B20: Ecosystems
Communities, ecosystems and interdependence
📋 Key Concepts
Ecosystems: An ecosystem is the interaction of a community of living organisms with the non-living (abiotic) parts of their environment. Organisms are interdependent — they rely on each other for survival.
Key Terms
- ecosystem
- community
- population
- habitat
- interdependence
- competition
- predation
- producer
- primary consumer
- secondary consumer
- tertiary consumer
- decomposer
- trophic level
- pyramid of biomass
📝 Key Definitions
| Term | Definition |
|---|---|
| Ecosystem | The interaction of a community of living organisms with the non-living parts of their environment |
| Community | All the populations of different species that live together in the same area at the same time |
| Population | All the organisms of the same species living in the same area at the same time |
| Habitat | The place where an organism lives |
| Interdependence | Organisms within a community depend on each other for food, shelter, pollination, seed dispersal, etc. |
Example 1
In a woodland, there are 500 oak trees, 2000 caterpillars, 50 blue tits and 5 sparrowhawks. Identify the population, community and habitat.
Solution:
Population: all the organisms of one species, e.g. 500 oak trees or 2000 caterpillars.
Community: all the populations living together — oak trees, caterpillars, blue tits, and sparrowhawks (and all other species in the woodland).
Habitat: the woodland itself — the place where these organisms live.
📝 Competition and Predation
Competition: Organisms compete for resources that are in limited supply.
- Plants compete for: light (for photosynthesis), water (for photosynthesis and transport), minerals/nutrients (from soil), space (for root growth and light exposure)
- Animals compete for: food, mates (for reproduction), territory (for nesting, feeding), water
Predation: A predator-prey relationship where one organism (the predator) kills and eats another (the prey). Predator and prey populations are linked — when prey numbers increase, predator numbers increase (with a time lag), then prey numbers decrease, followed by predator numbers decreasing.
Example 2
Describe and explain the typical predator-prey population cycle.
Solution:
1. When prey population increases, there is more food available for predators.
2. Predator population increases (with a time lag, as predators need time to reproduce).
3. More predators eat more prey, so prey population decreases.
4. With less prey available, predators have less food and their population decreases.
5. With fewer predators, prey population increases again.
This creates a repeating cycle of rise and fall in both populations.
📝 Food Chains and Food Webs
Food chain: Shows the flow of energy from one organism to the next. Each stage is called a trophic level.
Food chain:
Producer → Primary consumer → Secondary consumer → Tertiary consumer
(plant) (herbivore) (carnivore) (top carnivore)
Producer → Primary consumer → Secondary consumer → Tertiary consumer
(plant) (herbivore) (carnivore) (top carnivore)
Key points about food chains:
- Producers (plants) make their own food by photosynthesis
- Primary consumers are herbivores that eat producers
- Secondary and tertiary consumers are carnivores that eat other consumers
- Decomposers (bacteria and fungi) break down dead organisms and waste, returning nutrients to the soil
- Arrows show the direction of energy flow (from the eaten to the eater)
Food webs: Show how multiple food chains in an ecosystem are interconnected. They show that most animals eat more than one type of food and are eaten by more than one predator. This makes ecosystems more stable — if one species decreases, others may still be available as food sources.
Example 3
Grass is eaten by rabbits, which are eaten by foxes. Grass is also eaten by caterpillars, which are eaten by blue tits, which are eaten by sparrowhawks. Draw this as a food web and identify the producers, primary consumers and secondary consumers.
Solution:
Producer: grass
Primary consumers: rabbits, caterpillars
Secondary consumers: foxes, blue tits
Tertiary consumer: sparrowhawks
Food web: Grass → Rabbits → Foxes; Grass → Caterpillars → Blue tits → Sparrowhawks
📝 Pyramids of Biomass
Pyramid of biomass: Shows the mass of living material at each trophic level. It is always pyramid-shaped because energy and biomass are lost at each trophic level.
Why biomass decreases at each trophic level:
- Not all of the organism is eaten (e.g. bones, roots, fur)
- Not all food eaten is absorbed — some is egested (passes through as faeces)
- Some absorbed material is used for respiration, which releases CO₂ and water (and energy as heat)
- Some absorbed material is excreted as waste (e.g. urea in urine)
- Only approximately 10% of biomass is transferred from one trophic level to the next
Energy transfer efficiency:
Only ~10% of biomass/energy is passed to the next trophic level.
The rest is lost through: respiration, excretion, egestion, and not all parts being eaten.
Only ~10% of biomass/energy is passed to the next trophic level.
The rest is lost through: respiration, excretion, egestion, and not all parts being eaten.
Example 4
A field of grass has a biomass of 10,000 kg. Calculate the approximate biomass of primary consumers and secondary consumers in this food chain.
Solution:
Producer (grass): 10,000 kg
Primary consumers (herbivores): ~10% of 10,000 = 1,000 kg
Secondary consumers (carnivores): ~10% of 1,000 = 100 kg
This shows why pyramids of biomass get narrower at each level and why food chains rarely have more than 4 or 5 trophic levels.
❓ Practice Questions
Q1: Define the terms: ecosystem, community, population and habitat.
Q2: Explain the difference between competition and predation, giving an example of each.
Q3: Describe the predator-prey cycle and explain why there is a time lag between changes in the prey population and changes in the predator population.
Q4: Explain why a pyramid of biomass is always pyramid-shaped. Give three reasons why biomass is lost between trophic levels.
Q5: A producer has a biomass of 50,000 kJ/m². Estimate the biomass available to tertiary consumers and explain why food chains are rarely longer than 4-5 trophic levels.
✅ Answers
- Ecosystem: the interaction of a community of living organisms with the non-living parts of their environment. Community: all the populations of different species living in the same area at the same time. Population: all organisms of the same species living in the same area at the same time. Habitat: the place where an organism lives.
