Ecosystem and Food Chain Notes for UPSC

An ecosystem is a community of living organisms (plants, animals, microbes) that interact with each other and with their non-living surroundings (air, water, soil, sunlight) as one system. A pond, a forest, a grassland, a desert and even a farm field are ecosystems. The term was first used by A. G. Tansley in 1935. In the UPSC Civil Services Examination, ecosystem concepts such as food chain, energy flow, trophic levels, ecological pyramids, productivity and succession are asked every year in Prelims, and they are the base for Mains answers on conservation and climate. This note explains everything in simple language.

Components of an Ecosystem

Every ecosystem has two types of components. The abiotic (non-living) components are sunlight, temperature, water, soil, air and minerals. The biotic (living) components are divided by function into three groups. Producers (autotrophs), mainly green plants and algae, make their own food by photosynthesis. Consumers (heterotrophs) depend on others for food: herbivores are primary consumers, carnivores that eat herbivores are secondary consumers, and those that eat other carnivores are tertiary consumers. Decomposers, mainly bacteria and fungi, break down dead matter and return minerals to the soil. Without decomposers, dead bodies would pile up and nutrients would not be recycled. Detritivores such as earthworms and termites help by feeding on dead matter.

Food Chain and Food Web

A food chain is a straight line showing who eats whom, such as grass to grasshopper to frog to snake to eagle. There are two main types. In a grazing food chain, the chain starts with green plants and moves to herbivores and carnivores; this is the main chain in most ecosystems. In a detritus food chain, the chain starts with dead organic matter that is eaten by detritivores and decomposers, and it is the main path of energy in many forests. In reality, animals eat more than one food, so many food chains join together to make a food web. A complex food web makes the ecosystem more stable, because if one species disappears, others can fill its role.

Trophic Levels and Energy Flow

Each step in the food chain is a trophic level. Producers are the first trophic level, herbivores the second, and so on. Energy flows in one direction, from the sun to producers and then to consumers, while nutrients move in a cycle. The 10 percent law, given by Raymond Lindeman (1942), says that only about 10 percent of the energy of one trophic level passes to the next, and the rest is lost as heat in respiration or used in life activities. Because of this loss, food chains are usually limited to four or five steps. This is also why eating at a lower level (plants) supports more people than eating at a higher level (meat).

Ecological Pyramids

An ecological pyramid shows the relationship between trophic levels graphically. The pyramid of numbers and of biomass is usually upright in a grassland or forest, but can be inverted in some cases: the number pyramid is inverted in a tree ecosystem where one tree supports many insects, and the biomass pyramid is inverted in a pond ecosystem because phytoplankton have small biomass but reproduce very fast. The pyramid of energy is always upright, because energy is lost at every level. This is a favourite question: which pyramid can never be inverted? The answer is the pyramid of energy.

Productivity

Primary productivity is the rate at which producers capture solar energy and store it as organic matter. Gross Primary Productivity (GPP) is the total amount of organic matter produced. Net Primary Productivity (NPP) is GPP minus the respiration losses of the plants (NPP = GPP minus R). NPP is the food available to consumers. Tropical rainforests, estuaries and coral reefs are among the most productive ecosystems, while deserts and the open ocean are the least productive per unit area. Even so, the open ocean contributes a large part of the total productivity of the Earth because of its huge area.

Nutrient Cycles (Biogeochemical Cycles)

Nutrients move between living things and the environment in cycles. In a gaseous cycle, such as the carbon and nitrogen cycles, the main reservoir is the atmosphere. In a sedimentary cycle, such as phosphorus and sulphur, the main reservoir is the Earth's crust. In the nitrogen cycle, bacteria like Rhizobium in the root nodules of legumes fix nitrogen from the air into usable forms, and denitrifying bacteria return it to the air. Human activities, such as burning fossil fuels and over-using fertilisers, upset these cycles and cause pollution and climate change.

Ecological Succession

Succession is the gradual and orderly change in the community of an area over time. Primary succession begins on a bare area with no soil, like a new rock surface or lava, and it takes hundreds of years; the first living things are called pioneer species, such as lichens. Secondary succession begins in an area where a community has been destroyed but soil is still there, like an abandoned farm or a burnt forest, and it is faster. The final stable community is the climax community.

Types of Ecosystems

Ecosystems are divided into terrestrial (forests, grasslands, deserts, tundra) and aquatic (freshwater like lakes and rivers, and marine like oceans, estuaries and coral reefs). An ecotone is a transition zone between two ecosystems, like the area between a forest and a grassland, and it shows the edge effect, a higher variety of species. Mangroves protect coasts and are nurseries for fish. Wetlands act like sponges and filters. Coral reefs are called the rainforests of the sea, and need warm, clear and shallow water. India's Sundarbans is the largest mangrove forest in the world.

