Predator-prey relationships and population cycles
Biology · Key Stage 3 · The School
How the two populations are linked
When PREY are plentiful, PREDATORS have plenty to eat. More predators survive and breed, so the predator group grows. As predators grow in number, they eat more prey, so the prey group falls. With less prey around, some predators starve or fail to breed. So the predator group falls too. That lets the prey group recover, and the cycle can start again.
Reading a predator-prey graph
On a typical graph, the PREY peak comes FIRST. A PREDATOR peak follows a little later. It takes time for more prey to turn into more surviving, breeding predators. The predator peak then makes the prey group fall. That, in turn, makes the predator group fall a little later still. This time-lag between the two lines is the key thing to spot on such a graph.
Why populations cycle rather than settle
The two groups do not settle at one fixed number. Each one keeps changing the conditions the other depends on. More prey creates the conditions for more predators. More predators then wreck the conditions that let prey grow. Each group overshoots a little before the other catches up and pulls it back. That push and pull is why the pattern repeats in a cycle, rather than settling flat.
A simple lynx-and-hare cycle moves through four stages, repeating. Stage 1: hares are plentiful, lynx have plenty of food, and lynx numbers grow. Stage 2: more lynx means more hares get eaten, so hare numbers start to fall. Stage 3: with fewer hares around, lynx begin to starve or fail to breed, so lynx numbers fall too. Stage 4: with fewer lynx hunting them, hare numbers recover, and the system is back near Stage 1. Real data on lynx and hares, kept over many decades from fur records, shows almost exactly this pattern. The lynx peak reliably follows the hare peak by a year or two — real evidence for a pattern that also makes good biological sense.