Which of the following is an example of density independent limitation on a population?

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.

In population ecology, limiting factors are factors in the environment that control various aspects of a population. Some limiting factors come into play depending on the density of the population, and others are unrelated to the population density. The latter are referred to as density-independent factors. Density dependent limiting factors are related to living organisms while density-independent limiting factors are related to the environment.

Density Dependent Limiting Factors

Density dependent limiting factors cause the per capita (per individual) growth rate of a population to change as the population gets larger. Limiting factors that are density dependent usually cause the per capita growth rate to decrease, acting as a negative feedback loop to control the size of the population. The maximum number of individuals that can live in an area based on the density dependent limiting factors is called the carrying capacity.

Competition

Individuals in a population are always competing for limited resources like food, mates, shelter, and water. As the population size increases, the competition becomes more intense causing some individuals to die over time, not mate, etc. This feedback makes a correction by reducing the population size to a level that can be supported by the environment.

Predation

Areas with high populations attract predators that kill and eat individuals, helping to keep the population under control. By feeding on these individuals, predators may end up increasing their own numbers, resulting in natural cyclical changes in populations.

Diseases, Parasites, and Accumulation of Waste

Diseases and parasites have more opportunities to spread and infect individuals in larger populations, such as through contaminated water supplies. Also, waste can accumulate quickly in large populations and this leads to death from disease and parasites and can also impair reproduction, reducing the size of the population.

Density Independent Limiting Factors

Limiting factors that fall into this category affect the per capita growth rate independent of the population density. These factors don’t make continual corrections to keep the population size under control because the strength of their effectiveness is not rooted in the number of individuals present. Density independent limiting factors cause abrupt and erratic shifts in population size. Small populations are particularly at risk of being wiped out by density independent limiting factors.

The category of density independent limiting factors includes fires, natural disasters (earthquakes, floods, tornados), and the effects of pollution. The chances of dying from any of these limiting factors don’t depend on how many individuals are in the population. In addition, individuals may not die directly from the limiting factor but from the effects of it such as from the loss of habitat or a primary food source resulting from a flash flood.

Density independent limiting factors also cause population sizes to increase. For example, the water from a flash flood increases the growth of vegetation, thereby providing more food for primary consumers in the ecosystem.

References

• Density dependence. (2017, October 16). In Wikipedia. Retrieved from https://en.wikipedia.org/w/index.php?title=Density_dependence&oldid=805682938
• Population regulation. (n.d.). Retrieved May 11, 2018, from https://www.khanacademy.org/science/biology/ecology/population-growth-and-regulation/a/mechanisms-of-population-regulation

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Biologydictionary.net Editors. "Density Dependent and Independent Limiting Factors." Biology Dictionary, Biologydictionary.net, 17 Sep. 2018, https://biologydictionary.net/density-dependent-and-independent-limiting-factors/.

Biologydictionary.net Editors. (2018, September 17). Density Dependent and Independent Limiting Factors. Retrieved from https://biologydictionary.net/density-dependent-and-independent-limiting-factors/

Biologydictionary.net Editors. "Density Dependent and Independent Limiting Factors." Biology Dictionary. Biologydictionary.net, September 17, 2018. https://biologydictionary.net/density-dependent-and-independent-limiting-factors/.

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