B40: Ecosystems and Interdependence
Communities, ecosystems and how organisms depend on each other
Key Definitions
Ecosystem: A community of living organisms (biotic) interacting with each other and their physical environment (abiotic) in a self-supporting system.
Community: All the populations of different species that live together in the same habitat at the same time and interact with each other.
Population: All the organisms of the same species living in the same habitat at the same time and capable of interbreeding.
Habitat: The place where an organism lives, providing food, shelter and the conditions it needs to survive.
Interdependence: The way in which organisms within a community depend on each other for food, shelter, pollination, seed dispersal and other survival needs.
Stable community: A community where the populations of all species remain roughly constant over time, with balanced predator-prey relationships and competition.
Competition
Organisms compete for limited resources. Competition can be interspecific (between different species) or intraspecific (within the same species). Intraspecific competition is usually more intense because organisms of the same species need identical resources.
Plant Competition
Plants compete for:
- Light – taller plants shade shorter ones; trees in a forest canopy outcompete seedlings below
- Water – roots spread to absorb moisture; cacti outcompete other desert plants by storing water
- Minerals/ions – nitrates, phosphates and potassium from the soil; plants with deeper or more extensive root systems absorb more
- Space – for root growth and leaf spread; climbing plants like ivy reach light by growing over others
Example 1: Plant Competition in Woodland
In an oak woodland, oak trees in the canopy layer compete for light and grow tall to capture sunlight. Below them, bluebells flower early in spring before the canopy closes, avoiding competition for light. This is an adaptation to reduce competition.
Animal Competition
Animals compete for:
- Food – lions and hyenas compete for the same prey on the African savanna
- Mates – male deer fight (rut) to win breeding females; peacocks display to attract peahens
- Territory – robins sing to defend feeding and nesting territories from other robins
- Water – desert animals compete at rare water sources
Example 2: Intraspecific Competition in Red Deer
Male red deer compete fiercely for mates during the autumn rut. The strongest stags win territories and harems of females, passing on their genes. Weaker males are excluded from breeding. This intraspecific competition ensures only the fittest reproduce.
Predation
Predation is when one organism (the predator) kills and eats another (the prey). Predator and prey populations are linked in a predator-prey cycle:
- When prey numbers increase, predators have more food so predator numbers increase
- More predators eat more prey, so prey numbers decrease
- With less prey available, predator numbers decrease due to starvation and less breeding
- With fewer predators, prey numbers increase again – and the cycle repeats
Predator-prey cycles are always out of phase: The predator population peaks after the prey population peaks because it takes time for the extra food to translate into more predators through reproduction.
Example 3: Lynx and Snowshoe Hare Cycle
One of the best-documented predator-prey cycles is between the Canada lynx and the snowshoe hare. Records from the Hudson's Bay Company over 200 years show regular peaks and troughs in the fur returns of both species, with the lynx population always lagging behind the hare population by about 1–2 years.
Food Chains
A food chain shows the transfer of energy and biomass from one organism to the next:
| Trophic Level | Organism Type | Role | Example |
|---|---|---|---|
| 1 | Producer | Makes its own food by photosynthesis | Grass |
| 2 | Primary consumer | Eats producers (herbivore) | Rabbit |
| 3 | Secondary consumer | Eats primary consumers (carnivore) | Fox |
| 4 | Tertiary consumer | Eats secondary consumers (carnivore) | Eagle owl |
Decomposers (bacteria and fungi) break down dead organisms and waste material, returning minerals and nutrients to the soil. They are essential for recycling matter in ecosystems but are not usually shown on food chains.
Food chains always start with a producer (a plant or alga that photosynthesises). Arrows in a food chain show the direction of energy flow (from the eaten to the eater), not "who eats whom".
Example 4: A Four-Level Food Chain
Phytoplankton → Zooplankton → Herring → Cod
Phytoplankton (microscopic algae) are the producers, zooplankton are the primary consumers, herring are the secondary consumers and cod are the tertiary consumers.