- Competition is when organisms compete for limited resources (e.g. plants competing for light). Predation is when one organism (predator) kills and eats another (prey) (e.g. foxes eating rabbits). Competition is between organisms of the same or different species; predation is between predator and prey species.
- When prey increase, predators have more food and their population increases (with a time lag because predators need time to reproduce and grow). More predators eat more prey, so prey decrease. Less prey means predators decrease. Fewer predators means prey increase again. The time lag exists because reproduction takes time — predators cannot increase instantly when prey becomes more available.
- A pyramid of biomass is always pyramid-shaped because biomass is lost at each trophic level. Reasons for loss: (1) Not all parts of an organism are eaten (bones, fur, roots). (2) Not all food eaten is absorbed — some is egested as faeces. (3) Absorbed material is used for respiration, producing CO₂, water and heat. (4) Some material is excreted as waste (urea).
- Producer: 50,000 kJ/m². Primary consumers: ~10% = 5,000 kJ/m². Secondary consumers: ~10% = 500 kJ/m². Tertiary consumers: ~10% = 50 kJ/m². Food chains are rarely longer than 4-5 trophic levels because there is not enough energy/biomass remaining to support another level — only about 10% is transferred at each step.
🎯 Exam Tips
- Learn the definitions of ecosystem, community, population and habitat precisely — these are frequently tested
- In food chains, arrows go FROM the eaten TO the eater (showing energy flow)
- Decomposers are NOT shown in food chains but are essential for recycling nutrients
- When describing predator-prey cycles, always mention the TIME LAG
- Only ~10% of energy/biomass transfers between trophic levels — use this in calculations
- Pyramids of NUMBER can be inverted (e.g. one tree with many insects), but pyramids of BIOMASS are always pyramid-shaped
- When explaining biomass loss, mention at least 3 reasons: not all eaten, egestion, respiration, excretion
🔢 Maths Skills
Mathematical Skills
You need to calculate population sizes from quadrat data, interpret pyramids of biomass, and calculate percentage energy transfer between trophic levels.
Maths Example
A 1 m × 1 m quadrat is placed 15 times in a 200 m² field. The mean number of daisies per quadrat is 6. Estimate the total population.
Population = mean per quadrat × (total area ÷ quadrat area) = 6 × (200 ÷ 1) = 1,200 daisies
Maths Example 2
A producer has 50,000 kJ/m² of energy. Calculate the energy at the primary, secondary and tertiary consumer levels assuming 10% transfer efficiency.
Primary consumer: 50,000 × 0.10 = 5,000 kJ/m²
Secondary consumer: 5,000 × 0.10 = 500 kJ/m²
Tertiary consumer: 500 × 0.10 = 50 kJ/m²
⚠️ Common Misconceptions
Watch Out!
1. Wrong: A population is all the organisms in an area Correct: A population is all the organisms of ONE species in an area — a community includes all the different species
2. Wrong: Arrows in a food chain show who eats whom Correct: Arrows show the direction of energy flow, FROM the eaten TO the eater (e.g. grass → rabbit, not rabbit → grass)
3. Wrong: Pyramids of number are always pyramid-shaped Correct: Pyramids of NUMBER can be inverted (e.g. one tree with many insects) — only pyramids of BIOMASS are always pyramid-shaped
4. Wrong: Decomposers are shown in food chains Correct: Decomposers are NOT shown in food chains but are essential for recycling nutrients back into the soil
✍️ 6-Mark Question
Extended Answer
6 marks: Explain why pyramids of biomass are always pyramid-shaped and why food chains rarely have more than four or five trophic levels.
Pyramids of biomass are always pyramid-shaped because biomass is lost at each trophic level. Firstly, not all parts of an organism are eaten — for example, bones, fur, roots and hooves are left behind. Secondly, not all food that is eaten is absorbed; some is egested as faeces. Thirdly, absorbed material is used for respiration, which releases CO₂, water and heat energy. Fourthly, some absorbed material is excreted as waste, such as urea in urine. Only approximately 10% of the biomass is transferred from one trophic level to the next. Because so much energy is lost at each stage, there is not enough biomass remaining to support further trophic levels beyond four or five. At the tertiary consumer level, the biomass available may be only 0.1% of the producer biomass, which is insufficient to sustain another level.
Mark scheme: 1 mark for not all parts eaten; 1 mark for egestion; 1 mark for respiration; 1 mark for excretion; 1 mark for ~10% transfer; 1 mark for explanation of why food chains are limited in length
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Students investigated the distribution of dandelions in a school field. They used a belt transect from the edge of a shady woodland into the open field, placing 1 m² quadrats every 5 metres. Their results were:
- 0 m (woodland edge): 0 dandelions per m²
- 5 m: 2 dandelions per m²
- 10 m: 8 dandelions per m²
- 15 m: 14 dandelions per m²
- 20 m (open field): 18 dandelions per m²
(a) Describe the trend in the data.
(b) Suggest an explanation for the trend.
(c) The students only placed quadrats along one transect line. Evaluate how this affects the reliability of their results.
Answers: (a) The number of dandelions increases as distance from the woodland increases — from 0 at the woodland edge to 18 per m² in the open field. (b) Dandelions need light for photosynthesis. Near the woodland, trees shade the ground, reducing light availability. In the open field, there is more light, so dandelions can photosynthesise more and grow better. Dandelions also compete with tree roots for water and minerals near the woodland. (c) One transect line may not be representative of the whole field — it might pass through an unusually bare or dense patch. Using multiple transect lines in different parts of the field and calculating a mean would improve reliability. However, the transect does show a clear and consistent trend, which suggests the pattern is real.
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