KS3 Science Study Pack: Ecosystems

Key Knowledge

An ecosystem is made from all the living organisms in an area and the non-living conditions around them. Living organisms include plants, animals, fungi, bacteria and algae. Non-living conditions include temperature, light, water, soil, oxygen, rocks and pollution levels. Ecosystems can be large, such as a woodland or rocky shore, but they can also be small, such as a pond, hedgerow, garden, school field or rotting log.

Ecosystems work because organisms are interdependent. This means they depend on each other and on their environment. A bee may depend on flowers for nectar. A flowering plant may depend on bees for pollination. A robin may depend on insects for food. Fungi and bacteria may depend on dead leaves for energy, while plants benefit when decomposition returns useful substances to the soil.

Energy enters most ecosystems when green plants and algae photosynthesise. They are called producers because they make their own food. Consumers get energy by eating other organisms. Food chains and food webs show feeding relationships and energy transfer. The arrows always show the direction that energy moves, from food to feeder.

Healthy ecosystems usually have good biodiversity. Biodiversity means the variety of living organisms in an area. This includes plants, animals, fungi, microorganisms and the variety of habitats. A biodiverse ecosystem is often more stable because if one food source decreases, some organisms may have other food sources available.

Human activity can damage ecosystems through habitat destruction, pollution, pesticides, overfishing, hunting, climate change and introducing non-native species. Humans can also protect ecosystems by creating nature reserves, planting hedgerows and wildflowers, reducing pollution, using sustainable fishing, building wildlife corridors and protecting pollinators.

What Is an Ecosystem?

An ecosystem includes:

• all the living organisms in a place
• all the non-living conditions around them
• the interactions between organisms and their environment

Examples of ecosystems include:

• a pond with pondweed, algae, water fleas, snails, fish, frogs and bacteria
• a woodland with oak trees, brambles, caterpillars, mice, owls, foxes and fungi
• a garden with flowering plants, bees, aphids, ladybirds, robins and decomposers
• a hedgerow that provides food, shelter and routes for animals to move between habitats
• a rocky shore with seaweed, limpets, mussels, crabs, small fish and gulls
• a grassland or school field where plants compete for light and space

An ecosystem is not just the place. It is the living organisms, the physical conditions, and the relationships between them.

Pond Ecosystem Diagram

                  light
                   |
                   v
        reeds and pondweed (producers)
          |                  |
          v                  v
     snails and tadpoles   water fleas
          |                  |
          v                  v
     dragonfly nymphs ---> sticklebacks
          |                  |
          +---------> frogs -+
                         |
                         v
                       heron

Non-living parts: water, mud, rocks, oxygen, temperature, light
Decomposers: bacteria and fungi break down dead plants and animals

In this pond, the living parts include pondweed, snails, tadpoles, fish, frogs, herons, bacteria and fungi. The non-living parts include water, oxygen, mud, light and temperature.

Habitats, Populations, Communities, and Species

These words are often mixed up, but they have different scientific meanings.

Term	Meaning	Example
Species	A group of organisms that can reproduce with each other to produce offspring of the same species	Blue tit, oak tree, common frog
Habitat	The place where an organism lives	A pond, hedge, tree bark, leaf litter
Population	All the organisms of one species living in an area	All the frogs in one pond
Community	All the populations of different species living in an area	Frogs, pondweed, snails, water fleas and fish in a pond
Ecosystem	A community plus the non-living conditions around it	A pond community with water, oxygen, mud, light and temperature
Environment	The surroundings and conditions an organism lives in	A shady, damp woodland floor
Niche	The role an organism has in its ecosystem, including how it gets food and where it lives	A bee feeds on nectar and helps pollinate flowers

A habitat is part of an ecosystem. For example, a hedgerow can be a habitat for a robin, a mouse and many insects. The hedgerow ecosystem also includes the soil, light, temperature, moisture and interactions between organisms.

Producers, Consumers, Predators, Prey, and Decomposers

Every organism has a role in its ecosystem.

Feeding role	Meaning	Example
Producer	An organism that makes its own food, usually by photosynthesis	Grass, algae, oak tree, pondweed
Primary consumer	A consumer that eats producers	Rabbit eating grass, caterpillar eating oak leaves
Secondary consumer	A consumer that eats primary consumers	Blue tit eating caterpillars
Predator	An animal that hunts, kills and eats other animals	Fox, owl, spider, ladybird
Prey	An animal that is hunted and eaten by a predator	Rabbit eaten by fox, aphid eaten by ladybird
Herbivore	An animal that eats plants or algae	Snail, rabbit, caterpillar
Carnivore	An animal that eats other animals	Sparrowhawk, fox, crab
Omnivore	An animal that eats both plants and animals	Mouse, blackbird, human
Scavenger	An animal that feeds on dead animals	Crow, some crabs, some beetles
Decomposer	An organism that breaks down dead material and waste	Bacteria, fungi, earthworms

Producers are very important because they make food using light energy. Consumers depend directly or indirectly on producers. A fox does not eat grass, but it may eat a rabbit that ate grass. This means the fox depends indirectly on grass.

Decomposers are also essential. Bacteria, fungi and earthworms break down dead plants, dead animals and animal waste. This returns useful substances to the soil, where plants can use them for growth. Decomposers are not simply dirty or harmful; without them, dead material would build up and soil would become less fertile.

Predators are not always bigger than their prey. A ladybird is a predator of aphids. A dragonfly nymph is a predator in a pond. A spider is a predator of flies. The key point is that a predator catches and eats another animal.

