Interactions in an Ecosystem (Cambridge (CIE) IGCSE Environmental Management): Revision Note
Exam code: 0680
Biotic interactions within an ecosystem
Biotic interactions are relationships between living organisms in an ecosystem
Every organism depends on other living things in some way
These interactions decide:who survives
which populations grow or fall
how energy moves
and how stable the ecosystem is
Biotic interactions are important because they:
keep populations under control
help plants reproduce
shape food chains and food webs
maintain biodiversity
allow ecosystems to stay balanced over time
Without these interactions, ecosystems would collapse
Types of biotic interactions
Species
Native species
Native species are those that naturally inhabit a particular area
They fit into food webs and cycles perfectly because they evolved there
Invasive species
Invasive species are introduced from elsewhere (often by humans)
They spread quickly because they usually have no predators in the new environment
They outcompete native species for food, water, and space
They have the ability to drastically alter the structure of food webs
Why it matters:
Invasive species can reduce biodiversity and weaken the ecosystem’s ability to cope with change
Competition
Competition is when organisms fight for the same resources, such as food, water, mates, and territory
Competition can be divided into:
Intraspecific competition
Interspecific competition
Intraspecific competition
This is when the same species compete (e.g. two grey squirrels fighting for food)
Interspecific competition
This is competition between individuals from different species e.g. grey vs red squirrels)
Why it matters:
Competition controls population sizes and decides which species survive in a habitat
Predation
One organism (predator) hunts and eats another (prey)
Why it matters:
It ensures that prey populations do not grow excessively large
This prevents predators from multiplying excessively
Creates predator–prey cycles that stabilise ecosystems
Predators also remove weak or sick prey, keeping populations healthy
The graph below shows some of the key patterns of predator-prey cycles:
The number of predators increases as there is more prey available
The number of prey then decreases as there are now more predators
The number of predators then decreases as there is now less prey available
The number of prey then increases as there are now fewer predators
The cycle now repeats
Pollination
Pollination is a special biotic interaction where a pollinator (for example, a bee) carries pollen between flowers
Why it matters:
It allows plants to reproduce by producing seeds and fruit
Supports food chains (plants are producers)
Many major crops depend on insect pollination
If pollinators decline, food production drops
Examiner Tips and Tricks
Think of biotic interactions as nature’s way of keeping everything balanced.
Explain why an interaction matters — don’t just describe it.
Use examples to help show understanding.
Remember that ecosystems stay healthy when no single population becomes too large.
Focus on cause → effect → consequence.
Pollination in an ecosystem
What is pollination?
Pollination is a biotic interaction that helps flowering plants reproduce
It involves the transfer of pollen from the anther (the male part of a flower) to the stigma (the female part)
Examiner Tips and Tricks
You don't need to memorise flower part names for this syllabus (other than pollen, anther and stigma), but you do need to understand the process and why it matters for ecosystems.
Why pollination is important:
It allows fertilisation → seeds → fruit
It supports food chains because plants are producers
It keeps plant populations healthy and diverse
Many human food crops depend on pollinators, especially insects like bees
Without pollination, plant reproduction slows down, and ecosystems can collapse
Types of pollination
1. Insect pollination
Insects (such as bees, butterflies, moths, and beetles) move pollen from one flower to another
How it happens:
The insect visits a flower for nectar
Pollen sticks to its body
It flies to another flower
The pollen brushes onto the stigma
Fertilisation can now take place → seeds develop
Why insect pollination matters:
Many plants rely on insect pollination to produce fruit and seeds
Insect pollination plays a crucial role in maintaining vast food webs
Essential for crops like apples, tomatoes, strawberries, and almonds
2. Wind pollination
Some plants rely on the wind to blow pollen from one flower to another
Wind-pollinated plants produce lots of tiny, light pollen grains so they can travel far
Examples include:
Wheat
Maize
Many grasses
Some trees (e.g., birch)
Why wind pollination matters:
Important for major cereal crops
Large areas, such as grasslands, benefit from wind pollination
3. Pollination → fertilisation → seed and fruit formation
Once pollen reaches the stigma:
A pollen tube forms
Fertilisation takes place inside the flower
Seeds develop inside a fruit
Examiner Tips and Tricks
You only need a simple idea:
Pollination → allows fertilisation → leads to seed and fruit formation.
