14.0.4 14.5: Some populations cycle between large and small.

Nature is not always tidy. Exponential and logistic growth equations, for instance, help us understand the concept of population growth, but real populations don’t always show such “textbook” growth patterns. Sometimes they vary greatly in their rates and patterns of growth.

Locust swarms of biblical proportions certainly attest to the unpredictability of population growth (FIGURE 14-9). In northwest Africa, the desert locusts (migrating grasshoppers) normally live as solitary individuals in relatively small, scattered populations. In 2004, however, the population size increased rapidly, probably due to unusually good rains and mild temperatures. As the rainy season ended and green areas gradually shrank, the locusts became progressively concentrated in smaller and smaller areas. At this point, for reasons related to overcrowding but not completely understood, the locusts behaved like a mob, rather than solitary individuals. Giant swarms—some including tens of millions of insects—began flying across huge expanses of land in search of food. They completely consumed the crops on giant swaths of farmland, causing more than $100 million in damage.

There is more than one way for population growth to deviate from the standard pattern. The explosive locust population growth, for example, occurs at unpredictable intervals. Another unusual pattern is the oscillations of the lynx and snowshoe hare populations of Canada. As seen in FIGURE 14-10, rather than undergoing smooth logistic growth, the populations of both the snowshoe hare and its predators, the lynx, cycle regularly between increases to very large numbers and crashes to much smaller numbers.

Thanks to the Hudson’s Bay Company, which kept detailed records for decades on the number of pelts it purchased from fur trappers, this population cycling is well documented. Although its cause is not fully certain, to some extent, the predator and the prey cause their own cycling:

• 1. The hare population size grows,
• 2. providing more food for the lynx,
• 3. which then reproduce at a higher rate,
• 4. causing them to eat great numbers of the hares,
• 5. thereby reducing the hare population (and the lynx’s food source),
• 6. causing the lynx population to crash,
• 7. enabling the hare population to grow, and the cycle to begin anew.

In discussing these examples of unusual population growth, we must not lose sight of the fact that, for the most part, regularly cycling populations and populations with periodic outbreaks of huge numbers are more the exception than the rule. In general, the logistic growth pattern describes populations better than any other model.

One population-growth myth that demands debunking is the story of lemmings and their supposed suicides. Do lemmings jump off cliffs when their population becomes too large? In a word: no. Lemming populations do occasionally experience large increases in size. Then, just as locust behavior changes when population density gets too great, lemming behavior changes, and many individuals migrate in search of less crowded habitats and more food. As they enter unfamiliar territory, some lemmings may suffer increased rates of death. But these deaths are not suicides, and they do not occur in large groups.

How did the myth of lemming mass suicides become so prevalent? In the filming of a 1958 Disney documentary, White Wilderness, many lemmings were placed on a giant snow-covered turntable so that it would appear they were migrating. In a later scene, lemmings were filmed on a cliff overlooking a river and were then chased into the water as if in a mad rush. None of these scenes were of actual lemming migrations, and in nature, lemmings never behave like this.

There are practical reasons for predicting population sizes and growth rates, as we’ll see. But it can be difficult to translate simple growth models into feasible management practices.