Ecosystem Services

Ecosystems give us services that we often take for granted. Provisioning services give food, water, timber and medicine. Regulating services control the climate, floods, disease and water quality. Cultural services give recreation, beauty and spiritual value. Supporting services include soil formation and nutrient cycling. Protecting ecosystems is therefore not only about saving animals; it is about protecting the basic support system of human life and economy.

Human Impact and Bioaccumulation

When harmful chemicals such as pesticide DDT or heavy metals like mercury enter a food chain, they are not broken down. They get stored in the bodies of organisms (bioaccumulation) and become more concentrated at each higher level (biomagnification), so top predators like eagles and humans are hurt the most. Eutrophication is another problem, where too many nutrients from fertilisers and sewage cause algae to bloom in water bodies, use up the oxygen and kill fish. Managing waste and farm chemicals is therefore central to protecting ecosystems.

Ecosystems of India: A Short Tour

India has a wide range of ecosystems because of its different climates and landforms. The Himalayas have alpine meadows and cold forests, the Western Ghats have evergreen rainforests, the Thar has a desert ecosystem, the Gangetic plain has fertile agro-ecosystems and the long coastline has mangroves, lagoons such as Chilika and coral reefs in the Gulf of Mannar and Lakshadweep. The Sundarbans in the delta of the Ganga and the Brahmaputra is home to the Royal Bengal Tiger, and Keoladeo and Loktak are famous wetlands. Each of these places has its own food web and its own threats, such as sea-level rise for the Sundarbans and pollution for the rivers. Linking the concepts above with such Indian examples will help you write answers that are specific and not generic.

Common Mistakes in Ecology Questions

Students often say that energy is recycled, but only nutrients are recycled, while energy flows in one direction and is lost as heat. They also confuse the biomass pyramid and the energy pyramid. Another mistake is to think that the climax community never changes; in reality, it is stable only until a big disturbance such as fire, flood or human activity occurs. Remember too that a producer is not always a green plant: some bacteria produce food by chemosynthesis in deep sea vents. Careful reading of such small points helps you eliminate wrong options in the statement-type questions of Prelims.

Quick Revision

• Ecosystem: term by A. G. Tansley (1935); energy flows one way, nutrients cycle.
• 10 percent law by Lindeman; energy pyramid is always upright.
• NPP = GPP minus respiration.
• Primary succession starts on bare rock; secondary on existing soil.

Ecosystems are the working units of nature. Draw one food web, one pyramid of each type and the main nutrient cycles on a single page, revise it weekly, and you will answer almost any ecology question with ease.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• Environmental Pollution Notes for UPSC
• Protected Areas, Ramsar Sites and Wildlife Laws Notes for UPSC
• Renewable Energy in India Notes for UPSC
• All Environment and Ecology notes

Frequently Asked Questions (FAQ)

What is an ecosystem?

An ecosystem is a community of living organisms interacting with each other and with the non-living environment as a functional unit.

Who coined the term ecosystem?

The term ecosystem was coined by the British ecologist A. G. Tansley in 1935.

What is the difference between a food chain and a food web?

A food chain is a single straight path of energy transfer, while a food web is a network of many interconnected food chains in an ecosystem.

What is the 10 percent law?

Given by Raymond Lindeman, it states that only about 10 percent of the energy at one trophic level is passed on to the next, and the rest is lost as heat.

What are trophic levels?

Trophic levels are the steps in a food chain: producers, primary consumers, secondary consumers and so on.

Which ecological pyramid is always upright?

The pyramid of energy is always upright because energy is lost at every trophic level.

What is the difference between GPP and NPP?

GPP is the total organic matter produced by producers, while NPP is GPP minus the energy used by the plants in respiration.

What are decomposers?

Decomposers are organisms such as bacteria and fungi that break down dead organic matter and return nutrients to the soil.

What is ecological succession?

It is the gradual and orderly replacement of one community by another in an area until a stable climax community is reached.

What is the difference between primary and secondary succession?

Primary succession starts on a bare area with no soil, while secondary succession starts in an area where soil remains after the earlier community was destroyed.

What is biomagnification?

It is the increasing concentration of a harmful substance, such as DDT, in organisms at each higher level of the food chain.

What is an ecotone?

An ecotone is a transition zone between two different ecosystems, which often has a higher variety of species due to the edge effect.