Trophic levels and types of consumers
In a school canteen, the cook makes food, children eat it, and the leftovers go to the compost pit. In nature too, some living things make food, some eat it and some break down what is left. Each has a place in the ecosystem.
A trophic level (पोषण पातळी) is each step at which an organism obtains its food in a food chain.
In the figure : In the figure each step of the chain is a trophic level.
Rule 1: Producers, consumers, decomposers — By mode of nutrition, organisms are producers (उत्पादक), consumers (भक्षक), saprotrophs and decomposers (विघटक). Each step at which an organism gets its food in the chain is a trophic level.
- Easy : Grass is a producer; a grasshopper is a consumer.
- Medium : A mushroom growing on a dead log breaks it down, so it is a decomposer.
- Exam-type : Which trophic level is a plant on? The first, because it makes its own food.
Rule 2: Types of consumers — Primary consumers (प्राथमिक भक्षक) are herbivores like the grasshopper, squirrel and elephant; they depend directly on producers. Secondary consumers (द्वितीयक भक्षक) are carnivores like the frog, owl and fox; they eat herbivores. Apex or top consumers (अंतिम भक्षक), like the tiger and lion, eat herbivores and carnivores. Omnivores (सर्वभक्षक), like humans and the bear, eat plants and animals.
- Easy : An elephant eats leaves: primary consumer.
- Medium : A frog eats insects (herbivores), so it is a secondary consumer.
- Exam-type : A tiger eats deer and also foxes. It sits at the top of the chain, so it is an apex consumer.
Producers make food; consumers eat; decomposers break down. Trophic level = step in the chain.
Calling a human only a herbivore or only a carnivore. Humans are omnivores.
Memory clue: "P-P-S-A": Producer, Primary, Secondary, Apex.
Spot the difference
| Level | Name | Example |
|---|---|---|
| 1 | Producer | Grass |
| 2 | Primary consumer | Grasshopper |
| 3 | Secondary consumer | Frog |
| 4 | Apex consumer | Tiger |
Practice questions
- 1What is a trophic level?
- 2Name the four types of organisms by mode of nutrition.
- 3Which consumers are herbivores?
- 4Name two secondary consumers.
- 5What are omnivores? Give an example.
- 6In grass → grasshopper → frog → snake, name the trophic level of the frog.
- 7In that chain, how many trophic levels are there?
- 8Why are decomposers important even though they are not part of the usual eating order?
Show answers (try them yourself first)
- 1A step at which an organism gets its food in a food chain.
- 2Producers, consumers, saprotrophs and decomposers.
- 3Primary consumers.
- 4Frog and owl (or fox).
- 5Animals that eat both plants and animals, e.g. humans.
- 6Third level (secondary consumer).
- 74
- 8They break down dead remains so that nutrients return to the soil, air and water for plants.
Food chain and food web
In a village pond, a grasshopper eats leaves, a frog eats the grasshopper, a snake eats the frog and an eagle eats the snake. That is one line. But the frog also eats other insects, and the eagle also eats rats. Many lines join into a net.
A food chain (अन्नसाखळी) is a definite sequence in which organisms eat one another. A food web (अन्नजाळे) is many food chains joined together.
In the figure : In the figure food chains join to make a food web.
Rule 1: Food chain — A food chain is the definite sequence of interactions among producers, consumers and saprophytes. Each chain has four, five or more links.
- Easy : Grass → grasshopper → frog → snake is a chain of 4 links.
- Medium : Adding an eagle to that chain makes 5 links: grass, grasshopper, frog, snake, eagle.
- Exam-type : Arrows in a food chain point from the eaten to the eater, because they show the direction in which food energy goes.
Food chain and food web
In the app you can move this figure with your finger.
Open in the appRule 2: Food web — An ecosystem has many food chains linked at different levels, forming a food web. An organism may be prey of many others: an insect that eats leaves is eaten by frogs, wall lizards and birds. Food webs are found everywhere in nature, and balance in the web is needed for the ecosystem.
- Easy : A leaf-eating insect is eaten by a frog, a lizard and a bird: three links from one insect.
