Food chains and food webs

1. Overview

This topic explores how energy flows through ecosystems, starting from the sun and moving through various organisms. Understanding food chains and webs allows us to predict how changes to one population—whether through human activity or natural shifts—can impact an entire biological community.

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Key Definitions

• Producer: An organism that makes its own organic nutrients (usually glucose), usually using energy from sunlight through photosynthesis.
• Consumer: An organism that gets its energy by feeding on other organisms.
• Herbivore: An animal that gets its energy by eating plants.
• Carnivore: An animal that gets its energy by eating other animals.
• Decomposer: An organism that gets its energy from dead or waste organic material (e.g., bacteria and fungi).
• Food Chain: A chart showing the transfer of energy from one organism to the next, beginning with a producer.
• Food Web: A network of interconnected food chains showing the complex feeding relationships in an ecosystem.
• Trophic Level: The position of an organism in a food chain, food web, or ecological pyramid.
• Biomass: The total mass of living material in a specific area or trophic level.

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Core Content

The Food Chain

A food chain is a linear sequence that shows how energy is transferred.

• Direction of Arrows: The arrows in a food chain represent the flow of energy. They point from the organism being eaten to the organism that eats it.
• Structure:
  • Producer $\rightarrow$ Primary Consumer $\rightarrow$ Secondary Consumer $\rightarrow$ Tertiary Consumer $\rightarrow$ Quaternary Consumer.
• 📊A linear sequence starting with Grass (Producer), followed by an arrow pointing to a Grasshopper (Primary Consumer), an arrow to a Frog (Secondary Consumer), and an arrow to a Hawk (Tertiary Consumer).

Producers and Consumers

• Producers are the foundation of almost all food chains. They convert light energy into chemical energy (carbohydrates) via photosynthesis.
• Consumers are categorized by what they eat and their position:
  • Primary consumers are herbivores (eat producers).
  • Secondary consumers are carnivores (eat primary consumers).
  • Tertiary/Quaternary consumers are usually top predators.

Food Webs

In the wild, most animals eat more than one type of food. A food web is a more realistic model of an ecosystem.

• Interdependence: If one species is removed (e.g., through overhunting), it affects many others.
• Human Impact:
  1. Overharvesting: Removing too many of a species (e.g., overfishing) can cause their prey to overpopulate and their predators to starve.
  2. Invasive Species: Introducing a foreign species can disrupt the web as the new species may have no natural predators but may compete with native species for food.

Ecological Pyramids

1. Pyramid of Numbers: Shows the number of individual organisms at each trophic level.
   • Shape: Can be "inverted" or irregular. For example, one large oak tree (producer) can support thousands of insects (primary consumers).
   • 📊A pyramid with a very narrow base (one tree), a wide middle section (many insects), and a narrower top (few birds).
2. Pyramid of Biomass: Shows the total mass of living material at each level.
   • Shape: Almost always "pyramid" shaped (widest at the bottom).
   • Advantages: More accurate than a pyramid of numbers because it accounts for the size of organisms.

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Extended Content (Extended Curriculum Only)

Pyramid of Energy

This shows the total energy available at each trophic level over a specific period (e.g., per year).

• Shape: Always a true pyramid shape; it can never be inverted because energy is lost at every level.
• Advantages: The most accurate representation of energy flow; it accounts for the rate of production and seasonal changes.

Efficiency of Energy Transfer

Energy transfer between trophic levels is inefficient. Usually, only about 10% of the energy is passed to the next level.

• Why is energy lost?
  1. Respiration: Energy is used for movement and metabolism, eventually lost as heat to the surroundings.
  2. Excretion/Egestion: Energy remains in waste products (faeces/urine).
  3. Uneaten parts: Not all of an organism is eaten (e.g., bones, roots, fur).

Food Chain Length

Food chains rarely have more than five trophic levels. This is because there is insufficient energy remaining to support a viable population at a higher level after so much energy has been lost at each previous step.

Human Food Efficiency

It is more energy-efficient for humans to eat crop plants than livestock.

• Short Chain: Sunlight $\rightarrow$ Crop $\rightarrow$ Human (1 energy transfer).
• Long Chain: Sunlight $\rightarrow$ Crop $\rightarrow$ Livestock $\rightarrow$ Human (2 energy transfers).
• By eating plants directly, we eliminate a trophic level and avoid the 90% energy loss associated with raising livestock.

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Key Equations

Efficiency of Energy Transfer (%) $$\text{Efficiency} = \left( \frac{\text{Energy available to the next level}}{\text{Energy received from the previous level}} \right) \times 100$$

• Units: Expressed as a percentage (%).
• Note: "Energy" can be substituted with "Biomass" in similar calculations.

