Students should be able to interpret graphs used to model these cycles.

The idea

Predator-prey cycles are often shown as line graphs with time on the x-axis and population size on the y-axis. Two lines are plotted, one for the predator and one for the prey. To interpret such a graph, read the axes and key first, then trace each line. Identify peaks and troughs and note which line peaks first. The prey line usually peaks before the predator line, because predators need time to find more food, survive and reproduce. Where the prey line falls, the predator line often rises shortly afterwards, and where the predator line is high, the prey line falls. Compare the heights of peaks and depths of troughs to judge the size of each population. Use the graph to describe trends and to explain the link between the two populations.

What earns the marks

• Reads the axes and key correctly, identifying time on the x-axis and population size on the y-axis and matching each line to its organism.
• Identifies peaks and troughs on both lines and states which population is larger at a given time.
• Recognises that the prey population peaks before the predator population, showing a time lag.
• Links a rise in predator numbers to a later fall in prey numbers, and a fall in predator numbers to a later recovery of prey.
• Describes the overall pattern as repeating cycles rather than a single change.
• Uses data from the graph, such as values at a stated time, to support the interpretation.

Common mistakes and how to put them right

• Mixing up the two lines.

  Put it right: Check the key and label each line before describing any trend.

• Reading the x-axis as population size.

  Put it right: Population size is on the y-axis and time is on the x-axis.

• Claiming the lines rise and fall together.

  Put it right: The predator line lags behind the prey line, so their peaks occur at different times.

Exam tips

• Quote specific values or dates from the graph to support each point you make.
• Use comparative language such as higher, lower, steeper and later when describing the two lines.
• If asked to explain, link each change in the predator line to a change in food supply from the prey.