Food Chains and Energy Transfer

A food chain shows one pathway for energy transfer through feeding. It begins with a producer.

grass -> rabbit -> fox
producer  prey     predator

The arrow means energy is transferred from the food to the feeder. In the chain grass -> rabbit, energy moves from the grass to the rabbit because the rabbit eats the grass. The arrow does not mean "points to what gets eaten". A good way to read the arrow is "is eaten by".

Worked Example: Building a Food Chain

Description: In a field, rabbits eat grass. Foxes eat rabbits.

Step 1: Find the producer.

Grass is a green plant, so it is the producer.

Step 2: Find the organism that eats the producer.

The rabbit eats grass, so the rabbit is a consumer and a herbivore.

Step 3: Find the predator.

The fox eats the rabbit, so the fox is a predator. The rabbit is the fox's prey.

Step 4: Write the food chain using arrows for energy transfer.

grass -> rabbit -> fox

Energy passes from grass to rabbit to fox.

Worked Example: Arrow Direction

oak leaf -> caterpillar -> blue tit -> sparrowhawk

This means:

• energy passes from the oak leaf to the caterpillar
• energy passes from the caterpillar to the blue tit
• energy passes from the blue tit to the sparrowhawk

It does not mean that the caterpillar feeds the oak leaf. The caterpillar eats the oak leaf. The arrow shows the direction of energy transfer.

Food Chain Examples

Food chain	Producer	Primary consumer	Predator-prey pair
grass -> rabbit -> fox	Grass	Rabbit	Fox and rabbit
algae -> water flea -> stickleback	Algae	Water flea	Stickleback and water flea
oak leaf -> caterpillar -> blue tit -> sparrowhawk	Oak leaf	Caterpillar	Sparrowhawk and blue tit
seaweed -> limpet -> crab -> gull	Seaweed	Limpet	Crab and limpet; gull and crab
pondweed -> snail -> frog -> heron	Pondweed	Snail	Frog and snail; heron and frog

Food chains are not circular. They show one direction of energy transfer. Energy is eventually transferred to the surroundings, so it is not recycled in the same way that useful substances can be recycled by decomposers.

Food Webs and Interdependence

A food web is made from many connected food chains. Food webs are more realistic than food chains because most organisms eat more than one type of food and may be eaten by more than one predator.

Woodland Food Web

                         owl
                          ^
                          |
oak leaves -> caterpillar -> blue tit -> sparrowhawk
    |             ^             ^
    |             |             |
    v             |             |
  aphids -------> ladybird      |
    ^                           |
    |                           |
brambles -> rabbit ----------> fox
    |          ^
    v          |
   mouse ------+

Dead leaves and waste -> fungi, bacteria and earthworms

In this food web:

• oak leaves and brambles are producers
• caterpillars, aphids, rabbits and mice are consumers
• caterpillars, aphids and rabbits are herbivores
• blue tits, ladybirds, foxes, owls and sparrowhawks are predators
• the blue tit has more than one possible food source
• decomposers break down dead leaves and waste

Worked Example: Identifying Organisms in a Food Web

Use the woodland food web above.

One producer: oak leaves or brambles.

One herbivore: caterpillar, aphid or rabbit.

One carnivore: owl, fox, sparrowhawk, blue tit or ladybird.

One predator-prey pair: ladybird and aphid, or fox and rabbit.

One organism with more than one food source: blue tit, because it can eat caterpillars and ladybirds in this simplified web.

Interdependence

Interdependence means organisms rely on each other. They may depend on each other for:

• food
• shelter
• pollination
• seed dispersal
• decomposition
• population control

In a garden ecosystem, flowering plants provide nectar for bees and butterflies. Bees and butterflies can transfer pollen between flowers, helping plants reproduce. Aphids feed on plant sap. Ladybirds eat aphids, helping to control aphid numbers. Robins eat insects and worms. Hedgehogs eat slugs, beetles and other invertebrates. Decomposers break down dead leaves and return useful substances to the soil.

If one population changes, other populations may change too. Some effects are direct, such as a predator having less prey. Other effects are indirect, such as a plant increasing because the herbivore that ate it has decreased.

Worked Example: Predicting Population Changes

Situation: Slug numbers decrease in a garden.

Step 1: Identify organisms that eat slugs.

Hedgehogs and thrushes may eat slugs.

Step 2: Predict the direct effect.

Hedgehogs and thrushes may have less food, so their populations may decrease or they may need to eat other food.

Step 3: Identify organisms eaten by slugs.

Slugs eat many garden plants.

Step 4: Predict the indirect effect.

Some plants eaten by slugs may increase because fewer slugs are feeding on them.

Step 5: Remember uncertainty.

The exact result depends on other food sources, weather, disease and human actions such as gardening or pesticide use.

Competition and Limited Resources

Competition happens when organisms need the same limited resource. A resource is something an organism needs to survive or reproduce.

Plants may compete for:

• light
• water
• mineral substances in soil
• space

Animals may compete for:

• food
• water
• shelter
• territory
• mates

In a school field, grass plants and daisies may compete for light, water and space. If part of the field is mown often, taller plants may not survive well because they are cut before flowering. In an unmown wildflower area, there may be more plant species because different plants can grow, flower and provide food for insects.

Competition can happen within a species or between different species. Rabbits competing with other rabbits for grass is competition within a species. Rabbits and deer competing for some of the same plants is competition between species.

An organism that is well suited to its environment is more likely to survive and reproduce. At KS3, "survival of the fittest" means being well adapted to the environment, not simply being the strongest.