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Common Mistakes to Avoid

• ❌ Wrong: Drawing arrows pointing to the animal being eaten.
• ✅ Right: Arrows must point in the direction of energy flow (into the mouth of the consumer).
• ❌ Wrong: Thinking a pyramid of numbers must always be a perfect triangle.
• ✅ Right: A pyramid of numbers can be any shape (e.g., a "top-heavy" pyramid if the producer is a single large tree).
• ❌ Wrong: Assuming 100% of energy is transferred between levels.
• ✅ Right: Most energy (approx. 90%) is lost as heat, waste, or unused material.

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Exam Tips

• Command Words: If asked to "Construct" a food chain, ensure you start with a producer and use arrows correctly. If asked to "Interpret" a food web, look for how a change in one population affects others "upstream" or "downstream."
• Trophic Levels: Remember that one organism can occupy different trophic levels in a food web depending on which food chain you are following.
• Calculation Questions: You are often asked to calculate the percentage efficiency. Always show your working and remember to multiply by 100 at the end.
• Real-world Context: Be prepared to discuss "Sustainability." Exams often ask why vegetarianism is more sustainable for a growing human population (refer back to the energy efficiency of short food chains).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A simplified food chain in a grassland ecosystem is shown below:

Grass $\rightarrow$ Grasshopper $\rightarrow$ Frog $\rightarrow$ Snake $\rightarrow$ Hawk

(a) Define the term 'food chain'. [2]

(b) Identify the producer and a secondary consumer in this food chain. [2]

(c) State what the arrows in the food chain represent. [1]

Worked Solution:

(a)

1. A food chain is a sequence of organisms describes the order of organisms

2. Through which energy and nutrients are transferred explains what moves through the chain

How to earn full marks:

• Both the sequence of organisms AND the transfer of energy/nutrients must be present for full marks.
• 'What eats what' is an acceptable simplification of energy/nutrient transfer.

(b)

1. The producer is Grass. correctly identifies the producer

2. A secondary consumer is Frog. correctly identifies the secondary consumer

How to earn full marks:

• Correctly identify both the producer and the secondary consumer.
• Accept "the frog" for the secondary consumer.

(c)

1. The arrows represent the flow of energy (or transfer of energy/biomass) from one organism to the next. clearly states energy flow

How to earn full marks:

• "Energy flow" or "transfer of energy" is essential. "What eats what" is also acceptable.
• "Matter" or "nutrients" may be included, but energy MUST be present.

Common Pitfall: Many students only mention "what eats what" without explicitly stating the transfer of energy. Remember that food chains are about energy transfer, not just who consumes whom. Also, be careful when identifying consumers; primary consumers eat producers, secondary consumers eat primary consumers, and so on.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

A marine ecosystem contains the following organisms: phytoplankton, zooplankton, small fish, large fish, and seals.

(a) Construct a food chain using these organisms. [2]

(b) Place each organism into its appropriate trophic level. [3]

(c) Explain why the number of organisms usually decreases as you move up a food chain. [1]

Worked Solution:

(a)

1. Phytoplankton $\rightarrow$ Zooplankton $\rightarrow$ Small fish $\rightarrow$ Large fish $\rightarrow$ Seals correct order of organisms

How to earn full marks:

• The organisms must be in the correct sequence.
• Arrows must show the direction of energy flow.

(b)

1. Phytoplankton: Producer identifies the producer

2. Zooplankton: Primary consumer identifies the primary consumer

3. Small fish: Secondary consumer identifies the secondary consumer

4. Large fish: Tertiary consumer identifies the tertiary consumer

5. Seals: Quaternary consumer identifies the quaternary consumer

How to earn full marks:

• 1 mark for each correct trophic level and correct organism.
• No marks deducted for incorrect/missing arrows in this part.

(c)

1. Energy is lost at each trophic level (e.g., through respiration, excretion, undigested material). states the loss of energy

How to earn full marks:

• Must mention energy loss, not just a decrease in numbers without explanation.
• Acceptable examples include respiration, excretion, movement, heat, or undigested material.

Common Pitfall: Students often forget to mention the loss of energy when explaining why the number of organisms decreases up a food chain. Simply stating that organisms are eaten is not enough; you need to explain that energy is used and lost as heat, waste, or during respiration.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A forest ecosystem contains a food web including trees, caterpillars, small birds, foxes, and decomposers.

