Ecology Part 1- Key concepts

Key Ecology Concepts

The study of the interactions between living things and their environments is known as ecology. In ecology, everything is connected to everything else and there is a constant interaction between organisms and their environment.

Autecology and Synecology

Autecology & Synecology are two main branches of ecology. Autecology is the study of individual organism or individual species. It is also known as population ecology. Synecology is the study of group of organisms of different species which are associated together as a unit in form of a community. It is also known as community ecology.

Autecology helps us to understand the relationships between individual plants and environment. Synecology helps us to understand the relationships between communities and environment.

Various Ecological Units

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 partsof 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:

Ecological Niche

Ecological niche of an organism is the physical space occupied by it, its functional role in the community i.e. tropic position, its position in environment and the conditions of existence.

Difference between Niche and Habitat

A niche describes how an organism makes its living and responds to the distribution of resources and competitors. The ecological niche involves both the place where an organism lives as well as the roles that an organism does in its habitat. For example, various habitats of house sparrow include woodlands, grasslands, and deserts; houses, factories, warehouses, zoos etc. However, when we talk about its niche, it would include – eating insects, grains, seeds etc.; making nests in houses, trees and shrubs etc. Thus, niche is a broader concept than habitat and its focus is on functional roleplayed by the species rather than only the place it needs to live. For any organism, the niche includes both the physical habitat and how it has adapted to life in that habitat. In summary, Habitat is “address” while Niche is “profession”.

Ecological Equivalents

Organisms that occupy the same or similar ecological niches in different geographical regions are known as Ecological Equivalents. For examples, owls and cats, both feed on mice; but owls are found in deserts or forests while cats are around human habitations. In this context, owls and cats are ecological equivalents in terms of their feeding role. Similarly, Kangaroos of Australia perform the same functions (herbivores) as antelopes or Bison of North America. Both live in similar habitats of different regions but have similar profession of herbivores.

Can two species share the same niche?

Each species has one Niche; no two species can share the same niche for long because then the competition for resources will drive the inferior species out. This is called character displacement or niche shift or niche displacement. Further, please note that competition for food, light and space is most severe between two closely related species occupying same niche.

Narrow Niche and Broad Niche

When an organism is adapted various kinds of environmental conditions for its survival, it has a generalized or broad niche. Such organisms are called generalists. On the other hand, if the organism needs specific set of conditions for its survival, it has a specialized niche or narrow niche. Such organisms are called specialists.

There are many species of plants and animals which can survive only a narrow range of climatic or environmental conditions. For example, Giant Panda has a very specialized niche because 99% of its food is bamboo plants. The destruction of bamboo in China has led the Giant Panda to reach stage of near extinction. Thus, narrow niche is one of the reasons behind the extinction of species.

  • Examples of Narrow Niche: Giraffes, Lemurs, Ganges River Dolphin etc.
  • Example of Broad Niche: Humans, Rodents, House Sparrows etc.

Food Chain and Food Webs

Food chain refers to a linked feeding series in an ecosystem. A food chain illustrates the order in which a chain of organisms feed upon each other and the sequence of organisms through which energy and materials are transferred, in the form of food, from one tropic level to another. The following graphics shows a simple food chain.

The food chains are not isolated and are inter-linked to each other. For example, a Hawk can eat snakes as well as other smaller birds. A mouse can eat grass, bread or even grasshoppers. A Lizard can eat insects of different types. Thus, various food chains are intertwined in each other making a food web. A Foodweb is thus a system of interlocking and interdependent food chains. A typical Foodweb is shown in the below graphics.

Trophic Levels

There are basically four levels to the food chain viz. producers, primary consumers, secondary consumers, and tertiary consumers.

First Trophic Level (Plants)

Producers or Autotrophs produce all of the available food. They make up the first trophic (feeding) level. They possess the highest biomass (the total weight of all the organisms in an area) and the greatest numbers. This is evident from the fact that the plants make around 99 percent of the earth’s total biomass.

Second Trophic Level (Herbivores or Primary Consumers)

Primary consumers (Herbivores) are organisms that directly feed on producers. A good example is a cow. They make up the second trophic level.

Third Trophic Level (Carnivores or Secondary Consumers)

The next level consists of organisms that feed on primary consumers. They are the secondary consumers, and they make up the third trophic level. They are called carnivores and omnivores

Fourth Trophic Level (Apex Predators or Tertiary Consumers)

Apex predator species occupy the highest trophic level(s) and have a crucial role in maintaining the health of their ecosystems. Apex predators affect prey species’ population dynamics. Where two competing species are in an ecologically unstable relationship, apex predators tend to create stability if they prey upon both. Inter-predator relationships are also affected by apex status.

Ecological Pyramids

An ecological pyramid shows the relationship of biomass, productivity or energy at different trophic levels. The primary producers are generally shown at the bottom and apex predators at the top. The pyramids are different for different ecosystems. There are three types of Ecological Pyramids as follows:

  • Pyramid of numbers: This shows the number of individual organisms at successive trophic levels. It can be upright or inverted.
  • Pyramid of Biomass: This shows the biomass at successive trophic levels. It can be upright or inverted.
  • Pyramid of energy: It shows the rate of energy flow and/or productivity at successive trophic levels. It is always upright.


