Food Chains and Food Webs

IGCSE Edexcel Biology
4.6–4.9 Feeding Relationships and Energy Transfer

Section A: Identifying Trophic Levels

1. The aye-aye is a nocturnal primate that eats insects, fruit, and nectar. In a food chain: mango tree → insect pest → aye-aye [3]
(a) Identify the producer in this food chain
(b) What is the trophic level of the insect pest?
(c) Is the aye-aye a herbivore, carnivore, or omnivore? Explain your answer.
2. Meerkats live in the Kalahari Desert and primarily eat insects, scorpions, and small vertebrates. [4]
(a) Construct a food chain that includes: acacia tree, locust, scorpion, meerkat, and martial eagle
(b) What trophic level is the meerkat in this chain?
(c) What would happen to the locust population if meerkats were removed?
(d) Explain why there are fewer eagles than locusts in this ecosystem

Section B: Aquatic Food Chains

3. The axolotl is an aquatic salamander found in Mexican lakes. Consider this food chain:
green algae → pond snail → axolotl → great blue heron [3]
(a) Identify the primary consumer
(b) If the green algae contains 10,000 kJ of energy, calculate how much energy would be available to the axolotl (assume 10% energy transfer)
(c) How much energy would reach the heron?
4. Marine ecosystems contain complex food chains. [5]
Producers: phytoplankton
Primary consumers: krill
Secondary consumers: octopus, dolphin
Tertiary consumers: orca
(a) Write a food chain that includes all organisms above
(b) What is the role of phytoplankton in this ecosystem?
(c) Both orcas and dolphins are marine mammals. Suggest why they occupy different positions in some food chains
(d) If octopus populations decreased dramatically, explain two possible effects on the ecosystem

Section C: Terrestrial Ecosystem Food Chains

5. The ocelot is a wild cat found in Central and South American rainforests. It hunts small mammals, birds, and reptiles. The agouti is a rodent that feeds on seeds and Brazil nuts from rainforest trees (including the Brazil nut tree). The jaguar is an apex predator that can prey on ocelots and other mammals. [4]
(a) Create a food chain using these organisms
(b) What is the trophic level of the ocelot in this chain?
(c) Rainforest ecosystems are highly complex with many interconnected feeding relationships. Draw a food web showing the connections between these organisms:
Producers: Brazil nut tree, Fruit trees, Rainforest trees
Primary consumers: Agouti (eats nuts/seeds), Fruit bat (eats fruit), Caterpillars (eat leaves)
Secondary consumers: Ocelot, Harpy eagle
Tertiary consumer: Jaguar
(d) Explain how the toco toucan and agouti might compete in this ecosystem
6. African savanna ecosystems support diverse wildlife with complex predator-prey relationships. The acacia tree, giraffe, zebra, wildebeest, lion, hyena, and wild dog all inhabit this ecosystem. [5]
(a) Giraffes can reach leaves high in acacia trees. Explain how this is an adaptation that reduces competition
(b) What would be the effect on the acacia tree population if giraffe numbers increased significantly?
(c) African savanna ecosystems show complex predator-prey relationships. Complete the food web by drawing arrows to show ALL feeding relationships between these organisms:
Producers: Acacia tree, Grass
Primary consumers: Giraffe, Zebra, Wildebeest
Secondary consumers: Lion, Hyena, Wild dog
Note: Hyenas and wild dogs hunt medium-sized herbivores; lions hunt all three herbivores
(d) Lions are apex predators. Explain what this term means

