Ecosystem and its components
A school garden has plants, insects, birds, soil, water and sunshine. Take away any one of them and the whole garden changes, because every part depends on the others.
An ecosystem is all the living things of an area together with the non-living parts around them, working as one system. The living part is the biotic component; the non-living part (sunlight, temperature, rain, soil, minerals) is the abiotic component. Biotic components are producers (green plants and some bacteria that make food by photosynthesis), consumers (herbivores, carnivores, omnivores and parasites that depend on producers) and decomposers (bacteria and fungi that break dead remains into simple substances which plants reuse). Forests, ponds and lakes are natural ecosystems; crop fields, gardens and aquariums are human-made.
Equation / formula : Ecosystem = biotic + abiotic · producers → consumers → decomposers · matter is recycled
In the figure : Producers, consumers and decomposers are the living parts; sunlight, water and soil are the non-living parts.
Rule 1 : Biotic or abiotic? — Ask: is it alive, or part of a living thing? Living organisms are biotic. Physical factors such as light, water, temperature, soil and minerals are abiotic.
- Easy : In a pond, algae and tadpoles are biotic; water, mud and sunshine are abiotic.
- Medium : A garden has rose bushes, bees, soil and sunshine. Biotic: rose bushes and bees. Abiotic: soil and sunshine. So 2 of the 4 are abiotic.
- For exams : A student lists dissolved oxygen in a pond as biotic. This is wrong: oxygen is a non-living factor, so it is abiotic, even though living things need it.
Rule 2 : Roles of organisms — Producers make food from sunlight. Consumers eat producers or other consumers. Decomposers break dead matter down and return nutrients to the soil. Without decomposers, dead matter piles up and the soil runs short of nutrients.
- Easy : Wheat plant = producer; goat = consumer; mould on stale bread = decomposer.
- Medium : On a lawn: grass → grasshopper → lizard. Grass is the producer; the grasshopper and the lizard are consumers; fungi on dead leaves are decomposers.
- For exams : A forest has no decomposers for a year. Fallen leaves pile up and the soil loses nutrients, so new plants grow poorly.
An aquarium needs people to feed it and clean it. A pond looks after itself because it has many decomposers and a balanced food supply.
Calling a fungus a producer because it grows on wood. Fungi cannot photosynthesise; they are decomposers.
Alive = biotic; physical factor = abiotic; makes food = producer; eats = consumer; cleans up = decomposer. Memory clue : "P-C-D": Producers Prepare food, Consumers Chew it, Decomposers Digest the dead.
Spot the difference
| Point | Biotic components | Abiotic components |
|---|---|---|
| Nature | living organisms | non-living physical factors |
| Examples | plants, animals, fungi, bacteria | sunlight, rain, soil, temperature |
Practice questions
- 1What do we call all the living things of an area together with the non-living parts around them?
- 2Name the three kinds of biotic components.
- 3Give two examples of abiotic components.
- 4Why are decomposers called the cleaners of an ecosystem?
- 5A pond has algae, fish, mud, water and bacteria. How many of these five are biotic?
- 6Classify a wheat field, a lake and an aquarium as natural or human-made.
- 7Why is a predator not kept with the fish in a home aquarium?
- 8Dead leaves in a forest vanish in a few months, but a plastic bottle stays. Which group of organisms causes the first change?
Show answers (try them yourself first)
- 1An ecosystem.
- 2Producers, consumers and decomposers.
- 3Any two of sunlight, temperature, rain, soil, minerals.
- 4They break down dead remains and wastes and return nutrients to the soil.
- 5Three: algae, fish and bacteria.
- 6Wheat field: human-made; lake: natural; aquarium: human-made.
- 7It would eat the fish and upset the balance.
- 8Decomposers (bacteria and fungi).
Food chains, trophic levels and food webs
A sparrow eats a caterpillar, the caterpillar eats a leaf, and a hawk may later catch the sparrow. Each one eats the one before it, like links in a chain.
A food chain is a series of organisms in which each one feeds on the one before it and food energy moves along. Every step is a trophic level. Producers form the first level; herbivores (primary consumers) the second; small carnivores (secondary consumers) the third; large carnivores (tertiary consumers) the fourth. The arrow always points from the one that is eaten to the one that eats it. In nature, most organisms eat more than one kind of food, so many chains join into a net called a food web. A web is more realistic and more stable than one chain: if one food source disappears, an animal can switch to another.
Equation / formula : Producer → herbivore → small carnivore → large carnivore · arrow = eaten → eater · food web = many chains joined
In the figure : Each arrow shows who is eaten and who eats; the producer is always at the start.
Food chains, trophic levels and food webs
In the app you can move this figure with your finger.