- Medium : If snakes vanish from a field, rats multiply, since fewer are eaten, and then grain stores suffer.
- Exam-type : In a web, if a frog population drops, insects can rise and snakes may eat more rats, so the whole web shifts. Balance needs every part.
Food chain and food web
In the app you can move this figure with your finger.
Open in the appFood chain: a single line. Food web: many chains joined. Arrows go from eaten to eater.
Drawing arrows from predator to prey. Arrows follow the flow of food energy.
Memory clue: "Chain = single line, Web = many lines".
Spot the difference
| Point | Food chain | Food web |
|---|---|---|
| Shape | Line | Net |
| Paths | One | Many |
| Stability | Weak | Higher |
Practice questions
- 1What is a food chain?
- 2What is a food web?
- 3How many links does a food chain usually have?
- 4In which direction do the arrows point in a food chain?
- 5Write a food chain with 4 links from a field.
- 6Why is a food web more stable than a food chain?
- 7If the number of snakes falls, what happens to rats and to eagles?
- 8A frog is eaten by a snake, a bird and a lizard. How many consumers does the frog support in the web, and why is it called prey?
Show answers (try them yourself first)
- 1The definite sequence in which organisms eat one another.
- 2Many food chains joined at different levels.
- 3Four, five or more.
- 4From the eaten to the eater.
- 5Grain plant → rat → snake → eagle.
- 6If one source of food is lost, an animal can eat something else.
- 7Rats increase; eagles may have less to eat.
- 83 consumers; it is prey because it is hunted and eaten by others.
The energy pyramid and one-way flow of energy
When you drink milk that a cow made from grass, you get only a small part of the energy that was in the grass. Most is used by the cow to walk, breathe and keep warm. The same happens at every step of a chain.
The pyramid of energy (ऊर्जा पिरॅमिड) shows that energy falls from producers to top consumers.
In the figure : In the figure only about one tenth of the energy goes to the next level.
Rule 1: The energy pyramid — The amount of matter and energy decreases from producers to top consumers, and so does the number of organisms. This pattern is the pyramid of energy. Charles Elton proposed the ecological pyramid in 1927, and Lindeman studied food chains and energy flow in 1942. In an aquatic example, phytoplankton hold 10,000 kcal, zooplankton 1000 kcal, fishes 100 kcal and humans 10 kcal: about one tenth at each step.
- Easy : If plants hold 1000 kcal, the next level gets about 100 kcal (one tenth).
- Medium : Grass 5000 kcal → grasshopper 500 → frog 50 → snake 5. Each step is one tenth of the one before.
- Exam-type : In the aquatic example 10,000 → 1000 → 100 → 10 kcal, the humans at the top get 10 ÷ 10,000 = 1/1000 of the energy of the phytoplankton.
The energy pyramid and one-way flow of energy
In the app you can move this figure with your finger.
Open in the appRule 2: One-way flow of energy — The sun is the most important source of energy. Green plants store some solar energy as food. Decomposers release some energy as heat, and no energy returns to the sun, so energy flow is one way. After the death of apex consumers, their energy is available to decomposers (fungi and microbes), which convert the remains into simple carbon compounds that mix with air, water and soil and are absorbed again by plants.
- Easy : Sun → plant → herbivore → carnivore → heat lost. Energy never returns to the sun.
- Medium : Because of one-way flow, an ecosystem needs sunlight all the time to run.
- Exam-type : Why are there few lions and many deer? Little energy remains at the top, so it can support only a few lions.
Energy falls about one tenth at each step. Energy flow is one way; nutrients cycle.
Thinking energy goes round and round like water. Energy is lost as heat and does not return.
Memory clue: "10 percent rule": each level keeps about 10.
Spot the difference
| Level | Energy in the example |
|---|---|
| Phytoplankton | 10,000 kcal |
| Zooplankton | 1000 kcal |
| Fishes | 100 kcal |
| Humans | 10 kcal |
Practice questions
- 1What is the pyramid of energy?
- 2Who proposed the ecological pyramid and when?