Food Webs
A food web shows all the interconnected food chains in an ecosystem. Most animals eat more than one type of food and are eaten by more than one predator, so food webs give a more realistic picture than single food chains.
Interdependence in food webs: If one species disappears, it affects all the other species connected to it. Removing a predator can cause its prey to increase, which then reduces the prey's own food source. This is why changes in food webs have knock-on effects.
Example 5: Impact of Removing a Species
If farmers kill all the foxes in an area, the rabbit population increases rapidly because there are fewer predators. The rabbits eat more grass, leaving less for other herbivores like deer and insects. The insect population may fall, reducing food for birds. One change ripples through the entire food web.
Pyramids of Biomass
A pyramid of biomass shows the total mass of living material at each trophic level in a food chain. The bar for producers is always the widest at the bottom, and each subsequent bar is narrower.
Why are pyramids of biomass always pyramid-shaped? Because energy is lost at each trophic level. Only about 10% of the biomass (and energy) is transferred from one trophic level to the next. The rest is lost through:
- Respiration – organisms use glucose to release energy, producing CO₂ and water
- Excretion – waste products contain energy that is not passed on
- Not all parts of the organism are eaten (bones, fur, roots)
- Some material is not digested and is egested in faeces
Example 6: Biomass Pyramid for a Grassland
Grass (1,000,000 kg) → Rabbits (100,000 kg) → Foxes (10,000 kg) → Eagles (1,000 kg)
At each step, roughly 90% of the biomass is lost. This is why food chains rarely have more than four or five trophic levels – there is simply not enough energy left to support another level.
Stable Communities
A stable community is one where population sizes remain relatively constant over time. This happens when:
- Predator-prey relationships are balanced
- Competition does not eliminate any species
- Decomposers recycle nutrients efficiently
- Environmental conditions remain suitable
Examples of stable communities include ancient woodlands, coral reefs and tropical rainforests. Human activities such as deforestation, pollution and climate change can destabilise communities.
Comparison: Food Chains vs Food Webs vs Pyramids of Biomass
| Feature | Food Chain | Food Web | Pyramid of Biomass |
|---|---|---|---|
| Shows | Single pathway of energy flow | All feeding relationships in a community | Total biomass at each trophic level |
| Realism | Simplified – most organisms eat multiple foods | More realistic – shows complex interactions | Quantitative – shows actual amounts |
| Arrows | Show direction of energy flow | Show direction of energy flow | Not applicable – bars show quantity |
| Shape | Linear | Network / web | Always pyramid-shaped |
| Use | Quick identification of trophic levels | Understanding interdependence | Showing energy loss between levels |
Practice Questions
Q1: Foundation Define the terms ecosystem and community.
Q2: Foundation Describe three resources that plants compete for and give an example of how one plant species may outcompete another.
Q3: Higher Explain why predator-prey population cycles are always out of phase. Use an example in your answer.
Q4: Foundation Construct a food chain with four trophic levels, labelling each trophic level. Explain why the arrows point in the direction they do.
Q5: Higher Explain why a pyramid of biomass is always pyramid-shaped. Why do food chains rarely exceed five trophic levels?
Q6: Foundation Describe what a stable community is and explain how removing one species from a food web could destabilise it.
Answers
- Ecosystem: A community of living organisms interacting with each other and their physical (abiotic) environment in a self-supporting system. Community: All the populations of different species living together in the same habitat at the same time and interacting with each other.
- Plants compete for light (e.g. tall oak trees shade out smaller plants), water (e.g. plants with deeper roots access water in dry soil that shallow-rooted plants cannot reach), and minerals/ions (e.g. legumes with nitrogen-fixing bacteria gain nitrates that other plants cannot obtain from the air). A species may outcompete another by growing taller and blocking light, preventing the other from photosynthesising effectively.
- Predator-prey cycles are out of phase because it takes time for changes in prey population to affect predator population. When prey numbers increase, predators have more food but they must first reproduce before their population grows – this takes weeks or months. So the predator peak always comes after the prey peak. For example, when snowshoe hare numbers rise, it takes 1–2 years for lynx numbers to respond, creating a lag between the two population curves.
