There are basically two different types of population regulation - classified according to the types of factors that control the size of the population.

1. Density-dependent Control

a) Definition:

A density-dependent factor is one where the effect of the factor on the size of the population depends upon the original density or size of the population. A disease is a good example of a density-dependent factor. If a population is dense and the individuals live close together, then each individual will have a higher probability of catching the disease than if the individuals had been living farther apart. Not only will a greater number of individuals be affected, but, more importantly, a greater proportion of the population will be affected if they are living close together. For example, bird populations are often regulated more by this type of regulation.

b) Characteristic of the factor:

In general, density-dependent factors are biological factors, such as diseases, parasites, competition, and predation.

c) Characteristics of populations controlled primarily by density-dependent factors:

A population being controlled primarily by density-dependent factors will have the first growth form I showed you called the "self-limiting" growth form. Furthermore, in populations being controlled by density-dependent factors, growth rates are usually inversely proportional to population density. For example, if the population density is high, the growth rate is low. Conversely, if the density is low, the growth rate is high.

d) Characteristics of ecosystems having populations controlled by density-dependent factors:

Because of the nature of these biological factors, such as disease and competition, this type of regulation will usually occur in

i) ecosystems where the communities have MANY species, i.e., where many biological interactions are taking place,

ii) ecosystems NOT usually stressed periodically by physical factors (such as periodic flooding through the area), i.e., ecosystems that are usually more stable.

2. Density-independent Control

a) Definition:

A density-independent factor is one where the effect of the factor on the size of the population is independent of and does NOT depend upon the original density or size of the population. The effect of weather is an example of a density-independent factor. A severe storm and flood coming through an area can just as easily wipe out a large population as a small one. Another example would be a harmful pollutant put into the environment, e.g., a stream. The probability of that harmful substance at some concentration killing an individual would not change depending on the size of the population. For example, populations of small mammals are often regulated more by this type of regulation.

b) Characteristic of the factor:

In general, density-independent factors are physical factors, such as weather factors (e.g., severe winter) or the presence of harmful chemicals.

c) Characteristic of populations controlled primarily by density-independent factors:

Many populations controlled by density-independent factors have the second growth form, the "resource-limited" type. There is much less biological control and the control is a more haphazard, physical control. The population size often goes over the carrying capacity before some other physical factor decreases the population size. Unlike the case for density-dependent factors, in populations being controlled by density-independent factors, growth rates do not seem to show any trend at all relative to population density.

d) Characteristics of ecosystems having populations controlled by density-independent factors:

This type of regulation will usually occur in

i) ecosystems where the communities have FEW species, i.e., where fewer biological interactions are taking place,

ii) ecosystems ARE usually stressed periodically by physical factors (such as periodic flooding through a flood plain).

NOTE: Population regulation factors, then, can be classified as above into two types, but what usually happens in nature is that a population is actually controlled by a COMBINATION of density-dependent and density-independent factors. Some populations will be primarily controlled by one type and other populations will be controlled primarily by the other type. Note that both types of factors are external forces on the population.

 

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