Limits to Growth

A female cottontail rabbit (Sylvilagus floridanus) can give birth as often as seven times a year. A female American toad (Anaxyrus americanus) can lay thousands of eggs every spring. So why are the meadows and forests of the eastern United States not literally hopping with rabbits and toads? In nature, the size of a population and the rate of population growth are influenced by what ecologists call “limiting factors.”

Take It to the Limit

Think about all the different resources that two common animals need to stay alive. Cottontail rabbits need food to eat (grasses and other plants), water to drink, and a safe place to raise their young. American toads eat insects and, though they often live in forest habitat, need ponds or puddles to lay their eggs. Both toads and rabbits have to watch out for predators. But even if they avoid a hungry hawk or snake, they face other potentially deadly dangers, including diseases, forest fires, or drought.

Any of these factors—food, shelter, breeding sites, predators, and more—may serve to limit the growth of a rabbit or toad population. Often, the population is affected by several limiting factors that act together.

Density Matters—Unless It Does Not

Limiting factors fall into two broad categories: density-dependent factors and density-independent factors. These names mean just what they say: Density-independent factors have an impact on the population, whether the population is large or small, growing or shrinking. For example, a wildfire that sweeps through a dense forest in the Everglades has a big impact on every population in the community, regardless of the density of any one population.

Wildfire is abiotic (nonliving), and most density-independent limiting factors fall in this category. Other density-independent factors include hurricanes, pollutants, and seasonal climate extremes.

Density-dependent limiting factors tend to be biotic—having to do with living organisms. Competition and predation are two important examples of density-dependent factors.

Mountain chickadees (Parus gambeli) compete for a special kind of nest site—tree holes. These little cavities are excavated and then abandoned by woodpeckers. Scientists who added new nest sites in one expanse of forest saw the chickadee nesting population increase significantly, suggesting that nest sites are a density-dependent limiting factor.

A small furry rodent found in eastern Greenland called the collared lemming (Dicrostonyx groenlandicus) is a good example of how predation can be a density-dependent limiting factor. The population goes through a boom-and-bust cycle every four years. The lemming population grows to as much as 1,000 times its initial size, then crashes.

The cause is stoats (Mustela erminea), a type of weasel that hunts and eats lemmings almost exclusively. Stoats do not reproduce as fast as lemmings, so after a crash, when both stoat and lemming numbers are low, stoats do not have much impact on the lemming population. But by the fourth year, after the stoat population has had time to grow to greater numbers, the stoats—together with other predators—cause another lemming crash, and the cycle continues.

Carrying Capacity

If a population is small and resources are plentiful, a population may grow quickly. But over time, because of limiting factors, population growth tends to slow and then stop. The population has reached the “carrying capacity” of the ecosystem.