Ecosystems and InterdependenceOxford AQA IGCSE Biology: Revision notes
Section 1
What are the key definitions in an ecosystem?
Understanding the terminology is essential for describing how organisms interact and survive together.
Ecosystem is a community of organisms and the physical environment they live in, functioning together as a system.
Community refers to all the different populations of species living and interacting in an area.
Population is a group of organisms of the same species living in the same area at the same time.
Habitat is the place where an organism lives, including its physical and biological surroundings.
Niche describes the role of an organism in its environment – what it eats, where it lives, and how it interacts with other species. No two species can occupy the same niche in a stable ecosystem.
Interdependence occurs when organisms in an ecosystem depend on each other for survival, whether for food, shelter, pollination, or other needs.
Examiners test whether you can distinguish between habitat and niche. Habitat is the address (where it lives); niche is the job (what it does and how it interacts). Always mention both the physical location and the organism's role.
A habitat is like an apartment building (the location), while a niche is like a resident's job and daily routine (their specific role in that building).
Section 2
What are the roles of producers, consumers, and decomposers?
Energy flows through an ecosystem via different trophic levels, each with a distinct role.
Producers are autotrophic organisms (usually plants) that capture energy from the sun through photosynthesis and convert it into chemical energy stored in organic compounds. They form the base of all food chains and webs.
Consumers are heterotrophic organisms that obtain energy by feeding on other organisms. They are classified by their position in the food chain:
| Consumer Type | Definition | Example |
|---|---|---|
| Primary consumer | Herbivores that feed directly on producers | Rabbit eating grass |
| Secondary consumer | Carnivores or omnivores that feed on primary consumers | Fox eating rabbit |
| Tertiary consumer | Carnivores that feed on secondary consumers | Eagle eating fox |
Decomposers are organisms (bacteria, fungi) that break down dead organic matter and return nutrients to the soil. They feed on dead plants, animals, and waste products, making nutrients available for producers again.
Always remember: decomposers are NOT consumers. While consumers eat living organisms, decomposers break down dead material. The exam will test whether you know decomposers return nutrients to the environment, completing the cycle.
Students often forget that decomposers are essential. A common error is omitting them when discussing energy flow, but examiners expect you to explain how nutrients are recycled back to producers.
Section 3
How do we construct and interpret food chains and food webs?
Food chains show the linear transfer of energy from one organism to the next. Each organism is at a different trophic level, and the arrows always point in the direction of energy flow (towards the consumer).
Example food chain: Grass → Rabbit → Fox → Eagle
In this chain:
- Grass is the producer
- Rabbit is the primary consumer
- Fox is the secondary consumer
- Eagle is the tertiary consumer
Food webs show multiple interconnected food chains in an ecosystem, illustrating the complex feeding relationships between organisms. A food web is more realistic because most organisms have more than one food source and may be preyed upon by several predators.
How to construct a food web:
- Identify all the organisms in the ecosystem
- Use arrows to show the direction of energy flow from food source to feeder
- Ensure each organism has at least one arrow pointing towards it (food it eats) and away from it (organisms that eat it)
- Include producers, all types of consumers, and decomposers
How to interpret a food web:
- Trace the pathway from any organism to find what it eats and what eats it
- Count the number of food chains present
- Identify which organisms are most dependent (have few food sources) – these are vulnerable if that source is lost
When drawing arrows, always point them towards the organism doing the eating, not the food source. Examiners mark this carefully – arrows in the wrong direction loses marks.
In a food web with grass, insects, rabbits, foxes and birds: Grass has arrows pointing to both insects and rabbits; insects have arrows to both birds and rabbits; rabbits have arrows to foxes and birds. This shows each predator has multiple prey options and some prey are eaten by multiple predators.
Section 4
How are organisms interdependent in ecosystems?
Interdependence means organisms depend on each other for survival, and the removal or change of one organism affects others in the ecosystem.
Types of interdependence:
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Food relationships: Organisms depend on others for food. If a producer is removed, all consumers that feed on it (directly or indirectly) are affected.