Open in the appRule 1 : Naming the trophic level — Count from the producer: producer = level 1, then each step adds one. A primary consumer eats producers; a secondary consumer eats primary consumers; a tertiary consumer eats secondary consumers.
- Easy : Paddy → rat → snake → eagle: paddy is level 1 and the eagle is level 4.
- Medium : Algae → water flea → small fish → heron. The small fish is at level 3, so it is a secondary consumer.
- For exams : A man eats rice and also fish. As a rice-eater he is at level 2; as a fish-eater he is at a higher level. So an omnivore can sit at more than one trophic level.
Rule 2 : Food chain or food web? — One straight line of eating is a chain. When an organism is eaten by, or eats, more than one kind, the chains cross and form a web. If one link of a web is lost, other paths keep the rest alive.
- Easy : Grass → deer → tiger is a chain. If the tiger also eats wild boar, which eats roots, the chains form a web.
- Medium : A snake eats both rats and frogs. It belongs to two chains at once, so the snake links two chains into a web.
- For exams : Deer vanish from a forest. A leopard that eats only deer would starve, but one that also eats hares and goats can switch. Food webs make an ecosystem more stable than a single chain.
Decomposers act on dead remains from every level of the chain, so they are not placed as a step in it.
Drawing the arrow from the eater to the eaten. The arrow shows the flow of food energy: from the eaten to the eater.
Start at the producer, count the steps, arrow to the eater; many chains = web. Memory clue : "Point to the mouth": the arrow head always points to the mouth that eats.
Spot the difference
| Point | Food chain | Food web |
|---|---|---|
| Shape | one straight line | branching network |
| Stability | breaks if one link is lost | other paths keep it going |
Practice questions
- 1What is a trophic level?
- 2Which organisms always form the first trophic level?
- 3In which direction does the arrow point in a food chain?
- 4In wheat → mouse → owl, name the primary consumer.
- 5In algae → tadpole → fish → heron, at which trophic level is the fish?
- 6Why is a food web more stable than a single food chain?
- 7If all herbivores of a grassland are removed, what happens to the carnivores?
- 8Is grass → tiger → deer a valid food chain? Give a reason.
Show answers (try them yourself first)
- 1Each step or feeding level of a food chain.
- 2Producers (green plants).
- 3From the organism that is eaten to the one that eats it.
- 4The mouse.
- 5The third trophic level.
- 6Animals can switch to other food if one source disappears.
- 7They lose their main food and their numbers fall.
- 8No. A tiger does not eat grass, so the chain does not follow who eats whom.
Food chains, trophic levels and food webs
In the app you can move this figure with your finger.
Open in the appEnergy flow and biological magnification
A solar lamp charges all day, yet only part of the sunlight becomes stored energy and the rest is lost as heat. Plants are the same: they store only a small part of the sunlight that reaches them.
The Sun is the source of energy for almost every ecosystem. Green plants capture only about 1% of the sunlight falling on their leaves and store it as food energy. Energy then flows in one direction only: Sun → producers → consumers. It never goes back to the Sun or to the previous level. At each step only about 10% of the energy is passed on (the 10 per cent law); the rest is lost as heat in breathing and other activities. This is why food chains have only three or four steps, and why producers hold the most energy. In contrast, harmful chemicals such as some pesticides cannot be broken down. They enter plants through soil and water and then build up at every higher level. This is biological magnification: the concentration is least in producers and greatest in top carnivores, including humans.
Equation / formula : Next level gets about 10% · 100% → 10% → 1% → 0.1% · energy falls upward, harmful chemicals rise upward
In the figure : Energy shrinks to one-tenth at every step, so very little reaches the top of a long chain.
Rule 1 : Applying the 10 per cent law — Multiply by 0.1 for each step you go up the chain. To go down, divide by 0.1, which means multiply by 10.
- Easy : Producers hold 10000 J: the primary consumers get 10000 × 0.1 = 1000 J.
- Medium : Producers hold 50000 J. The secondary consumers get 50000 × 0.1 × 0.1 = 500 J.
- For exams : The tertiary consumers receive 8 J. Working back: secondary 80 J, primary 800 J, producers 8000 J.
Rule 2 : Biological magnification — A non-degradable chemical cannot be broken down or easily removed from the body, so it gathers at every step. It is lowest in producers and highest in the top carnivores, including humans.
- Easy : Algae → small fish → large fish → eagle: the pesticide is lowest in the algae and highest in the eagle.
- Medium : A pesticide reads 0.02 units in water plants, 0.4 in small fish and 5 in a fish-eating bird. The reading rises up the chain: this is biological magnification.
- For exams : Energy falls as we go up, while a non-degradable chemical rises. A student who says both fall is wrong.
Washing does not fully remove pesticide residues from grain or vegetables, because part of the chemical is absorbed into the plant.
