iGCSE Biology (0610)-19.2 Food chains and food webs

Question

(a) A student recorded the information about an aquatic habitat.

Fig. 4.1 shows the student’s notes.

(i) Construct a food web to show the feeding relationships described in Fig. 4.1.

Do not draw the organisms.

(ii) Complete Table 4.1 using the information in Fig. 4.1 by identifying the names of the missing trophic levels and one organism at each different trophic level.

(iii) Outline how the energy in the primary consumers in this aquatic food web is used to produce biomass in the secondary consumers.

(iv) Humans also eat salmon. Predict the impact on the feeding relationships shown in Fig. 4.1 of overharvesting of salmon.

(b) Describe what is meant by the term decomposer.

(c) Animals such as salmon can be farmed for meat. Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants.

▶️ Answer/Explanation

Solution

(a)(i)

The food web should show:

aquatic plants → midges → stoneflies → salmon → herons
aquatic plants → mayflies → salmon → herons
aquatic plants → freshwater shrimps → salmon → herons

Explanation: A food web illustrates all the feeding relationships in an ecosystem. The arrows should point from the food source to the consumer. In this case, aquatic plants are the producers at the base, with multiple pathways leading up to the top predator (herons).

(a)(ii)

Name of the trophic level	Organism in Fig. 4.1
producer	aquatic plants
primary consumer	mayflies / midges / freshwater shrimps
secondary consumer	salmon / stoneflies
tertiary consumer	salmon / herons
quaternary consumer	herons

Explanation: Trophic levels represent the position an organism occupies in a food chain. Producers (aquatic plants) are at the base, followed by primary consumers (herbivores), then secondary consumers (carnivores that eat herbivores), and so on up to the top predators.

(a)(iii)

Energy transfer occurs through the following process:

Primary consumers (like mayflies) obtain chemical energy from eating aquatic plants
When secondary consumers (like salmon) eat primary consumers, they gain some of this energy
Energy is used for respiration, growth, reproduction, and other life processes
Only about 10% of the energy is transferred between trophic levels
The remaining energy is lost as heat, in undigested material, or used for metabolism

Explanation: Energy flow through ecosystems is inefficient. Most energy is lost at each transfer, which is why food chains rarely have more than 4-5 trophic levels. The energy that isn’t lost is incorporated into the biomass of the secondary consumers.

(a)(iv)

Overharvesting salmon would likely cause:

Decrease in heron population (less food available)
Increase in mayflies, stoneflies, and freshwater shrimps (reduced predation)
Possible decrease in midges (if stoneflies increase)
Variable impact on aquatic plants (may increase or stay stable)

Explanation: Removing a key predator like salmon disrupts the entire food web. Prey species may increase dramatically, which can then affect other species they interact with. This demonstrates the interconnectedness of ecosystems.

(b) A decomposer is an organism that breaks down dead organic matter and waste products, returning nutrients to the ecosystem.

Explanation: Decomposers like bacteria and fungi play a crucial role in nutrient cycling. They break down complex organic compounds into simpler forms that plants can reuse, completing the nutrient cycle.

(c) It’s more energy efficient because:

Energy transfer between trophic levels is only about 10% efficient
Eating plants directly means accessing more of the original solar energy
Less energy is lost to respiration, movement, and heat in plant-based diets
Livestock production requires more land, water, and resources

Explanation: The “10% rule” of energy transfer explains why shorter food chains are more efficient. Each time energy moves up a trophic level, 90% is lost. Therefore, eating producers (plants) gives humans access to more of the original energy captured through photosynthesis.

Question

Phytoplankton are microorganisms that photosynthesise.
(a) State the chemical equation for photosynthesis.

(b) A scientist made notes about a marine food web. Fig. 5.1 shows the notes she made.

(ii) State the principal source of energy for this food web.

(iii) Table 5.1 describes features of the food web in Fig. 5.2. Complete Table 5.1, using the information in Fig. 5.2

(iv) Fig. 5.3 is a photograph of an orca.

Using the information in Fig. 5.2, predict and explain the most likely effect of a decrease in the orca population on the population size of:

(c) Krill and seals can be harvested to produce omega‑3 fatty acids. Many people take omega‑3 fatty acids as a dietary supplement as they are thought to have health benefits in humans.
(i) List the chemical elements found in all fats.

(ii) Using the information in Fig. 5.2, explain why it is more energy efficient to use krill as a source of omega‑3 fatty acids rather than seals.

(d) Describe how fats ingested by humans are digested and absorbed.

▶️ Answer/Explanation

Solution

Ans:

(a) The chemical equation for photosynthesis is:
\(6CO_2 + 6H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6O_2\)

(b)(ii) The principal energy source is sunlight, which phytoplankton convert into chemical energy via photosynthesis.

(b)(iii) Completed Table 5.1

(b)(iv)
Krill: Population would decrease due to increased predation by crabeater seals and Adélie penguins (orca’s prey competitors).
Leopard seals: Population would increase due to reduced predation pressure from orcas.

(c)(i) All fats contain carbon (C), hydrogen (H), and oxygen (O).

(c)(ii) Krill are more energy-efficient because:
1. They occupy a lower trophic level (less energy lost).
2. Energy is lost at each trophic level through metabolism, excretion, and other processes (~90% loss per level).

(d) Fat digestion and absorption:
1. Emulsification by bile salts (in small intestine).
2. Enzymatic breakdown by lipase (from pancreas) into fatty acids + glycerol.
3. Absorption via lacteals in villi (small intestine).
4. Transported as chylomicrons to lymphatic system.

iGCSE Biology (0610)-19.2 Food chains and food webs – iGCSE Style Questions Paper 4

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