Predator-Prey Population Changes

Predator and prey populations often affect each other. If prey numbers increase, predators may have more food and their numbers may later increase. If predator numbers increase, more prey may be eaten and prey numbers may later decrease. If prey numbers decrease, predators may have less food and predator numbers may later decrease.

The predator population often changes after the prey population. This delay is called a time lag.

Predator-Prey Population Graph

Population size
  ^
  |       prey
  |      /\        /\        /\
  |     /  \      /  \      /  \
  |    /    \    /    \    /    \
  |   /      \  /      \  /      \
  |  /        \/        \/        \
  |       predator
  |        /\        /\        /\
  |       /  \      /  \      /  \
  |______/____\____/____\____/____\____> time

Worked Example: Explaining a Predator-Prey Graph

Imagine a graph showing rabbits as prey and foxes as predators.

Step 1: Identify which population rises first.

The rabbit population rises first.

Step 2: Explain why the predator population rises after the prey.

When there are more rabbits, foxes have more food. More foxes survive and reproduce, so the fox population rises after a time lag.

Step 3: Explain why the prey population may fall.

When there are more foxes, more rabbits are eaten, so the rabbit population may decrease.

Step 4: Explain why the predator population may later fall.

If rabbit numbers decrease, foxes have less food. Fewer foxes survive and reproduce, so the fox population may decrease.

Step 5: Use careful wording.

Populations usually fluctuate. They do not stay perfectly constant in real ecosystems because weather, disease, competition and human activity also affect them.

More predators do not always mean prey instantly disappear. Predator and prey populations interact over time.

Biodiversity and Ecosystem Stability

Biodiversity is the variety of living organisms in an area. It includes:

• variety of species
• variety within species
• variety of habitats
• plants, animals, fungi and microorganisms

High biodiversity can make an ecosystem more stable and resilient. Resilient means able to recover after a change. In a biodiverse food web, a predator may have several prey species. If one prey species decreases, the predator may survive by eating another. A habitat with many plant species may support more insects, birds and decomposers than a habitat with only one plant species.

Low biodiversity can make ecosystems more vulnerable. If a field contains only one crop species, a disease affecting that crop may damage the whole field. If a food web has few species, removing one species may have a large effect.

Common UK Ecosystem Organisms and Their Roles

Ecosystem	Organism	Role
Pond	Algae	Producer and food for water fleas
Pond	Water flea	Consumer and prey for fish
Pond	Dragonfly nymph	Predator of small pond animals
Pond	Bacteria and fungi	Decomposers
Woodland	Oak tree	Producer, habitat and food source
Woodland	Caterpillar	Herbivore and prey for birds
Woodland	Owl	Predator of mice
Garden	Bee	Pollinator and consumer of nectar
Garden	Ladybird	Predator of aphids
Hedgerow	Hawthorn	Producer, shelter and nesting site
Rocky shore	Seaweed	Producer
Rocky shore	Limpet	Herbivore that grazes on algae

Biodiversity Comparison Table

Students sampled two habitats using quadrats. They counted the number of different plant species in each quadrat.

Quadrat number	Mown lawn: number of plant species	Wildflower area: number of plant species
1	2	6
2	3	7
3	2	5
4	4	8
5	3	6
Total	14	32
Mean per quadrat	2.8	6.4

The wildflower area has higher plant biodiversity because it has a higher mean number of plant species per quadrat. The evidence is that the wildflower area had a mean of 6.4 species per quadrat, compared with 2.8 species per quadrat in the mown lawn.

Worked Example: Calculating a Mean

Question: Find the mean number of plant species in the mown lawn.

Values: 2, 3, 2, 4, 3

Step 1: Add the values.

2 + 3 + 2 + 4 + 3 = 14

Step 2: Count how many quadrats were used.

There were 5 quadrats.

Step 3: Divide the total by the number of quadrats.

14 / 5 = 2.8

Mean number of plant species = 2.8 species per quadrat.

Environmental Factors and Adaptations

Environmental factors are conditions that affect organisms. They can be living or non-living.

Important environmental factors include:

• temperature
• light intensity
• water availability
• soil quality
• oxygen level
• pollution
• disease
• predators
• competition

Organisms have adaptations that help them survive in their environment. An adaptation is a feature or behaviour that helps an organism survive and reproduce.

Examples:

• Cacti have features that reduce water loss, but UK plants such as marram grass are also adapted to dry, sandy conditions.
• Fish have gills to take oxygen from water.
• Earthworms have bodies suited to moving through soil.
• Deciduous trees in the UK lose leaves in autumn, which can reduce water loss and damage during winter.
• Some animals hibernate in winter when food is limited.
• Some birds migrate when seasons change and food supply changes.

Seasonal changes in UK ecosystems include flowering in spring, leaf growth in summer, migration, hibernation, leaf fall in autumn and changes in food supply. These changes affect feeding relationships. For example, fewer insects in winter can mean less food for insect-eating birds.

Natural changes can also affect ecosystems. Drought may reduce plant growth, which can reduce food for herbivores. Flooding may wash away nests or change oxygen levels in water. Storms may damage trees but also create dead wood habitats for decomposers and insects. Disease can reduce a population quickly. A new predator can change prey numbers.

Light Level and Plant Results

Students counted daisies in five areas of a school field with different light levels.

Area	Light level	Number of daisies in one quadrat
A	20%	1
B	40%	4
C	60%	8
D	80%	13
E	100%	15

Pattern: As light level increases, the number of daisies increases.

Evidence: At 20% light there was 1 daisy, but at 100% light there were 15 daisies.