Saying energy is recycled like nutrients. Nutrients go round and round; energy flows one way and is lost as heat.
1% caught, 10% passed on; energy falls upward, harmful chemicals pile up. Memory clue : "1 in, 10 on": plants catch 1% of sunlight, then each step passes on 10%.
Spot the difference
| Point | Energy | Harmful chemical |
|---|---|---|
| Going up the chain | decreases | increases |
| Reason | lost as heat at each step | non-degradable, so it builds up |
Practice questions
- 1What is the source of energy for almost all ecosystems?
- 2About what percentage of sunlight falling on leaves do plants capture?
- 3State the 10 per cent law.
- 4Why do food chains rarely have more than four steps?
- 5Producers hold 20000 J. How much energy can reach the large carnivores at the fourth level?
- 6Secondary consumers hold 150 J. How much did the primary consumers hold?
- 7Define biological magnification.
- 8In algae → small fish → large fish → bird, where is a pesticide most concentrated, and why?
Show answers (try them yourself first)
- 1The Sun.
- 2About 1%.
- 3Only about one-tenth of the energy passes from one trophic level to the next.
- 4Energy is lost at each step, so too little is left to support more levels.
- 520000 × 0.1 × 0.1 × 0.1 = 20 J.
- 6150 ÷ 0.1 = 1500 J.
- 7The build-up of a harmful non-degradable chemical at each higher trophic level.
- 8In the bird, because it is at the top and the chemical builds up at every step.
Ozone layer and its depletion
On a sunny day we wear a cap or put on sunscreen, because the Sun’s ultraviolet rays can burn the skin. High in the sky, a thin layer of ozone acts like a huge sunscreen for the whole planet.
Ozone (O3) has three oxygen atoms; the oxygen we breathe (O2) has two. High in the atmosphere, ultraviolet (UV) radiation splits an O2 molecule into two free oxygen atoms, and each free atom joins another O2 to form O3. This ozone layer absorbs most of the Sun’s harmful UV rays, which can cause skin cancer and damage other living things. Near the ground, though, ozone is a poisonous gas. In the 1980s the amount of ozone began to fall sharply. The cause was chlorofluorocarbons (CFCs), synthetic chemicals used in refrigerators and fire extinguishers. In 1987 the UNEP agreement froze CFC production at 1986 levels, and manufacturers now make CFC-free refrigerators.
Equation / formula : O2 → O + O (UV) · O + O2 → O3 · CFCs destroy O3 · less ozone = more UV reaches the earth
In the figure : Strong UV high in the sky makes ozone; the ozone layer then shields the earth from UV.
Rule 1 : How ozone forms — There are two steps. Step 1: UV splits O2 into two free atoms (O2 → O + O). Step 2: each free atom joins an O2 molecule (O + O2 → O3). Strong UV is needed, so ozone forms high in the atmosphere.
- Easy : Write the two steps: O2 → O + O, then O + O2 → O3.
- Medium : Two free O atoms join two O2 molecules to make two O3 molecules. Atoms before: 2 + 4 = 6. Atoms after: 2 × 3 = 6. The atoms are conserved.
- For exams : Ozone does not form in a dark room, because the first step needs strong UV to split O2. That is why it forms high in the atmosphere.
Rule 2 : Ozone depletion and protection — CFCs from old refrigerators and similar sources rise and destroy ozone, so the layer thins and more UV reaches the earth. We protect the layer by using CFC-free products and by following agreements such as the 1987 UNEP agreement.
- Easy : A new fridge labelled CFC-free does not harm the ozone layer.
- Medium : The thinner the ozone layer, the more UV reaches the ground, so the risk of skin cancer rises.
- For exams : A student says ozone is good everywhere. This is wrong: high above, it protects us, but at ground level it is a poisonous gas.
The 1987 agreement shows that countries acting together can reduce damage to the environment.
Mixing up O2 and O3, or saying CFCs make ozone. CFCs destroy ozone.
O3 high up = shield; O3 near the ground = poison; CFC = ozone destroyer. Memory clue : "Ozone is the sky’s umbrella": it blocks the UV rain, and CFCs poke holes in it.
Spot the difference
| Point | Oxygen (O2) | Ozone (O3) |
|---|---|---|
| Atoms in a molecule | 2 | 3 |
| Role | needed for breathing | shield high up; poison near the ground |
Practice questions
- 1What is the chemical formula of ozone?
- 2Where in the atmosphere is the ozone layer found?
- 3Which harmful radiation does the ozone layer absorb?
- 4Name the chemicals mainly responsible for the fall in ozone.
- 5Write the two steps by which ozone forms.
- 6Why did UNEP ask countries in 1987 to freeze CFC production?
- 7Why is ozone useful high up but harmful at ground level?
