Fig. 1.1 shows a food web for a coral reef ecosystem.
The world population has grown rapidly, reaching over 8 billion people, with most growth in developing countries. Population growth creates increased demand for food, water, energy and other natural resources.
Global energy demand continues to rise as populations grow and economies develop, with fossil fuels still providing the majority of the world's energy supply. Renewable energy sources including solar, wind, hydroelectric and geothermal power are increasingly being used to generate electricity without producing greenhouse gases.
(a) State the names of two producers shown in the food web. [2]
(b) From the food web, describe two food chains, each containing at least four organisms. [4]
(c) Explain what would happen to the populations of other organisms in the food web if a disease killed most of the sea urchins. [6]
(a) Two producers from the food web [2]
Producers are organisms that make their own food through photosynthesis. From the coral reef food web, any two of the following are acceptable:
- Algae [1]
- Phytoplankton [1]
- Seagrass [1]
These are all photosynthetic organisms that form the base of the food web, converting light energy into chemical energy stored in organic molecules.
(b) Two food chains each containing at least four organisms [4]
A food chain traces the flow of energy from a producer through a series of consumers. Each chain must have at least four organisms linked by arrows showing "is eaten by".
- Phytoplankton → zooplankton → butterfly fish → reef shark [2]
- Algae → sea urchin → grouper → reef shark [2]
Any valid chain of four or more organisms taken from the food web is acceptable. The arrows must follow the direction of energy flow (from prey to predator).
(c) Effect on other populations if a disease killed most sea urchins [6]
Removing sea urchins disrupts multiple feeding relationships in the food web. The effects ripple through the ecosystem:
- The sea urchin population decreases sharply due to the disease. [1]
- Algae populations increase because fewer sea urchins are grazing on them. With reduced herbivory, algae can grow unchecked. [1]
- Butterfly fish populations decrease because they lose sea urchins as a food source (assuming the food web shows butterfly fish feeding on sea urchins). [1]
- Grouper populations decrease because groupers feed on sea urchins and now have less prey available. [1]
- Reef shark populations may decline because their prey (butterfly fish, grouper) have decreased in number, reducing the food available to the top predator. [1]
- The overall balance of the ecosystem is disrupted. Organisms that depended on sea urchins as food decline, while organisms that competed with sea urchins for resources may increase. [1]
This illustrates the concept of interdependence: the removal of one species has cascading effects throughout the food web because of the complex feeding relationships between organisms.