(a) Draw a food web to illustrate the relationships between these organisms. [3]

(b) Describe the role of decomposers in this ecosystem. [3]

(c) Explain how overharvesting of small birds by humans could affect the population of caterpillars and trees in this ecosystem. [2]

Worked Solution:

(a)

1. 📊A food web showing: Trees being eaten by caterpillars; Caterpillars being eaten by small birds; Small birds being eaten by foxes; Decomposers feeding on dead trees, caterpillars, small birds, and foxes. Arrows must clearly show the direction of energy flow.
   *correctly includes all organisms*

2. correctly shows feeding relationships

3. arrows show correct direction of energy flow

How to earn full marks:

• All organisms MUST be included in the food web.
• The food web MUST show all relationships (e.g., trees eaten by caterpillars, caterpillars eaten by birds, etc.).
• Arrows MUST clearly show the direction of energy flow (from food to consumer).

(b)

1. Decomposers break down dead organisms and waste materials. states that decomposers break down dead matter

2. They release nutrients (or minerals/elements) back into the soil. states release of nutrients

3. These nutrients are then used by plants (trees) for growth. explains how nutrients are recycled

How to earn full marks:

• Must mention breaking down dead matter/waste.
• Must mention the release of nutrients back into the environment (e.g., soil).
• Must mention that these nutrients are used by plants/producers.

(c)

1. Overharvesting small birds would lead to an increase in the caterpillar population. predicts increase in caterpillars

2. This could lead to increased consumption of trees, potentially harming them. explains impact on trees

How to earn full marks:

• Must state that the caterpillar population would increase.
• Must explain the impact on the trees, linking the increased caterpillar population to increased consumption of the trees.

Common Pitfall: When drawing food webs, make sure all organisms are included and that the arrows point in the correct direction, indicating the flow of energy. Also, remember that decomposers recycle nutrients, which is essential for plant growth. In impact questions, clearly state the initial effect (e.g., increase in caterpillars) and then explain the subsequent consequences.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A scientist is studying a freshwater lake ecosystem. They measure the biomass of different organisms in the lake and obtain the following data:

• Phytoplankton: 500 kg/hectare
• Zooplankton: 80 kg/hectare
• Small fish: 15 kg/hectare
• Large fish: 3 kg/hectare

(a) Define 'biomass'. [1]

(b) Construct a pyramid of biomass for this lake ecosystem, labeling each trophic level. [3]

(c) Explain why a pyramid of biomass is often considered a better representation of energy flow than a pyramid of numbers. [3]

(d) Suggest two reasons why the transfer of energy between trophic levels is not 100% efficient. [2]

Worked Solution:

(a)

1. Biomass is the total mass of living organisms mass of living organisms

2. In a given area or volume. states the area/volume aspect

How to earn full marks:

• Must mention the mass of living organisms.
• Must mention the area or volume (e.g., per unit area).

(b)

1. 📊Pyramid of biomass with four levels: Phytoplankton (base, 500 kg/hectare), Zooplankton (80 kg/hectare), Small fish (15 kg/hectare), Large fish (3 kg/hectare). Each level must be correctly labelled with the organism.
   *correctly draws a pyramid shape*

2. correctly labels each trophic level with the correct organism

3. correctly represents the relative biomass of each level (e.g., base is largest, top is smallest)

How to earn full marks:

• Pyramid must be drawn generally to scale, reflecting biomass.
• Each level MUST be labeled with the correct organism.
• Pyramid shape MUST be clear (wider at the bottom, narrower at the top).

(c)

1. A pyramid of biomass takes into account the size of individual organisms. states that biomass accounts for size

2. A pyramid of numbers only counts the number of organisms, regardless of their size. states that numbers only counts individuals

3. Therefore, biomass gives a more accurate representation of the total energy stored at each trophic level (or energy flow through the ecosystem). relates to energy stores

How to earn full marks:

• Must state that biomass takes into account the size of organisms.
• Must state that numbers only considers the quantity of organisms.
• Must link this to a more accurate representation of energy/energy flow.

(d)

1. Some energy is lost as heat through respiration. heat from respiration

2. Some energy is lost in waste products (e.g., feces, urine). energy lost in waste

How to earn full marks:

• Must state two valid reasons for energy loss.
• Acceptable reasons include: respiration, excretion, undigested food, heat loss, movement.
• Simply stating "energy is lost" is insufficient; a specific mechanism is required.

Common Pitfall: When defining biomass, remember to include both "living organisms" and "per unit area/volume." When explaining energy transfer inefficiency, be specific about how energy is lost (e.g., respiration, excretion) rather than just saying "energy is lost." Many students also forget to label the trophic levels correctly on the pyramid of biomass.