Pyramid of Numbers

This Pyramid shows the numbers of the producers, herbivores and the carnivores at their successive trophic levels. This pyramid can be either upright, or inverted or partially upright.

Pyramid of numbers in grassland Ecosystem

In a grassland ecosystem, the number of producers (mainly grasses) is always maximum, followed by decreasing numbers at second trophic level (herbivores), third trophic level (carnivores) and least number of apex predators. Thus, a pyramid of numbers in grassland is upright.

Pyramid of Numbers in Pond Ecosystem

In a pond ecosystem, the producers are phyto-planktons such as algae, bacteria etc. They are maximum in number. The small herbivorous fishes, rotifers etc. are smaller in number than producers, while the small carnivorous fishes are even less in number. Finally, the apex consumers or biggest carnivorous fishes are least in number. Thus, the Pyramid of numbers in a Pond Ecosystem is also upright.

Pyramid of Numbers in Forest Ecosystem

In a forest ecosystem, the producers are large size trees which make the base of Pyramid. The herbivores such as fruit eating birds, deer, elephants etc. make the primary consumers and are less than primary producers. After that, the number goes down at each successive level. Thus, a Pyramid of numbers in a Forest Ecosystem is partially upright or spindle shaped.

Pyramid of Numbers in Parasitic Food Chain

A single plant or animal in a parasitic food chain may support numerous parasites, which might be further supporting a larger number of hyperparasites. Thus, the pyramid of numbers in a parasitic food chain is inverted.

Pyramid of Biomass

Pyramid of Biomass in Forests and Grasslands

Pyramid of biomass for terrestrial ecosystems (grasslands, forests) the biomass generally decreases at each higher trophic level from plants via herbivores to carnivores. This is evident from the fact that the terrestrial producers’ viz. grasses, trees and shrubs have a much higher biomass than the animals that consume them, such as deer, zebras and insects. The level with the least biomass is the highest predators in the food chain, such as foxes and eagles. Thus, in terrestrial ecosystems, the Pyramid of Biomass is upright.

Pyramid of Biomass in Aquatic Ecosystems

In the aquatic system, the biomass can increase at higher trophic levels. For example, in Oceans, the food chain typically starts with phytoplankton and ends at predatory fish, which has largest biomass. Thus, the pyramid of biomass is inverted in the aquatic systems including marine, ponds etc.

Pyramid of Energy

Energy cannot be recycled and during the flow of energy from one trophic level to other, there is a considerable loss in the form of heat, respiration, mechanical energy etc. Thus, highest energy is available to primary producers and lowest to tertiary consumers. Thus, the pyramid of energy is always upright and vertical. In this pyramid, the energy is minimum as the highest trophic level and is maximum at the lowest trophic level.

Since there is a successive reduction in energy flow at successive trophic levels, shorter the food chain, greater would be the available food energy. The reason is with an increase in the length of food chain, there is a corresponding more loss of energy.

Biomass Productivity

The rate of generation of biomass in an ecosystem is called Productivity, which is expressed in units of energy (example: joules per meter² per day) or in units of dry organic matter (example: kg per meter² per year).

Primary Productivity

Primary production is the synthesis of new organic material from inorganic molecules such as water and CO2 via photosynthesis or chemosynthesis. The rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producers is called primary productivity. Further, the total energy fixed by plants via photosynthesis is called Gross Primary Productivity. A small fraction of this energy fixed is used in the respiration of plants, which gives them necessary energy for various physiological and morphological functions.  When this respiratory utilization is reduced from Gross Primary Productivity, what we get is Net Primary Productivity. Net Primary Productivity is the actual rate of biomass productivity, which refers to the balance between gross photosynthesis and respiration and other plant losses.  The primary productivity is also known as energy storage at producer level.

Highest Net Primary Productivity of Various Ecosystems

Kindly note that highest primary productivity is found in Tropical Forests, Estuaries and Swamps/ Marshes. Each of them produces around 9000 Kcal per meter² per year.

  • Tropical Rainforests have high primary productivity because of availability of plenty of solar light and water.
  • A typical estuary has high primary productivity because it is shallow (gets plenty of sunlight) and has turbulent water (which brings the nutrient rich material from sea bed).
  • Swamps and Marshes have high primary productivity because they have lots of nutrients and sunlight.
  • After Tropical Rainforests, Estuaries and Swamps / Marshes, the highest primary productivity is found in coral reefs, algal beds and temperate forests.
  • Least primary productivity is found in cold and hot deserts including tundra.

Secondary Productivity

Productivity of heterotrophs such as animals is called secondary productivity. It is also known as energy storage at consumer level.  Secondary productivity is done by consumers via assimilation of the food they take. Since not all the primary biomass is consumed; and since not all the consumed is digested; secondary production is only a fraction of primary production.

January 17, 2018

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