(a) Fig. 5.1 shows a food web from a desert ecosystem.
Identify, on Fig. 5.1:
a quaternary consumer ……………………………………………………………………………………………….
a herbivore ………………………………………………………………………………………………………………..
an organism that is both a secondary and tertiary consumer ……………………………………………
an organism that is in trophic level one. …………………………………………………………………………
(b) A food chain from the food web is shown:
grass → rabbits → wolves
(i) Sketch the pyramid of biomass for this food chain. Label the pyramid with the name of each species.
(ii) State the principal source of energy for food webs.
(iii) Humans harvest rabbits. The road runner population could increase or decrease as a result of humans overharvesting rabbits. Explain how both changes in the road runner population could occur.
(c) The population of hawks has decreased significantly in the past 20 years. One cause of this decrease is insecticides used in farming.
(i) Explain why farmers use insecticides.
(ii) Suggest three other ways that humans may cause the population of birds such as hawks to decrease.
▶️ Answer/Explanation
(a)
Quaternary consumer: Wolf (top predator).
Herbivore: Rabbit or grasshopper (feeds on plants).
Secondary and tertiary consumer: Road runner or wolf (consumes both herbivores and other carnivores).
Trophic level one: Grass (producer).
(b)(i) Pyramid of biomass:
– Bottom (largest block): Grass.
– Middle: Rabbits.
– Top (smallest block): Wolves.
(Shape must be pyramid-like with labeled tiers.)
(b)(ii) Principal energy source: The Sun (drives photosynthesis in producers).
(b)(iii) Road runner population changes:
Decrease: Fewer rabbits → wolves prey more on road runners.
Increase: Less rabbit grazing → more grass → more grasshoppers (food for road runners).
(c)(i) Farmers use insecticides to:
– Kill crop-damaging insects.
– Increase crop yield/quality.
(c)(ii) Other human impacts on hawk populations:
1. Deforestation (habitat loss).
2. Hunting/poisoning.
3. Pollution/climate change.
Fig. 2.1 shows part of a food web for a coral reef. Algae and plankton are producers.
(a) Using the information in Fig. 2.1, identify:
- an organism that feeds at the third trophic level,
- a herbivore,
- a carnivore,
- an organism that is a primary consumer and a secondary consumer.
(b) (i) State what the arrows in Fig. 2.1 represent.
(ii) Using the information in Fig. 2.1, construct a food chain containing five organisms. Do not draw the organisms.
(iii) State the name of the process used by some producers to convert energy from light into chemical energy.
(iv) State the name of the type of organism that gets its energy from dead or waste organic material.
(c) The large fish in the food web is the coral grouper, Cephalopholis miniata.
Fig. 2.2 is a photograph of a coral grouper on a coral reef. Coral groupers are a popular food fish for humans.
Overharvesting of the large fish would cause the turtle population to decrease.
Using the information in Fig. 2.1, explain why the turtle population would decrease.
(d) Coral groupers developed over time from a species of fish with very few spots on their bodies.
Complete the sentences to explain how coral groupers developed.
The fish species with few spots had genetic variation in their population.
When these fish ………………………………………….., some of the offspring were born with more spots than others.
Fish with more spots were better adapted to the ………….. because predators were less likely to see them.
Fish with more spots had a greater chance of passing on the ………….. for more spots to the next generation.
This process is called ……………. selection.
▶️ Answer/Explanation
(a)
Third trophic level: turtles / large fish / sea urchins / sharks / starfish
Herbivore: sea cucumbers / small fish / sea urchins
Carnivore: sea urchins / turtles / large fish / starfish / sharks
Primary & secondary consumer: sea urchin
(b)
(i) The arrows represent energy transfer/flow.
(ii) Example food chain: algae/plankton → small fish → starfish → large fish → shark
(iii) The process is photosynthesis.
(iv) Organisms that get energy from dead matter are decomposers.
(c)
Overharvesting large fish reduces their population, forcing sharks to eat more turtles. Additionally, fewer large fish mean more small fish survive, leading to overgrazing of algae. This reduces food for sea cucumbers, which turtles depend on, causing their population to decline.
(d)
When these fish reproduced, some offspring had more spots. Fish with more spots were better adapted to the environment/coral reef as predators couldn’t see them easily. They passed on the alleles for more spots, leading to natural selection.