Predators are a limiting factor for their prey, and prey are food, a limiting factor, for predators. So the two populations are tied together, as you saw in Limiting Factors and Carrying Capacity and Food Chains and Food Webs.
Key idea
When prey are plentiful, predators eat well and their numbers rise. More predators eat more prey, so the prey fall. With less food, predators fall next. With fewer predators, the prey recover. The two populations rise and fall in turns, and the predator peak comes after the prey peak.
Why it works
Predators need time to respond. A fox that eats well this year raises more kits, but those kits hunt only later. So the predator count keeps climbing for a while after the prey begin to fall, and keeps falling for a while after the prey start to recover. That delay is the lag between the curves.
Worked example: Read the lag
In a graph of snowshoe hares and lynx, the hares peak in year 4 and again in year 14. The lynx peak in years 6 and 16. How long is the lag?
- The lynx peak comes after each hare peak.
- 6 − 4 = 2 years; 16 − 14 = 2 years.
Answer: About 2 years.
Worked example: Predict the next step
Prey numbers are falling fast and predator numbers are at their highest. What happens next?
- Fewer prey means less food for each predator.
- Fewer predator young survive, and some predators starve or leave.
Answer: The predator population starts to fall.
Common mistake
Students think predators wipe out their prey, or that both populations peak at the same time. Predators usually cannot catch the last few prey, which hide or spread out, and starving predators decline first. On a graph, find the prey peak, then look a little later for the predator peak.
Try it in the Predators and Prey lab: change the number of foxes and rabbits and watch the cycles form.
A famous data set comes from fur-trading records in Canada, which show snowshoe hare and lynx numbers rising and falling for about a century. The hares' food supply plays a part in the cycle too.