Rabbits and foxes are locked in a loop: lots of rabbits feeds more foxes, more foxes means fewer rabbits, fewer rabbits starves the foxes, and fewer foxes lets the rabbits boom again. Round and round, in waves that chase each other forever.
Most people think removing a predator lets the prey flourish. In fact the prey explodes, strips its food, then collapses — wolves were a load-bearing part of Yellowstone, and the valleys were stripped bare for seventy years without them.
What's actually happening
An ecosystem has no manager, no plan, and no thermostat — yet it holds itself together for centuries. The secret is feedback. Every species' boom plants the seed of its own bust: more rabbits means more fox food means more foxes means fewer rabbits. Each arrow in that loop is just individuals eating and breeding, but the loop as a whole behaves like a regulator nobody built.
In the 1920s, Alfred Lotka and Vito Volterra wrote the loop down as a pair of equations (Volterra was explaining why WWI's fishing pause had boosted Adriatic sharks more than sardines). The equations' surprise: the populations never settle at a balanced number. They orbit it — prey waves and predator waves chasing each other, predators always cresting about a quarter-cycle late, because foxes can only multiply after the rabbit boom has fed them. The Hudson's Bay Company's fur ledgers, kept for ninety years, show Canadian lynx and snowshoe hare numbers doing exactly this dance on a roughly ten-year beat.
The sobering lesson is how the loop breaks. Remove the predator, as the US did to wolves in Yellowstone by 1926, and the prey doesn't flourish; it explodes, strips the vegetation, and crashes. Elk denuded Yellowstone's valleys for seventy years until wolves were returned in 1995, whereupon willows, beavers, and songbirds rebounded in a cascade. Stability wasn't the absence of the killer. The killer was a load-bearing part.
An ecosystem balances itself through feedback with nobody in charge, orbiting its equilibrium in waves — and the predator is essential, not optional.
- 1Start with 10 "rabbit" tokens and 2 "fox" tokens. Each round: every pair of rabbits adds one rabbit; every fox adjacent to rabbits converts one rabbit into half a fox; any fox that didn't eat is removed.
- 2Track both counts for fifteen rounds on a quick graph.
- 3You'll see the waves emerge by hand — and notice your fox line peaking a couple of rounds after every rabbit peak.
Common questions
The populations never settle at a balanced number — they orbit it. Predators always crest about a quarter-cycle after the prey, because foxes can only multiply once a rabbit boom has fed them.
The prey does not flourish; it explodes, strips the vegetation, then crashes. Removing wolves from Yellowstone let elk denude the valleys for seventy years until the wolves were returned in 1995.
Yes. The Hudson's Bay Company's fur ledgers, kept for ninety years, show Canadian lynx and snowshoe hare numbers cycling in offset waves on a roughly ten-year beat, exactly as the equations predict.