Despite the vicious rumours you may have heard, lemmings are not suicidal or even particularly conformist. They are predated upon by snowy owls, seabirds, arctic foxes and stoats. But where did this myth come from and how did we discover it was wrong?
In the 1958 Disney documentary White Wilderness lemmings are show hurling themselves off cliffs in mass suicides. Through the lens of complex systems this might seem like an example of a cascading failure. Maybe one or two lemmings accidentally slip off a cliff and because lemmings like to follow a few more leap off until a crowd of lemmings are jumping onto the new trend. Much like “the wave” can be started across an entire sports stadium with just six crowd members.
However, this explanation is unnecessary because the lemmings shown in the “documentary” where pushed off the cliffs. In truth, lemming populations would often explode and shrink in size. Perhaps these “boom and bust” cycles are what inspired White Wilderness to postulate such a fanciful story about lemming suicide. But what actually causes these cycles?
In 1925/1926 Lotka and Volterra independently developed models for explaining animal population cycles. In particular the Lotka-Volterra model – also known as the predator-prey model – represents the relationship between a predator and prey. For example, lynx and hares. It shows how the boom in a prey species (hares) causes the boom in a predator species (lynx). This leads to a bust in the hare population which causes a bust in the lynx population. Which leads to another boom in the hare population, starting the cycle over again.
The predator-prey model is made of two relatively simple equations. One which describes the prey population in terms of breeding and predation rates. And another which describes the predator population in terms of offspring per prey and mortality rates.
Notice how the equations do not describe how the hares use camouflage or how the lynx are territorial hunters? Yet the theoretical output of the model shows the same dynamics as the real world populations. This is the essence of modelling and why it is such a powerful tool for understanding complex systems.
So the lemmings are just another example of a predator prey cycle? Well, it’s a bit more complex than that. Lemmings have four distinct predators, snowy owls, seabirds, arctic foxes and stoats. Each of the predators is responsible for some amount of predation of the lemmings. However, the first two predators don’t appear to have the predator-prey cycle shown in Lotka-Volterra model. The arctic fox and particularly the stoat populations show the familiar predator-prey cycles.
In fact if we observe the predators in their habitat we will notice that they have different predation patterns. At high population numbers of lemmings the small rodents cannot dwell in their burrows. This is when the avian predators will strike. When the population of lemmings is low the mammals, particularly the stoat, can track the lemmings in the snow and their burrows.
Since the lemming population is mainly in bust are the main driver of lemming population. When the stoat population falls, the lemming population explodes.