- Grass → Caterpillar → Blue tit → Sparrowhawk. Trophic level 1 = grass (producer), trophic level 2 = caterpillar (primary consumer), trophic level 3 = blue tit (secondary consumer), trophic level 4 = sparrowhawk (tertiary consumer). The arrows show the direction of energy flow from the organism being eaten to the organism eating it – they do not mean "eats".
- A pyramid of biomass is always pyramid-shaped because energy (and therefore biomass) is lost at each trophic level. Only about 10% of biomass is transferred between levels; the rest is lost through respiration (energy used for life processes), excretion (waste), egestion (undigested material) and parts not eaten (bones, roots, fur). Food chains rarely exceed five trophic levels because after four or five transfers, so little energy remains that there is insufficient biomass to support another population of consumers.
- A stable community is one where population sizes of all species remain roughly constant over time because predator-prey relationships and competition are balanced. Removing one species can destabilise it: for example, removing a top predator like foxes would cause their prey (rabbits) to increase, leading to overgrazing of plants. This reduces food for other herbivores, whose populations then decline, affecting their predators in turn – a cascade of changes through the food web.
Exam Tips
- Food chain arrows show the direction of energy flow, NOT "who eats whom" – this is a very common exam mistake
- Always start a food chain with a producer (plant/alga) – examiners will penalise chains starting with consumers
- When explaining predator-prey cycles, always mention the time lag between prey increase and predator increase
- Use the correct terms: primary consumer, secondary consumer, tertiary consumer – not "first consumer" or "level 2 consumer"
- When asked why biomass pyramids are always pyramid-shaped, give at least three reasons for energy loss: respiration, excretion/egestion, and parts not eaten
- Remember that decomposers are not shown on food chains but are essential for recycling nutrients
🔢 Maths Skills
Mathematical Skills
Interpreting population data and predator-prey cycles: read values from graphs, identify peaks and troughs, and describe the time lag between predator and prey population peaks. Calculate percentage changes in population size.
Example: If the prey population rises from 500 to 800, the percentage increase = (800 − 500) / 500 x 100 = 60%. The predator population typically peaks 1–2 years after the prey peak due to the time needed for reproduction.
⚠️ Common Misconceptions
Watch Out!
Students often think predators always control prey populations. Wrong: Predators always control prey numbers Correct: Bottom-up control (food supply for the prey) is often more important; predators are just one factor among many
Students often think decomposers are at the top of food chains. Wrong: Decomposers are the top trophic level Correct: Decomposers break down dead material from all trophic levels — they are not part of a linear food chain but recycle nutrients back into the ecosystem
✍️ 6-Mark Question
Extended Answer
6 marks: Explain interdependence in a named community, including at least three different relationships between organisms.
In an oak woodland community, organisms are interdependent in multiple ways. Oak trees (producers) provide food for caterpillars (primary consumers) through their leaves, and for deer through their acorns. Caterpillars are food for blue tits (secondary consumers), which are in turn preyed upon by sparrowhawks (tertiary consumers). Bees and butterflies pollinate wildflowers, enabling seed production. Squirrels and jays disperse acorns, helping oak trees reproduce. Decomposers (fungi and bacteria) break down fallen leaves and dead organisms, returning minerals to the soil for the oak trees to absorb through their roots. If the oak trees were removed, all these organisms would be affected — showing the interdependence of the community.
Mark scheme: 1 mark for naming a community, 1 mark for each interdependent relationship explained (up to 3), 1 mark for explaining the consequence of removing one species, 1 mark for mentioning decomposers
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A graph shows the population of foxes and rabbits over 20 years. The rabbit population peaks in year 4, 10 and 16. The fox population peaks in year 5, 11 and 17. Explain the pattern shown in the graph. Why are the fox peaks always one year after the rabbit peaks? Suggest what would happen to both populations if a disease killed 80% of the foxes in year 8.
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