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

The idea

Graphs that model predator-prey cycles usually plot population size on the vertical axis against time on the horizontal axis, with a separate line for each species. To interpret them, read the axes and key first, then trace each line. A peak in the prey line is followed later by a peak in the predator line, because predators need time to find extra food and reproduce. A trough in the prey line is followed by a fall in the predator line as food becomes scarce. The vertical gap between a peak and the following trough shows the size of the population change, and the time between successive peaks shows the length of the cycle. Comparing the two lines lets you explain the cause of each change and predict what happens next.

What earns the marks

• Read the axes, units and key before describing any line on the graph.
• Identify peaks and troughs for each species and state the population size and time at which they occur.
• Show that a predator peak occurs after the corresponding prey peak, demonstrating a lag.
• Link a rising prey line to increased food for predators and a rising predator line to increased predation on prey.
• Use the graph to predict the next change, such as a fall in prey numbers after a predator peak.

Common mistakes and how to put them right

• Describing the shape of the lines without quoting values.

  Put it right by reading specific figures from the axes at named times.

• Assuming both lines peak together.

  Put it right by identifying the time lag between the prey peak and the predator peak.

• Ignoring the key and confusing which line represents which species.

  Put it right by checking the key before interpreting the graph.

Exam tips

• Quote data from the graph, such as the population size at a peak and the year it occurs, to support each point.
• Use comparative language such as 'higher than', 'later than' and 'falls by' when describing two lines.
• Structure answers as cause then effect, for example prey rise, so predators have more food, so predator numbers rise.