mixed mesophytic forests). In India, you can take Western Ghats and Eastern Himalayas as two distinct ecoregions.