Why pollination is essential for ecosystems
It keeps plant populations stable
Plants reproduce more successfully
Seeds spread → more habitats form
Many animals (including humans) rely on pollinated plants for food
If pollinators decline:
Fewer seeds are produced
Plant populations shrink
Food becomes scarce for herbivores
Entire food webs are affected
Worked Example
Explain why pollination is an essential biotic interaction for both natural ecosystems and human food production.
[6 marks]
Answer:
Pollination is the transfer of pollen from one flower to another, allowing for fertilisation and the production of seeds and fruits. [1]
In natural ecosystems, this action is essential because it helps plant populations grow and spread. [1] As producers form the base of food chains, successful pollination supports herbivores and the predators that feed on them. [1]
Insect pollination is especially important for many flowering plants. If pollinators decline, fewer seeds form, plant numbers fall, and biodiversity decreases. [1] Pollination is also vital for human food production because many crops depend on insects, particularly bees. [1]
Without effective pollination, crop yields fall, which threatens food security and affects economies. [1]
Marking guidance
1 mark for defining pollination that shows clear understanding of the process.
Up to 4 marks for explaining its role in ecosystems and food production.
1 mark for a reasoned conclusion.
Examiner Tips and Tricks
Remember: pollination starts the entire food chain.
Don’t just say 'pollination helps plants reproduce'—explain the impact on ecosystems.
Mention both insect and wind pollination if asked about different methods.
Use simple chains: pollination → fertilisation → seeds → new plants → stable ecosystem.
Link pollination to biodiversity, crop yields, and food security for full marks.
Photosynthesis in an ecosystem
Photosynthesis is a chemical process used by green plants and algae to make glucose, a type of sugar that stores energy
Plants use:
carbon dioxide from the air
water from the soil
light energy from the Sun
They produce:
glucose for energy and growth
oxygen, which is released into the atmosphere
carbon dioxide + water → glucose + oxygen
Why photosynthesis matters
Forms the base of every food chain
Provides oxygen for most living things
Stores energy from the Sun inside plant tissues
Fuels growth, reproduction and survival of nearly all life on Earth
Without photosynthesis, ecosystems would collapse because no new energy would enter the food chain
Examiner Tips and Tricks
You do not need a symbol equation for photosynthesis in the exam.
Where photosynthesis happens
Photosynthesis occurs mostly in the leaves, particularly in the palisade layer, because this layer contains
Many chloroplasts (chlorophyll is the green pigment in chloroplasts that absorbs light energy for photosynthesis)
Receives lots of sunlight
Leaves have a large surface area
Examiner Tips and Tricks
There is no need to name cell structures, but it's useful to mention that leaves are adapted for absorbing light and gases.
What plants do with the glucose they make
Plants must store or use the glucose to survive
They use it in respiration to release energy
Plants convert it into starch for storage
They build new cell walls and roots
Plants make fruits, seeds and flowers
They use it to support growth and repair
Why photosynthesis is essential for ecosystems
Plants are producers that make food for herbivores
Photosynthesis keeps oxygen levels stable
Photosynthesis removes carbon dioxide from the atmosphere
It supports entire food webs and nutrient cycles
When photosynthesis decreases:
Plant populations fall
Food chains weaken
Oxygen production drops
Ecosystems become less stable
Worked Example
Describe how the process of photosynthesis supports life in an ecosystem.
[4 marks]
Answer
Photosynthesis supports life in the ecosystem because plants act as producers in food chains. [1] The glucose they make is passed on to herbivores and then to higher consumers, moving energy through the ecosystem. [1] Plants also release oxygen as a waste product, which many organisms need for aerobic respiration. [1] Without photosynthesis, there would be no energy entering the food chain and ecosystems would collapse. [1]
Marking guidance
1 mark for describing how energy enters food chains.
Up to 2 marks for making clear links between energy and ecosystems.
1 mark for showing why photosynthesis is essential overall.
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