06 Population Ecology

Population size and density

A population is a group of individuals of the same species living in a particular area at a particular time. Population ecology examines how populations change in size and arrangement, and how environmental conditions influence those changes.

Population size is the total number of individuals. Population density is the number of individuals per unit area or volume. For example, 500 plants in a 1-hectare meadow have a density of 500 plants per hectare.

Ecologists may count every individual when feasible or estimate population size from samples. Plants can be counted in sample plots called quadrats. For mobile animals, mark-recapture involves capturing, marking, and releasing some individuals, then capturing another sample later. The fraction of marked individuals in the second sample helps estimate the population’s size. This method works best when samples represent the habitat and the population remains reasonably stable between captures.

Takeaway: Size is a total count; density relates that count to the area or volume occupied.

Population distribution

Population distribution describes how individuals are spaced within a habitat. The three common patterns are:

• Clumped: Individuals occur in groups, often because resources are patchy or organisms live socially. Herds and plants growing near where seeds fell are examples.

• Uniform: Individuals are spaced relatively evenly, often because of territorial behavior or competition.

• Random: Individuals are spaced without a predictable pattern when conditions and interactions do not strongly favor clustering or regular spacing.

Spacing can affect how easily individuals find mates, compete for resources, or avoid predators.

Takeaway: The pattern of spacing can influence interactions among individuals and their access to resources.

Population growth and carrying capacity

Population size changes through four processes: births and immigration add individuals, while deaths and emigration remove them. In equation form:

When resources are abundant, a population may grow rapidly. In exponential growth, the rate of increase rises as the population gets larger, producing a J-shaped curve. A simple model is:

Here, $N$ is population size, $t$ is time, and $r$ is the per-capita rate of increase—the balance of birth and death rates under the conditions being considered. Exponential growth is useful for describing short periods or unusually favorable conditions, but it cannot continue indefinitely in a finite environment.

As resources become harder to obtain, growth often slows. Carrying capacity, represented by $K$, is the population size an environment can support over time under particular conditions. The logistic growth model describes growth that slows as a population approaches carrying capacity:

This model produces an S-shaped curve. Growth is rapid when the population is small relative to $K$, slows as the population approaches $K$, and is zero at $K$ in the simplified model. Real populations may fluctuate around carrying capacity, which can itself change with seasons, weather, habitat conditions, or resource availability.

For example, rabbits in a field with plentiful food may initially increase quickly. As they consume more plants and compete for food and shelter, births may decline or deaths may increase, slowing population growth. A drought that reduces plant growth could also lower the field’s carrying capacity.

Takeaway: Exponential growth describes rapid increase under favorable conditions; logistic growth includes a slowdown as a population approaches the environment’s carrying capacity.

Limiting factors and population regulation

A limiting factor is a resource or environmental condition that restricts population growth, size, or distribution. Food, water, nutrients, light, nesting sites, and living space can all be limiting by affecting reproduction, survival, or both.

Limiting factors can be grouped by how their effects relate to population density:

• Density-dependent factors have stronger effects as density increases. Competition, disease, parasitism, and predation are common examples. Crowding can increase competition for food and make disease spread more readily.

• Density-independent factors affect populations regardless of density. Severe weather, fires, floods, and some forms of pollution can reduce a population whether it is sparse or crowded.

These categories describe how an effect relates to density, but they are not always absolute. Factors can interact: a harsh winter may affect deer at any density, while food scarcity after the winter may create stronger competition among survivors. Density-dependent regulation often slows growth as resources become scarce, but population sizes and carrying capacities can still change over time.

Takeaway: Density-dependent effects intensify as density rises; density-independent effects can occur at any density. Both may operate together, and environmental conditions can change over time.