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Habitat provision: Some organisms provide habitats for others (e.g. trees provide shelter and nesting sites for birds; aquatic plants provide shelter for fish).
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Pollination and seed dispersal: Plants depend on animals (insects, birds, mammals) to pollinate flowers and disperse seeds. Animals depend on plants for food rewards.
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Nutrient cycling: All organisms depend on decomposers to return nutrients to the soil, and plants depend on these nutrients to grow.
Effects of changes to an ecosystem:
When one species is removed or its population changes dramatically, other species are affected because of these interdependencies:
- If a producer is removed, all organisms that depend on it (directly or indirectly) lose a food source
- If a primary consumer is removed, secondary consumers lose a food source; the producer population may increase due to reduced grazing
- If a top predator is removed, secondary consumers may increase, leading to overconsumption of primary consumers, which then decline
- If a decomposer population declines, dead material accumulates and nutrients are not recycled
Examiners want to see you trace the consequences of species removal through the entire food web. Don't just say 'the fox population increases' – explain why and what happens next (increased predation on rabbits, rabbit population decreases, grass population increases).
If rabbits are removed from a food chain (Grass → Rabbit → Fox): Foxes lose their main food source and their population declines; grass population increases due to reduced grazing; if foxes can eat other prey, those prey populations decrease. This cascades through the entire ecosystem.
Section 5
What is the predator-prey relationship and population cycle?
Predator-prey relationships describe the dynamic interaction between a predator (consumer) and its prey (food source). These relationships drive cyclical changes in population numbers.
The predator-prey cycle:
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Prey population increases: With abundant food and few predators, the prey population grows.
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Predator population increases (delayed): As prey becomes abundant, predators have more food available, so they survive better and reproduce. However, this increase lags behind the prey increase due to reproduction time.
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Prey population decreases: As the predator population increases, more prey are eaten. Prey population declines.
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Predator population decreases: With less prey available, predators have insufficient food, so they starve or emigrate. The predator population declines, again with a time lag.
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Cycle repeats: With fewer predators, prey population increases again, starting the cycle anew.
Key features of predator-prey graphs:
- The prey population curve peaks first, then the predator population curve peaks slightly later (approximately one quarter of a cycle behind)
- The patterns are cyclical and repetitive – neither population reaches stability while predating on each other
- The amplitude (height of peaks) can vary depending on environmental factors
- When one population is at its maximum, the other is often at its minimum
Factors affecting the cycle:
- Availability of other food sources for predators (stabilises predator population)
- Disease or parasites in prey (amplifies the decline)
- Environmental conditions like harsh winters (affects both populations)
When interpreting a predator-prey graph, always explain the time lag: the predator population peaks AFTER the prey population because reproduction takes time. Use phrases like 'delayed response' or 'approximately one quarter of a cycle behind'.
The predator-prey cycle is like a delayed reaction: if you turn up the heating too high, the room gets hot, then you sweat and feel uncomfortable (prey increases, then predators increase), so you turn it down too far, then it gets cold (predators decline), so you turn it up again (prey increases).
Must Know
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Ecosystem: a community of organisms and its physical environment functioning together. Niche is an organism's role (job) in that ecosystem; habitat is where it lives (address).
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Trophic levels in energy flow: Producers (plants) → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers (top predators). Decomposers break down dead matter and recycle nutrients.
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Food chains show linear energy transfer with arrows pointing to the consumer. Food webs show multiple interconnected chains reflecting realistic feeding relationships.
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Interdependence means organisms rely on each other for food, habitat, pollination, and nutrient cycling. Removing one species cascades through the ecosystem – trace the full consequences through the food web.
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Predator-prey cycle is cyclical and repetitive: prey increases → predators increase (delayed) → prey decreases → predators decrease → cycle repeats. The predator population peaks approximately one quarter of a cycle after prey peaks due to reproduction time lag.
That's the notes covered.
Carry on to the next subtopic.