Interpreting a Food Chain After a Disease Outbreak
A farmer near Maun keeps goats and grows maize. A simplified food chain for the area is:
maize → goats → people
and a second chain is:
grass → goats → people
A disease kills most of the grass in the grazing area, but the maize field is unaffected. The farmer notices that the goats begin eating more maize plants and that the goats' body mass falls over two months, even though they are eating. A neighbour suggests killing all the goats to 'save the maize'. The farmer wants to understand the effects on the food chains before deciding.
1. Explain, using energy flow in food chains, why the goats' body mass falls when grass dies even though they eat more maize.
Read the worked answer
Food chains show energy flow from producers to consumers. Grass and maize are producers; goats are primary consumers; people are secondary consumers. Energy is lost at each step (through movement, heat, waste), so goats must eat a lot of plant material to gain mass.
When grass dies, the goats lose their main food source. They switch to maize, but:
- The amount of maize available is limited and may not replace the energy lost from grass.
- Maize and grass differ in nutrient content; a sudden change of diet can reduce digestion and growth.
- The goats may be competing with people for the maize, so the farmer may restrict how much they eat.
- Energy that would have gone into growth is used for movement, keeping warm and searching for food.
So even though the goats are eating, they may be taking in less usable energy than they use, so body mass falls. The food chain also shows that the effect passes upward: fewer or weaker goats mean less food for people.
2. Evaluate the neighbour's suggestion to kill all the goats. Give two reasons against it and one alternative action.
Read the worked answer
Killing all the goats is a poor decision:
- It removes a link in both food chains. Goats convert grass and maize into meat and milk for people. Without goats, people lose a food and income source, and the farmer loses animals that can be sold later.
- It does not solve the underlying problem. The grass died from disease, not from the goats. Removing goats may let grass recover temporarily, but it also removes the animals that could graze it once it regrows, and it may allow unpalatable plants to spread.
- It is irreversible. Killing the herd destroys breeding stock; rebuilding takes years and money.
An alternative is to reduce the herd temporarily (sell some animals), keep breeding animals, and protect or replant grass while feeding goats a measured supplement of maize or crop residues. This protects the maize and lets the grass recover without destroying the whole herd. The farmer should also find out what caused the grass disease so it can be managed.
3. Predict one effect on people if the number of goats falls sharply, and explain it using the idea of a food chain.
Read the worked answer
People are at the top of both chains (maize → goats → people and grass → goats → people). If goat numbers fall, there is less goat meat and milk available, so people have less food and protein from this source. The chain shows that a change at one level affects the level above it: fewer primary consumers means less energy and matter passing to secondary consumers. People may then rely more on maize, which increases demand for the crop and could raise its price or reduce the amount left to sell. In the longer term, poorer nutrition can affect health and work. The prediction is therefore a fall in food supply and possibly higher food costs for the community.