- 3What fraction of energy passes to the next level?
- 4What is the main source of energy in an ecosystem?
- 5Why is the flow of energy one way?
- 6Producers hold 20,000 kcal. How much reaches the secondary consumers (third level) by the 10% rule?
- 7If a top consumer gets 8 kcal, how much did the producers have in a four-level chain?
- 8Why do food chains rarely have more than four or five links?
Show answers (try them yourself first)
- 1The pattern of decrease of energy from producers to top consumers.
- 2Charles Elton, 1927.
- 3About one tenth.
- 4The sun.
- 5Some energy is lost as heat at every step and none returns to the sun.
- 620,000 → 2000 → 200 kcal.
- 78 × 10 × 10 × 10 = 8000 kcal
- 8Energy falls by about one tenth at each step, so little remains for further links.
Bio-geo-chemical cycles and the carbon cycle
A plastic bottle used again is useful, but a bottle thrown away is waste. In nature, the nutrients that plants and animals use are used again and again. Energy flows one way, but materials go round in cycles.
The bio-geo-chemical cycle (जैव-भू-रासायनिक चक्र) is the cyclic flow of nutrients between living and non-living parts of the earth.
In the figure : In the figure carbon keeps moving between air, plants, animals and decomposers.
Rule 1: Types of cycles — Energy flow is one way but nutrient flow is cyclical. The bio-geo-chemical cycle runs through the biosphere formed by the lithosphere, atmosphere and hydrosphere. In a gaseous cycle nutrients accumulate in the atmosphere (nitrogen, oxygen, carbon dioxide, water vapour). In a sedimentary cycle they accumulate in soil, sediment and sedimentary rocks (iron, calcium, phosphorus). The gaseous cycle is faster. Climate change and human activities affect the speed and balance of cycles.
- Easy : Nitrogen and oxygen cycle through the air, so they are gaseous cycles.
- Medium : Phosphorus is stored in soil and rocks, so its cycle is sedimentary and slower.
- Exam-type : Burning forests and fossil fuels increases CO₂ in the air, which speeds up the carbon going to the atmosphere and upsets the balance.
Rule 2: The carbon cycle — Plants convert CO₂ into carbohydrates by photosynthesis. Carbon passes from plants to herbivores, carnivores and apex consumers. After death decomposers release CO₂ again. CO₂ also returns by burning fossil fuels and wood, forest fires and volcanoes. Carbon occurs in coal, granite, diamond and limestone. Plants maintain the balance of oxygen and CO₂.
- Easy : Plant → herbivore → carnivore: carbon moves along the food chain.
- Medium : A dead leaf is broken by decomposers, which release CO₂ into the air.
- Exam-type : Carbon is found in coal, limestone and diamond, as well as in all living things, so it can sit locked for a long time in some of them.
Bio-geo-chemical cycles and the carbon cycle
In the app you can move this figure with your finger.
Open in the appEnergy one way; nutrients cycle. Gaseous cycle (faster) and sedimentary cycle (slower). Carbon cycle: CO₂ → plants → animals → decomposers → CO₂.
Thinking nutrients are used up like energy. They are recycled.
Memory clue: "Gas = Fast, Sediment = Slow".
Spot the difference
| Type | Stored in | Speed | Example |
|---|---|---|---|
| Gaseous | Atmosphere | Faster | N, O, C |
| Sedimentary | Soil, rocks | Slower | Fe, Ca, P |
Practice questions
- 1Which flow is one-way, energy or nutrients?
- 2What is a gaseous cycle?
- 3Name two elements of a sedimentary cycle.
- 4Which cycle is faster?
- 5How do plants take carbon from the air?
- 6Name two ways in which CO₂ goes back into the air.
- 7Where is carbon found other than in living things?
- 8Why do forest fires and burning of fossil fuels disturb the carbon cycle?
Show answers (try them yourself first)
- 1Energy.
- 2A cycle where nutrients accumulate in the atmosphere.
- 3Iron, calcium or phosphorus (any two).
- 4The gaseous cycle.