Possible reason: Daisies need light for photosynthesis, so they may grow better in brighter areas.

Limitation: Only one quadrat was used at each light level, so the results may not be reliable.

Improvement: Use several quadrats at each light level and calculate a mean.

Human Impacts on Ecosystems

Human activity can harm ecosystems directly and indirectly.

Human impact	Cause	Effect on organisms	Effect on food web	Possible conservation action
Deforestation	Trees removed for farming, roads or buildings	Woodland species lose habitat and food	Fewer leaves, insects, birds and decomposers	Plant trees, protect woodland, create wildlife corridors
Habitat destruction	Meadows built on or ponds filled in	Populations decrease or move away	Food chains are broken or simplified	Create ponds, protect local habitats, reduce mowing
Pollution	Chemicals, sewage or plastic enter habitats	Organisms may be poisoned, trapped or lack oxygen	Predators may lose prey; decomposers may increase in polluted water	Reduce waste, treat sewage, clean rivers
Pesticides	Chemicals used to kill pests	Insects decrease, including useful insects	Birds and bats may have less food	Use fewer pesticides, encourage natural predators, plant wildflowers
Overfishing	Fish removed faster than they reproduce	Fish populations decrease	Predators such as seabirds may have less food	Sustainable fishing limits and protected areas
Hunting	Animals killed for food, sport or trade	Target populations decrease	Predators, prey and competitors may be affected	Laws, protected species and monitoring
Non-native species	Species introduced by humans	Native species may be outcompeted or eaten	Food webs may change	Biosecurity, monitoring and careful management
Climate change	Greenhouse gas emissions change temperatures and weather	Habitats and food supply change	Seasonal timing may become mismatched	Reduce emissions, restore habitats

Cause-Effect Chain: Pesticide Use

farmer sprays pesticide
          |
          v
insect numbers decrease
          |
          v
less food for insect-eating birds
          |
          v
bird populations may decrease
          |
          v
plants pollinated by insects may produce fewer seeds

This shows direct and indirect effects. The direct effect is that insects decrease. An indirect effect is that birds may decrease because they have less food.

Pond Pollution and Oxygen

If fertiliser runs off fields into a pond, algae may grow quickly. When large amounts of algae die, decomposers break them down. Decomposers use oxygen from the water. If oxygen levels fall too low, pond animals such as fish, tadpoles and water fleas may die or move away. At KS3, the important idea is that pollution can change the non-living conditions in water, especially oxygen levels, and this affects living organisms.

Introduced Species

An introduced or non-native species is a species moved by humans to an area where it does not naturally live. Some introduced species cause few problems, but others can outcompete native species. For example, grey squirrels can compete with red squirrels for food and habitat. Some non-native plants can grow quickly and shade native plants. This reduces biodiversity and changes habitats.

Conservation and Protecting Biodiversity

Conservation means protecting and managing ecosystems and species. It does not always mean leaving everything untouched. Sometimes careful management improves habitats.

Positive human actions include:

• creating nature reserves
• rewilding some areas to allow more natural processes
• planting hedgerows as wildlife corridors
• creating ponds
• planting wildflowers for pollinators
• leaving dead wood for fungi and invertebrates
• reducing mowing in some areas
• reducing pollution
• using sustainable fishing
• protecting local nature reserves
• avoiding pesticides where possible

A hedgerow can be a wildlife corridor. It links habitats so animals can move between them. It also provides food, shelter and nesting sites. Removing hedgerows can isolate populations, making it harder for organisms to find food, mates and shelter.

Conservation actions are most useful when they match the problem. If pollinators are decreasing, planting nectar-rich flowers and reducing pesticide use can help. If pond organisms are decreasing because of pollution, reducing run-off and improving water quality can help.

Working Scientifically: Investigating Ecosystems

Fieldwork helps scientists collect evidence about organisms in habitats. In school, ecosystem investigations should be safe, fair and respectful to living organisms.

Investigation: Comparing Plant Biodiversity in a Mown Area and an Unmown Area

Aim: To compare plant biodiversity in a mown area and an unmown wildflower area using quadrats.

Equipment:

• quadrat
• tape measure
• identification key or plant guide
• results table
• pencil
• clipboard
• light meter if available
• gloves if instructed by the teacher

Quadrat Sampling Setup

School field

Mown grass area                    Unmown wildflower area
+-------------------+              +-------------------+
| []   []   []      |              | []   []   []      |
|                   |              |                   |
| []   []           |              | []   []           |
+-------------------+              +-------------------+

[] = quadrat placed at different sample positions

Method:

1. Choose two habitats to compare, such as a mown lawn and an unmown area.
2. Place a quadrat at a random position in the first habitat.
3. Count the number of different plant species inside the quadrat.
4. Record the result in a table.
5. Repeat at least five times in the same habitat.
6. Repeat the same method in the second habitat.
7. Calculate the mean number of plant species per quadrat for each habitat.
8. Compare the means and write a conclusion using evidence.

Safety and ethical care:

• Wash hands after fieldwork.
• Do not taste plants or soil.
• Handle living organisms gently and return them to their habitat.
• Avoid damaging habitats.
• Take care near water, nettles, thorns and uneven ground.
• Follow teacher instructions for equipment such as pond nets, sweep nets, pooters or pitfall traps.
• If using a pooter, use it carefully and only with teacher permission.