- 8Name two earlier uses of CFCs and one step you can take to protect the ozone layer.
Show answers (try them yourself first)
- 1O3.
- 2In the upper atmosphere.
- 3Ultraviolet (UV) radiation.
- 4Chlorofluorocarbons (CFCs).
- 5O2 → O + O (UV), then O + O2 → O3.
- 6To limit the damage to the ozone layer.
- 7High up it absorbs UV and protects living things; near the ground it is a poisonous gas.
- 8Refrigerators and fire extinguishers; buy CFC-free products.
Ozone layer and its depletion
In the app you can move this figure with your finger.
Open in the appWaste management and biodegradability
Banana peels buried in a pit turn into soil within weeks, while a plastic wrapper dropped next to them looks the same after a year. Some waste rots away; some stays.
Waste that bacteria, fungi and other decomposers can break down by biological processes is biodegradable, for example vegetable peels, dry leaves, paper, cotton and dung. Waste that these organisms cannot break down is non-biodegradable, for example plastic, glass, aluminium foil and metal cans. The reason is that enzymes are specific: each enzyme works only on certain substances, and no organism has an enzyme for most human-made materials. So non-biodegradable waste stays for a very long time, pollutes soil and water, harms animals that swallow it and blocks drains. Even biodegradable waste causes smell and pests when it piles up in large heaps. Good management means separating waste, making compost from biodegradable waste, recycling or reusing the rest, treating sewage before release, and handling e-waste (which has heavy metals) carefully. The 3 Rs are Reduce, Reuse and Recycle.
Equation / formula : Enzymes are specific: no enzyme, no breakdown · Reduce + Reuse + Recycle · Separate, compost, treat
In the figure : Sort waste into two groups, compost the first, reuse or recycle the second, and use less in the first place.
Rule 1 : Sorting waste — Ask: can bacteria and fungi break it down? Things that come from plants or animals usually can (biodegradable). Human-made plastics, glass and metals cannot (non-biodegradable).
- Easy : Tea leaves and dry leaves are biodegradable; a plastic straw and a glass jar are not.
- Medium : A school makes 40 kg of waste a day and 25% is non-biodegradable. That is 40 × 0.25 = 10 kg non-biodegradable and 40 − 10 = 30 kg biodegradable.
- For exams : A home makes 800 g of waste and 560 g of it is biodegradable. The percentage is 560 ÷ 800 × 100 = 70%.
Rule 2 : Managing waste — Separate the waste first. Compost biodegradable waste into manure; reuse or recycle non-biodegradable waste; treat sewage and factory waste before releasing it. Follow the 3 Rs: reduce, reuse, recycle.
- Easy : Carry a cloth bag instead of taking a new plastic bag each time: this reduces waste.
- Medium : Using an old glass jar to store things at home is reuse. Sending empty plastic bottles to a recycler is recycling.
- For exams : Dumping old computers on an empty plot is unsafe, because heavy metals can leak into soil and water. Giving them to an authorised e-waste collector is the safe choice.
Recycling helps, but it also uses energy, so reducing and reusing come before recycling.
Thinking all disposable cups are equal. A paper cup decays, a cloth bag can be reused many times, but a plastic cup stays for ages.
From plants or animals = decays; plastic, glass, metal = stays; Reduce first, then Reuse, then Recycle. Memory clue : "3 R" in order: Reduce (use less), Reuse (use again), Recycle (make new).
Spot the difference
| Point | Biodegradable | Non-biodegradable |
|---|---|---|
| Broken down by | bacteria and fungi | no organism has the right enzyme |
| Examples | vegetable peels, paper, dung | plastic, glass, aluminium cans |
Practice questions
- 1What is a biodegradable substance?
- 2Name two non-biodegradable materials.
- 3What are the 3 Rs of waste management?
- 4Why can no organism break down most plastics?
- 5A class makes 5 kg of waste a day and 2 kg of it is non-biodegradable. How much is biodegradable?
- 6A home makes 800 g of waste and 560 g is biodegradable. What percentage is biodegradable?
- 7Why can a big heap of kitchen waste still harm the surroundings, even though it is biodegradable?
- 8Why must sewage be treated before it is released into a river?
Show answers (try them yourself first)
- 1One that bacteria, fungi or other decomposers can break down by biological processes.
- 2Any two of plastic, glass, aluminium foil, metal cans.
- 3Reduce, Reuse and Recycle.
- 4Enzymes are specific, and no organism has an enzyme for them.
- 55 − 2 = 3 kg.
- 6560 ÷ 800 × 100 = 70%.
- 7It gives bad smell and attracts pests, and a huge amount rots faster than decomposers can handle.
- 8Untreated sewage pollutes the water and harms aquatic life.