Section D: Comparing Different Ecosystems

7. Compare food chains in different habitats: [4]
(a) Arctic: Arctic willow → Arctic hare → Arctic fox → Inuit people / Snowy owl
Temperate: Grass → European rabbit → red fox → wolf
Identify one similarity and one difference between these food chains
(b) Why are Arctic food chains typically shorter than tropical rainforest food chains?
8. The platypus is a unique Australian mammal that hunts underwater for invertebrates like insect larvae and crayfish. [5]
(a) Write a food chain: water lily → mayfly larvae → platypus → saltwater crocodile
(b) The platypus is described as a secondary consumer. Explain why this term is used
(c) Suggest what might happen to mayfly larvae populations if platypus numbers decreased
(d) Saltwater crocodiles are opportunistic predators. Explain what this means for food web complexity
(e) Suggest why the platypus and saltwater crocodile might not directly compete for food despite both living in Australian waterways
9. The nine-banded armadillo feeds on insects, grubs, and plant material. [5]
(a) Is the armadillo a herbivore, carnivore, or omnivore?
(b) Write two different food chains that both include the armadillo:
Chain 1 (armadillo as primary consumer):
Chain 2 (armadillo as secondary consumer):
(c) Explain why omnivores like the armadillo can be at different trophic levels
(d) Suggest an advantage of being an omnivore in terms of survival
10. Food Web Analysis: Desert Ecosystem [5]
Here is a partially completed food web from a desert ecosystem:
Producers: Desert plant, Seeds
Primary consumers: Locust, Lizard, Mouse
Secondary consumers: Roadrunner, Snake
Desert plant → Locust → Roadrunner
Desert plant → Lizard → Roadrunner
Desert plant → Lizard → Snake
Seeds → Mouse → Snake
Seeds → Mouse → ?
(a) Complete the food web by suggesting an organism that could eat the mouse (other than snake)
(b) Using the food web above, identify which organism is the producer
(c) The roadrunner eats both locusts and lizards. What term describes an organism that feeds at multiple trophic levels?
(d) Explain why removing all the snakes would have a greater effect on this food web than removing all the roadrunners
(e) Suggest one advantage of having a complex food web rather than simple food chains in this desert ecosystem

Section E: Food Web Analysis

11. A complex marine ecosystem [8]
Kelp forest ecosystem: A cold-water marine environment where giant kelp provides shelter and food
Producers: kelp, phytoplankton
Primary consumers: purple sea urchin (eats kelp), Antarctic krill (eats phytoplankton), anchovy (eats phytoplankton)
Secondary consumers: giant Pacific octopus (eats urchins, krill, fish), bottlenose dolphin (eats fish)
Tertiary consumer: orca (eats dolphins, octopus, and large fish)
(a) Draw a detailed food web showing the feeding relationships between all these organisms. Remember to include all possible feeding connections.
(b) Identify two organisms that might compete for the same food source
(c) If a disease killed most of the Antarctic krill population, explain the likely effects on:
(i) Phytoplankton populations
(ii) Bottlenose dolphin populations
(iii) Orca populations
(d) Explain why food webs provide a more realistic representation of ecosystems than food chains
Total marks: 58