Working scientifically table:

Part of investigation	Example for this practical
Independent variable	Habitat type: mown area or unmown area
Dependent variable	Number of different plant species per quadrat
Control variables	Quadrat size, number of quadrats, time of day, method of counting, weather where possible
Equipment	Quadrat, identification key, results table, pencil
Fair test	Use the same quadrat size and the same number of samples in both habitats
Reliability	Repeat with several quadrats and calculate a mean
Repeatability	Other groups should be able to repeat the same method and compare results
Accuracy	Identify plants carefully using a key
Precision	Record counts as whole numbers and use the same rules each time
Safety	Wash hands, avoid damaging habitats, follow teacher instructions

Recording Results

Quadrat	Mown area: plant species	Unmown area: plant species
1	2	5
2	1	6
3	3	7
4	2	5
5	2	8
Total	10	31
Mean	2.0	6.2

Conclusion: The unmown area had higher plant biodiversity. The evidence is that the mean number of plant species was 6.2 per quadrat in the unmown area, compared with 2.0 per quadrat in the mown area.

Evaluation

One limitation is that only five quadrats were used in each habitat. This may not represent the whole habitat. An improvement would be to use more quadrats placed randomly. This would make the results more reliable.

Another limitation is that some plants may be difficult to identify. An improvement would be to use an identification key or take photographs for checking later, if the teacher allows.

Other Fieldwork Equipment

Equipment	Use	Care and safety
Sweep net	Catches small invertebrates from long grass	Sweep gently and release organisms quickly
Pond net	Samples pond organisms	Take care near water and return organisms
Pooter	Collects tiny invertebrates	Use only as instructed; do not harm organisms
Pitfall trap	Samples small ground animals	Check often, provide shelter, release organisms
Quadrat	Counts organisms or species in a set area	Place carefully to avoid damage
Transect	Samples changes across a line through a habitat	Keep method consistent along the line
Identification key	Helps identify organisms	Use observable features carefully

Key Vocabulary

Word	Student-friendly definition	Example
Ecosystem	All the living organisms in an area and the non-living conditions around them	A pond with water, pondweed, fish, frogs and oxygen
Habitat	The place where an organism lives	A hedgerow is a habitat for birds and insects
Species	A group of organisms that can reproduce with each other to produce offspring of the same species	Common frog
Population	All the organisms of one species in an area	All the daisies in one field
Community	All the populations of different species in an area	Plants, insects, birds and fungi in a garden
Environment	The surroundings and conditions an organism lives in	A damp, shady woodland floor
Niche	The role of an organism in its ecosystem	Bees feed on nectar and pollinate flowers
Producer	An organism that makes its own food, usually by photosynthesis	Grass, algae, pondweed
Consumer	An organism that gets energy by eating other organisms	Rabbit, snail, fox
Herbivore	An animal that eats plants or algae	Caterpillar, rabbit
Carnivore	An animal that eats other animals	Owl, spider
Omnivore	An animal that eats plants and animals	Mouse, blackbird
Predator	An animal that hunts and eats other animals	Fox, ladybird
Prey	An animal hunted and eaten by a predator	Rabbit, aphid
Scavenger	An animal that feeds on dead animals	Crow, crab
Decomposer	An organism that breaks down dead material and waste	Bacteria, fungi, earthworms
Food chain	A diagram showing one pathway of energy transfer	grass -> rabbit -> fox
Food web	A diagram showing many connected food chains	A woodland web of oak, caterpillars, birds and foxes
Interdependence	Organisms depending on each other	Bees and flowering plants
Biodiversity	The variety of living organisms in an area	Many plant, insect, bird and fungi species in a meadow
Competition	When organisms need the same limited resource	Plants competing for light
Conservation	Protecting and managing ecosystems and species	Planting hedgerows or creating ponds
Non-native species	A species introduced to an area where it does not naturally live	Grey squirrels in some UK habitats
Reliability	How trustworthy results are when repeats are similar	Several quadrats giving similar patterns
Repeatability	Whether the method can be repeated to get similar results	Another group repeats the quadrat survey
Accuracy	How close a measurement is to the true value	Correctly identifying a plant species
Precision	How exact or detailed a measurement is	Measuring light level to the nearest percentage

Common Misconceptions

Misconception	Correct scientific idea
Food chains are circular.	Food chains show one direction of energy transfer, starting with a producer.
The arrow points to what gets eaten.	The arrow shows energy moving from food to feeder.
Predators are always larger than prey.	Predators can be small, such as spiders, ladybirds and dragonfly nymphs.
A consumer is only a shopper.	In biology, a consumer gets energy by eating other organisms.
Producers are animals that produce offspring.	In ecosystems, producers make their own food, usually by photosynthesis.
Decomposers are just dirty or harmful.	Decomposers recycle useful substances by breaking down dead material and waste.
Food webs are separate from food chains.	Food webs are made from many connected food chains.
Removing one organism only affects organisms that eat it directly.	Indirect effects can spread through a food web.
Biodiversity means lots of animals.	Biodiversity includes plants, animals, fungi, microorganisms and habitats.
Ecosystems are only huge places like rainforests.	A pond, hedgerow, rotting log, garden or school field can be an ecosystem.
All insects are pests.	Many insects are pollinators, decomposers, predators or food for other animals.
More predators mean prey instantly disappear.	Predator and prey populations interact over time and often fluctuate.
Human activity is always harmful.	Conservation and careful management can protect or improve ecosystems.
"Fittest" means strongest.	In ecology, it means well suited to the environment.