- 5By photosynthesis, as CO₂ becomes carbohydrates.
- 6Respiration and decay by decomposers, burning of fuel, volcanoes (any two).
- 7In coal, granite, diamond and limestone.
- 8They add large amounts of CO₂ to the air faster than plants can take it back.
Oxygen and nitrogen cycles
The air you breathe is mostly nitrogen, yet your body cannot use it directly. Farmers add urea or plant legumes to get nitrogen for crops. Oxygen, on the other hand, is used up when a candle burns, and plants give it back. These two gases go through cycles.
Nitrogen fixation (नायट्रोजन स्थिरीकरण) turns free nitrogen into usable compounds.
In the figure : In the figure most organisms cannot use free nitrogen; bacteria help to make it usable.
Rule 1: Oxygen cycle — Oxygen forms 21% of the atmosphere and is also in the hydrosphere and lithosphere. It is released by photosynthesis and used up in respiration, combustion, decomposition, corrosion and rusting. It is highly reactive and occurs as O₂, H₂O, CO₂ and other compounds, so its cycle is complex. Aerobes use oxygen; anaerobes do not. Ozone (O₃) is produced from oxygen.
- Easy : Air is 21% oxygen: 100 L of air holds 21 L of oxygen.
- Medium : A room of 60 m³ = 60,000 L holds 60,000 × 21 ÷ 100 = 12,600 L of oxygen.
- Exam-type : Oxygen is used by respiration, burning and rusting, and returned by photosynthesis. The balance depends on green plants, so cutting forests hurts.
Rule 2: Nitrogen cycle — Nitrogen forms 78% of the atmosphere and is part of proteins and nucleic acids. It is inactive, and most organisms cannot use free nitrogen. Processes: nitrogen fixation (nitrogen to nitrates and nitrites); ammonification (ammonia released from dead bodies and wastes); nitrification (ammonia to nitrite, then nitrate); denitrification (nitrogen compounds back to gaseous nitrogen).
- Easy : Nitrogen is 78% of the air, but plants take it in as nitrates.
- Medium : 100 L of air has 78 L of nitrogen and 21 L of oxygen: together 99 L, so 1 L is other gases.
- Exam-type : The steps in order: fixation (N₂ → nitrates), ammonification (dead matter → ammonia), nitrification (ammonia → nitrite → nitrate), denitrification (back to N₂).
Oxygen and nitrogen cycles
In the app you can move this figure with your finger.
Open in the appO₂ 21% of air, N₂ 78%. Oxygen: photosynthesis gives, respiration and burning use. Nitrogen: fixation, ammonification, nitrification, denitrification.
Saying animals and plants can breathe in nitrogen and use it. Most cannot; bacteria first change it.
Memory clue: "F-A-N-D": Fixation, Ammonification, Nitrification, Denitrification.
Spot the difference
| Gas | Share of air | Given by | Used by |
|---|---|---|---|
| Oxygen | 21% | Photosynthesis | Respiration, burning |
| Nitrogen | 78% | Denitrification | Plants as nitrates |
Practice questions
- 1What percent of air is oxygen?
- 2What percent of air is nitrogen?
- 3Which process puts oxygen into the air?
- 4Name two processes that use up oxygen.
- 5What is nitrogen fixation?
- 6What is denitrification?
- 7How much oxygen is there in 500 L of air?
- 8Why can most organisms not use the abundant nitrogen of air directly?
Show answers (try them yourself first)
- 121 percent
- 278 percent
- 3Photosynthesis
- 4Respiration and combustion (or rusting).
- 5Conversion of free nitrogen into nitrates and nitrites.
- 6Conversion of nitrogen compounds back into gaseous nitrogen.
- 7500 × 21 ÷ 100 = 105 L
- 8Free nitrogen is inactive; it must first be changed into nitrates by bacteria.
Quick revision
- Energy: one way (sun to producers to consumers, lost as heat). Nutrients: cyclic.
- Energy and numbers decrease up the pyramid.
- Cycles: carbon, oxygen, nitrogen; gaseous and sedimentary types.