Describing Populations
What Is the Difference between Density-Dependent and Density-Independent Factors?
Density-Dependent Factors
Teacher Note: Misconception
Students may believe that the size of one population does not affect the size of other populations. However, the size of one population can greatly affect the size of other populations in the same area or in the same food chain.
Population demographicsglossary term (opens in a new window) vary due to both density-dependent factorsglossary term (opens in a new window) and density-independent factorsglossary term (opens in a new window). Density-dependent factors affect populations more as the population densityglossary term (opens in a new window) increases. Competitionglossary term (opens in a new window) for resources becomes more intense as density increases. Resource availability is an example of a density-dependent factor.
Infectious diseases are diseases that can spread from one host to another. Infectious diseases are a density-dependent factor. An increase in density increases the rate of host interactions, which in turn hastens the spread of the disease. This axiom supports health professionals’ recommendations that people with infectious diseases—such as the flu—stay home and avoid interactions with other people.
Predation is also a density-dependent factor. Higher numbers of prey result in increases in the predator populationglossary term (opens in a new window). Higher predator numbers increase the predation rate. Higher numbers of prey can change predator behavior. Many predators prefer to hunt more abundant prey. As a particular prey population becomes less abundant, predators choose a different prey population (if available). Decreases in the number of prey will also decrease predator numbers. Decline in predators releases the predation stress on the prey population and their numbers increase again. These cyclic fluctuations in population sizes of predator and the prey result in the so-called “predator–prey relationship” and the co-evolution of predators and prey.
Density-dependent factors are generally biotic, or living, components. Resource availability is one notable exception as these resources can either be biotic, such as prey numbers, or abiotic, such as availability of water.
Density-Independent Factors
Density-independent factors are often abiotic. By definition, they do not vary with population density. Severe weather events such as fires and floods affect large and small populations regardless of population size. Human activities such as the release of pollutants and habitat destruction can also affect populations independent of density. Non-infectious diseases are an example of a biotic density-independent factor. Non-infectious diseases such as cancer, which are not spread from individual to individual, have similar rates in high- and low-density populations.
Teacher Note: Connections
In this item, students understand that the significance of a limiting factor on a population can be dependent on the density of the population in which it occurs. Before they begin, discuss with students that patterns of relationships between biotic and abiotic factors that exist at some population densities may not exist at other densities. After students complete this item, deepen their understanding of this concept by asking them to categorize the factors listed according to whether they have no effect, a small effect, or a large effect on a dense population and on a dispersed population. Then, students can work with partners or in small groups to compare their answers, explaining the reasoning behind their categorizations.