Diagram Interpretation

Food Web Interpretation Task

Use this food web.

                       heron
                         ^
                         |
algae -> water fleas -> stickleback
  |          ^              ^
  |          |              |
  v          |              |
snails ----> dragonfly nymph
  ^                         ^
  |                         |
pondweed -------------------+

dead material -> bacteria and fungi

Questions:

1. Name one producer.
2. Name one herbivore.
3. Name one predator.
4. Name one prey of the stickleback.
5. Name one organism with more than one food source.
6. Explain what could happen to sticklebacks if water fleas decreased.
7. Explain why bacteria and fungi are important in this pond ecosystem.

Model answers:

1. Algae or pondweed.
2. Water fleas or snails, because they feed on producers.
3. Heron, stickleback or dragonfly nymph.
4. Water fleas or dragonfly nymph.
5. Dragonfly nymph, because it can feed on snails and pondweed-linked small organisms in this simplified web; stickleback also has more than one food source.
6. Sticklebacks may decrease because water fleas are one of their food sources. If they have other food, they may not decrease as much.
7. Bacteria and fungi are decomposers. They break down dead material and waste, returning useful substances to the ecosystem.

Cause-Effect Chain Diagram

hedgerow removed
       |
       v
less shelter and fewer nesting sites
       |
       v
fewer insects, small mammals and birds
       |
       v
less food for predators such as owls and foxes
       |
       v
local biodiversity may decrease

This chain shows how one environmental change can affect several organisms, including organisms that do not use the hedgerow as their main food source.

Data and Graph Skills

When interpreting ecosystem data, use four steps:

1. Identify the pattern or key information.
2. Quote evidence from the data, using values.
3. Explain the biological reason.
4. Give a limitation, anomaly or improvement if asked.

Task 1: Habitat Comparison

Students counted invertebrates in leaf litter from two areas.

Sample	Under hedge: number of invertebrates	Open path edge: number of invertebrates
1	14	5
2	12	4
3	16	7
4	15	3
5	13	6

Questions:

1. Which habitat had more invertebrates overall?
2. Calculate the total number of invertebrates under the hedge.
3. Calculate the mean number of invertebrates at the open path edge.
4. Suggest one reason for the difference.
5. Suggest one improvement to make the results more reliable.

Model answers:

1. The habitat under the hedge had more invertebrates overall.
2. 14 + 12 + 16 + 15 + 13 = 70 invertebrates.
3. Open path edge total = 5 + 4 + 7 + 3 + 6 = 25. Mean = 25 / 5 = 5 invertebrates per sample.
4. The hedge may provide more shelter, moisture, dead leaves and food for decomposers and small animals.
5. Use more samples in each habitat and calculate a mean, or sample on more than one day.

Task 2: Predator-Prey Graph Data

The table shows a simplified predator-prey relationship.

Month	Aphids on rose plants	Ladybirds
April	20	2
May	70	4
June	120	12
July	80	20
August	35	15
September	25	7

Questions:

1. Which population increased first?
2. In which month did aphids reach their highest number?
3. In which month did ladybirds reach their highest number?
4. Explain why the ladybird peak happened after the aphid peak.
5. Describe one limitation of this dataset.

Model answers:

1. The aphid population increased first.
2. Aphids reached their highest number in June, with 120 aphids.
3. Ladybirds reached their highest number in July, with 20 ladybirds.
4. Ladybirds are predators of aphids. When aphids increased, ladybirds had more food, so more survived and reproduced. This caused a time lag before ladybird numbers peaked.
5. The data only covers six months and does not include other factors such as weather, disease or other predators.

Task 3: Simple Percentage Change

A pond survey found 40 water fleas before pollution and 10 water fleas after pollution.

Step 1: Find the decrease.

40 - 10 = 30 fewer water fleas.

Step 2: Divide the decrease by the original number.

30 / 40 = 0.75

Step 3: Convert to a percentage.

0.75 x 100 = 75%

The water flea population decreased by 75%.

Real-World Examples

UK Pond

A pond may contain algae, pondweed, water fleas, snails, tadpoles, dragonfly nymphs, sticklebacks, frogs, herons, bacteria and fungi. Algae and pondweed are producers. Water fleas, snails and tadpoles are consumers. Dragonfly nymphs, sticklebacks, frogs and herons are predators. Bacteria and fungi decompose dead material.

UK Woodland

In a woodland, oak trees and brambles provide food and habitats. Caterpillars and aphids feed on plants. Blue tits and blackbirds eat insects. Mice and rabbits feed on plant material. Foxes, owls and sparrowhawks are predators. Fungi and earthworms break down dead leaves and dead organisms.

Garden

A garden can be a small ecosystem. Flowering plants provide nectar for bees and butterflies. Aphids feed on plant sap. Ladybirds eat aphids. Spiders catch insects. Robins eat worms and insects. Hedgehogs eat slugs and beetles. Decomposers break down compost and dead leaves.

Hedgerow

A hedgerow provides berries, flowers, shelter and nesting sites. It can act as a wildlife corridor between fields, gardens and woodland. Removing hedgerows can reduce biodiversity because animals lose food, shelter and safe routes.

Rocky Shore

On a rocky shore, seaweed and algae are producers. Limpets graze on algae. Mussels filter small food particles from seawater. Crabs eat limpets, mussels and dead material. Small fish and gulls may feed on smaller shore organisms. Decomposers break down dead material.

Practice Questions and Model Answers

Multiple-Choice Questions

1. What is an ecosystem?

A. Only the animals living in a habitat
B. A community of organisms and the non-living conditions around them
C. A single species living in an area
D. A food chain with three organisms

Model answer: B. An ecosystem includes living organisms and non-living conditions.

