Protozoa are single-celled organisms. Describe two ways Figure 6 shows that the protozoa in the river feed on bacteria.
How to approach this question
Look for a predator-prey relationship pattern in the graph between protozoa (predator) and bacteria (prey).
1. The predator population should increase after the prey population increases because there is more food available.
2. The increase in predators should cause a decrease in the prey population as they are eaten.
Find evidence for both of these points by comparing the peaks and troughs of the two lines.
Full Answer
1. The population of protozoa increases after the population of bacteria increases.
2. As the protozoa population peaks, the bacteria population decreases.
The graph shows a classic predator-prey relationship.
1. The bacteria population increases first, providing a plentiful food source. The protozoa population then starts to increase, but with a time lag, as they reproduce. This is shown by the protozoa peak occurring after the bacteria peak.
2. As the number of protozoa (predators) increases, they consume more bacteria (prey), causing the bacteria population to decrease. This is shown on the graph where the bacteria line falls as the protozoa line is rising to its peak.
Common mistakes
✗ Only describing the shape of one curve without linking it to the other. For example, "The protozoa numbers go up and then down."
✗ Stating that the lines cross. This is an observation but doesn't explain the feeding relationship.
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