Mark Scheme

1. (a) Mango tree [1]
(b) Second trophic level / primary consumer [1]
(c) Omnivore - it eats both plant material (fruit, nectar) and animals (insects) [1]
2. (a) Acacia tree → locust → scorpion → meerkat → martial eagle [1]
(b) Fourth trophic level / tertiary consumer [1]
(c) Locust population would decrease / with no meerkat predation, scorpion populations increase, eating more locusts [1]
(d) Energy is lost at each trophic level / only 10% transferred / less energy available to support large populations at higher levels [1]
3. (a) Pond snail [1]
(b) 10,000 × 0.1 = 1,000 kJ to snail; 1,000 × 0.1 = 100 kJ to axolotl [1]
(c) 100 × 0.1 = 10 kJ [1]
4. (a) Phytoplankton → krill → octopus → orca [1]
(b) Producer / makes food through photosynthesis [1]
(c) Different prey preferences / different hunting methods / occupy different niches / dolphins eat fish while orcas can eat larger prey including marine mammals [1]
(d) Any two: prey species (krill/small fish) would increase [1]; predators (dolphin/orca) may decrease or switch to other prey [1]; ecosystem balance would be disrupted [1] (max 2)
5. (a) Brazil nut tree → agouti → ocelot → jaguar [1]
(b) Third trophic level / secondary consumer [1]
(c) Food web should show: Brazil nut tree → agouti; fruit trees → agouti AND fruit bat; rainforest trees → caterpillars; fruit bat → harpy eagle; caterpillars → harpy eagle AND birds; agouti → ocelot; ocelot → jaguar [2]
(d) Both may eat fruits/nuts from trees / compete for the same food resource [1]
Jaguar Ocelot Harpy eagle Birds Agouti Fruit bat Caterpillars Brazil nut tree Fruit trees Rainforest trees
6. (a) Long neck allows access to food other herbivores cannot reach / reduces competition for lower vegetation with other herbivores [1]
(b) Acacia population would decrease / more browsing pressure / could affect tree regeneration [1]
(c) Food web should show: both acacia and grass as producers; arrows from both to giraffe, zebra, and wildebeest; arrows from all herbivores to lion, hyena, and wild dog [2]
(d) Top predator / no natural predators / at the top of the food chain [1]
7. (a) Similarity: both have three trophic levels / both show plant producers and herbivores and carnivores / both show energy transfer [1]
Difference: Arctic uses Arctic hare/fox/owl while temperate uses rabbit/fox/wolf / different species adapted to different climates / Arctic animals have cold-adapted features [1]
(b) Less primary productivity in cold environments / shorter growing season / fewer plants mean fewer herbivores / harsh conditions limit number of species [1]
8. (a) Water lily → mayfly larvae → platypus → saltwater crocodile [1]
(b) Secondary consumer because it eats primary consumers (herbivorous invertebrates like mayfly larvae that feed on plants/algae) / it is at the third trophic level [1]
(c) Mayfly larvae would increase / less predation pressure / population would rise without control [1]
(d) They eat various prey at different trophic levels / increases number of connections in food web / makes food web more complex and interconnected [1]
(e) Different food preferences / eat different sized prey / occupy different habitats/depths / different hunting methods / niche separation [1]
9. (a) Omnivore [1]
(b) Chain 1 (armadillo as primary consumer): Plant → armadillo → predator (e.g., coyote) [1]
Chain 2 (armadillo as secondary consumer): Plant → insect/grub → armadillo → predator (e.g., coyote) [1]
(c) When feeding on plants they are primary consumers / when feeding on insects/animals they are secondary consumers / trophic position depends on their diet in that specific food chain [1]
(d) More food sources available / can switch diet if one food source becomes scarce / better survival in changing environmental conditions / increased adaptability [1]
10. (a) Any predator of mice: e.g., hawk, owl, fox, snake (already mentioned), coyote, etc. [1]
(b) Desert plant / producer [1]
(c) Omnivore / heterotroph eating at multiple trophic levels [1]
(d) Snakes eat mice and lizards (multiple food sources) so their removal removes a connection affecting multiple species / removal of roadrunners removes only one predator pathway / snakes have more impact on food web complexity [1]
(e) Any one: if one food source becomes scarce, organisms have alternatives / greater stability/resilience to environmental change / less vulnerable to extinction of single species / maintains ecosystem balance better [1]
11. (a) Food web should show: kelp and phytoplankton as producers with arrows to all primary consumers; urchin → octopus; krill → octopus, dolphin, orca; anchovy → octopus, dolphin, orca; octopus → orca [2]
(b) Antarctic krill and anchovy (both eat phytoplankton) [1]
(c) (i) Phytoplankton would increase / less grazing pressure by krill [1]
(ii) Bottlenose dolphins would decrease / lose food source / or switch to other prey [1]
(iii) Orcas would decrease / indirect effect from reduced dolphin/octopus numbers / less prey available [1]
(d) Shows multiple feeding relationships / organisms often eat more than one thing / shows interconnections / more realistic than linear chains / shows complexity of ecosystems [1]