2. In a food chain, what does the arrow show?

A. The direction of energy transfer
B. The direction an animal moves
C. The largest organism
D. The organism that is most important

Model answer: A. The arrow shows energy moving from food to feeder.

3. Which organism is usually a producer?

A. Fox
B. Caterpillar
C. Grass
D. Fungus

Model answer: C. Grass is a green plant that can photosynthesise.

4. Which statement about decomposers is correct?

A. They only cause disease.
B. They break down dead material and waste.
C. They are always predators.
D. They are not part of food webs.

Model answer: B. Decomposers break down dead material and recycle useful substances.

5. What does biodiversity mean?

A. The number of predators only
B. The variety of living organisms in an area
C. The amount of water in a habitat
D. The number of food chains drawn on a page

Model answer: B. Biodiversity includes variety of plants, animals, fungi, microorganisms and habitats.

6. A ladybird eating aphids is an example of:

A. predator and prey
B. producer and decomposer
C. pollination only
D. a non-living factor

Model answer: A. The ladybird is the predator and the aphids are prey.

Short-Answer Questions

7. Define habitat.

Model answer: A habitat is the place where an organism lives, such as a pond, woodland floor or hedgerow.

8. Define population.

Model answer: A population is all the organisms of one species living in an area, such as all the frogs in one pond.

9. Define community.

Model answer: A community is all the populations of different species living in an area.

10. Explain why a plant is called a producer.

Model answer: A plant is called a producer because it makes its own food by photosynthesis, using light energy.

11. Give two resources plants compete for.

Model answer: Plants compete for light and water. They may also compete for space and mineral substances in soil.

12. Give two resources animals compete for.

Model answer: Animals may compete for food and shelter. They may also compete for water, territory and mates.

FillBlank Questions

13. A __________ is an animal that hunts and eats another animal.

Model answer: predator.

14. The animal hunted and eaten by a predator is called __________.

Model answer: prey.

15. A __________ gets energy by eating other organisms.

Model answer: consumer.

16. A __________ breaks down dead material and waste.

Model answer: decomposer.

17. The variety of living organisms in an area is called __________.

Model answer: biodiversity.

18. Organisms depending on each other is called __________.

Model answer: interdependence.

Food Chain Questions

19. Write a food chain using these organisms: fox, grass, rabbit.

Model answer: grass -> rabbit -> fox.

20. In the food chain grass -> rabbit -> fox, name the producer.

Model answer: Grass is the producer because it makes its own food by photosynthesis.

21. In the food chain grass -> rabbit -> fox, name a predator-prey pair.

Model answer: Fox and rabbit. The fox is the predator and the rabbit is the prey.

22. In the food chain oak leaf -> caterpillar -> blue tit, explain what the first arrow means.

Model answer: The first arrow means energy is transferred from the oak leaf to the caterpillar when the caterpillar eats the oak leaf.

Food Web Questions

Use this food web.

grass -> rabbit -> fox
  |        ^       ^
  v        |       |
mouse -----+       |
  |                |
  v                |
 owl --------------+

23. Name one producer.

Model answer: Grass.

24. Name one organism that eats grass.

Model answer: Rabbit or mouse.

25. Name one predator of the mouse.

Model answer: Owl or fox.

26. Predict what may happen to foxes if rabbits decrease.

Model answer: Foxes may decrease because rabbits are one of their food sources. However, the effect may be smaller if foxes can eat other prey such as mice.

27. Explain why removing grass would affect the whole food web.

Model answer: Grass is the producer and supplies energy to rabbits and mice. If grass decreased, rabbits and mice would have less food, so their populations might decrease. Predators such as foxes and owls would then have less prey.

Data Questions

Students counted plant species in quadrats.

Quadrat	Short grass	Long grass
1	2	5
2	3	6
3	1	7
4	2	6

28. Calculate the total number of plant species counted in short grass.

Model answer: 2 + 3 + 1 + 2 = 8.

29. Calculate the mean number of plant species per quadrat in long grass.

Model answer: Long grass total = 5 + 6 + 7 + 6 = 24. Mean = 24 / 4 = 6 species per quadrat.

30. Which habitat has higher biodiversity? Use evidence.

Model answer: The long grass has higher biodiversity because it has a mean of 6 species per quadrat, compared with 2 species per quadrat in the short grass.

31. Suggest one reason long grass may have more plant species.

Model answer: Long grass may be cut less often, so more plants can grow, flower and survive.

32. Identify one limitation of the investigation.

Model answer: Only four quadrats were used in each habitat, so the sample may not represent the whole area.

Practical Method Questions

33. In a quadrat investigation comparing mown and unmown grass, what is the independent variable?

Model answer: The independent variable is the habitat type: mown grass or unmown grass.

34. What is the dependent variable?

Model answer: The dependent variable is the number of plant species counted in each quadrat.

35. Give two control variables.

Model answer: Use the same quadrat size and the same number of quadrats in each habitat. The method of counting should also be kept the same.

36. How could reliability be improved?

Model answer: Use more quadrats in each habitat and calculate a mean. Repeating the survey on another day could also help.

37. Give one ethical rule for ecosystem fieldwork.

Model answer: Handle living organisms gently and return them to their habitat. Students should also avoid damaging habitats.

Human Impact Questions

38. Explain one direct and one indirect effect of pesticide use in a field.

Model answer: A direct effect is that insect numbers decrease because the pesticide kills insects. An indirect effect is that insect-eating birds may decrease because they have less food.

39. Why can removing a hedgerow reduce biodiversity?

Model answer: A hedgerow provides food, shelter, nesting sites and a wildlife corridor. Removing it can reduce habitats and make it harder for organisms to move between areas.

40. Suggest one conservation action that could help pollinators.

Model answer: Planting wildflowers can help pollinators because it provides nectar and pollen. Reducing pesticide use can also help because fewer pollinators are killed.

41. Explain how plastic pollution could affect a river ecosystem.

Model answer: Animals may swallow plastic or become trapped in it. This can injure or kill them. If prey animals decrease, predators may also have less food.

42. Explain how pond creation can improve biodiversity.

Model answer: A pond creates a new habitat for algae, pondweed, invertebrates, amphibians and microorganisms. It can provide food and shelter, increasing the variety of species in the area.

Longer 8-Mark Question

Stimulus:

A farmer sprays pesticide on a field. Before spraying, the field food web included wheat, wildflowers, aphids, caterpillars, bees, ladybirds, spiders, skylarks and foxes. Aphids and caterpillars fed on plants. Bees fed on nectar and pollinated wildflowers. Ladybirds and spiders ate insects. Skylarks ate insects and seeds. Foxes sometimes ate skylarks.

Question:

Explain how spraying pesticide could affect the field ecosystem. Use food web evidence and include direct and indirect effects.

Model answer:

The pesticide would have a direct effect on insects because it is sprayed to kill them. Aphids and caterpillars may decrease, and useful insects such as bees may also decrease. This could affect pollination because bees feed on nectar and help pollinate wildflowers, so fewer bees may mean fewer wildflower seeds are produced.

There would also be indirect effects on predators. Ladybirds and spiders eat insects, so if insect populations decrease, ladybirds and spiders may have less food and their populations may decrease. Skylarks eat insects and seeds, so they may also have less food. If skylark numbers decrease, foxes may have less prey, although foxes may survive if they have other food sources.

The food web may become less biodiverse because several insect species and insect-eating predators decrease. The ecosystem may become less stable because there are fewer feeding relationships. A conservation action would be to reduce pesticide use, plant wildflower strips, or encourage natural predators such as ladybirds to control aphids.

This answer uses the scientific terms pesticide, population, pollination, predator, prey, food web and biodiversity, and it explains both direct and indirect effects.

Exam-Style Questions

1. Describe the difference between a habitat and an ecosystem.

Model answer: A habitat is the place where an organism lives. An ecosystem includes the living organisms in an area, the non-living conditions and the interactions between them.

2. A student writes this food chain: fox -> rabbit -> grass. Explain the mistake and correct it.

Model answer: The arrows are the wrong way round because they should show energy transfer from food to feeder. The correct chain is grass -> rabbit -> fox.

3. Explain why food webs are more realistic than food chains.

Model answer: Food webs are more realistic because most organisms eat more than one food and may be eaten by more than one predator. A food web shows many connected food chains.

4. A disease reduces rabbit numbers in a woodland. Predict two effects on the food web.

Model answer: Foxes may decrease because they have less rabbit prey. Some plants eaten by rabbits may increase because fewer rabbits are feeding on them.

5. Why can high biodiversity make an ecosystem more stable?

Model answer: High biodiversity means there are many species and feeding relationships. If one species decreases, some organisms may have alternative food sources, so the ecosystem may recover more easily.

6. Name one piece of equipment used to sample plant abundance and explain how it is used.

Model answer: A quadrat is placed on the ground and the organisms or plant species inside it are counted. Repeating this in several positions gives a more reliable estimate.

7. Explain why results from fieldwork may be more reliable if repeated.

Model answer: Repeats reduce the effect of unusual results and make the conclusion more trustworthy. A mean can be calculated from repeated samples.

8. A pond has low oxygen after pollution. Explain how this may affect animals.

Model answer: Pond animals such as fish and water fleas need oxygen for respiration. If oxygen decreases, some animals may die or move away, so predator populations may also decrease because there is less prey.

Revision Checklist

Use this checklist before a quiz or assessment.

I can...	Confident	Need more practice
Define ecosystem, habitat, population, community and species
Explain the difference between living and non-living parts of an ecosystem
Identify producers, consumers, predators, prey and decomposers
Explain why producers are important
Explain why decomposers are important
Draw a food chain with arrows in the correct direction
Explain that arrows show energy transfer from food to feeder
Interpret a food web and identify feeding relationships
Predict effects of increasing or decreasing one population
Explain predator-prey population changes and time lag
Define biodiversity and explain why it matters
Describe competition for limited resources
Give examples of environmental factors affecting organisms
Describe seasonal changes in UK ecosystems
Explain direct and indirect effects of human impacts
Suggest conservation actions and explain how they help
Plan a simple quadrat investigation
Identify independent, dependent and control variables
Calculate a total and a mean from fieldwork data
Use evidence from tables and graphs in answers
Evaluate a fieldwork method using reliability, fairness and limitations

Final Summary

Ecosystems are made from organisms, their physical environment and the relationships between them. Producers such as plants and algae make food by photosynthesis. Consumers get energy by eating other organisms. Predators hunt prey, while decomposers break down dead material and waste. Food chains show one pathway of energy transfer, and food webs show many connected feeding relationships.

Organisms are interdependent. They rely on each other for food, pollination, shelter, decomposition, seed dispersal and population control. Changes to one population can affect many others, sometimes indirectly. Biodiversity is important because ecosystems with a greater variety of species and habitats are often more stable and resilient.

Human activity can damage ecosystems, but careful conservation can protect and improve them. Good scientific fieldwork uses fair methods, clear variables, repeats, accurate recording and ethical care for living organisms.