Course: BIS 105 Environmental Biology

Unit 4 Learning Module

Population Distribution
Copyright
Adapted from CK-12 Foundation, - Biology I Honors (CA DTI-3) by Douglas Wilkin .

Measuring Populations

In biology, a population is a group of organisms of a single species living within a certain area. Ecologists study populations because they directly share a common gene pool. Unlike the species as a whole, members of a population form an interbreeding unit. Natural selection acts on individuals within populations, so the gene pool reflects the interaction between a population and its environment.

Biologists study populations to determine their health or stability, asking questions such as:

- Is a certain population of endangered grizzly bears growing, stable, or declining?

- Is an introduced species such as the zebra mussel or purple loosestrife growing in numbers?

- Are native populations declining because of an introduced species?

- What factors affect the growth, stability, or decline of a threatened population?

The first step in characterizing the health of a population is measuring its size. If you are studying the population of purple loosestrife plants on your block , you can probably count each individual to obtain an accurate measure of the population’s size. However, measuring the population of loosestrife plants in your county would require sampling techniques, such as counting the plants in several randomly chosen small plots and then multiplying the average by the total area of your county. For secretive, highly mobile, or rare species, traps, motion-detecting cameras, or signs such as nests, burrows, tracks, or droppings allow estimates of population size.

Purple loosestrife plant populations show patchiness due to uneven distribution of their wetland habitats, and clumped dispersion, due to local variation in soils.

Two problems with absolute size lead ecologists to describe populations in other terms. First, because your county may not be the same size as others, the total number of individuals is less meaningful than the population density of individuals – the number of individuals per unit area or volume. Ecologists use population densities more often for comparisons over space or time, although total number is still important for threatened or endangered species.

Concern about threatened and endangered species has led conservationists to attempt to define minimal viable population size for some species. A species’ MVP is the smallest number of individuals which can exist without extinction due to random catastrophic variations in environmental (temperature, rainfall), reproduction (birth rates or age-sex structure), or genetic diversity. In 1978, Mark Shaffer incorporated an estimate for grizzly bear MPV into the first Population Viability Analysis (PVA), a model of interaction between a species and the resources on which it depends. PVAs are species-specific, and require a great deal of field data for accurate computer modeling of population dynamics. PVAs can predict the probability of extinction, focus conservation efforts, and guide plans for sustainable management.

Patterns in Populations I: in Space (or Patterns in Space)

A second problem in measuring population size relates to the distribution of individuals within the population’s boundaries. If your county has extensive wetlands in the southern half, but very few in the north, a countywide population density estimate of purple loosestrife, which grows primarily in shallow freshwater pond edges, marshes, and fens, would be misleading. Patchy habitat – scattered suitable areas within population boundaries – inevitably leads to a patchy distribution of individuals within a population. On a smaller scale, plants within even a single wetland area may be clumped or clustered, due to soil conditions or gathering for reproduction. The characteristic pattern of spacing of individuals within a population is called dispersion or distribution. Clumped dispersion is the most common distribution, but species that compete intensely, such as cactus for water in a desert, show uniform, or evenly spaced, dispersion. Other species, whose individuals do not interact strongly, may show a random, or unpredictable, distribution. This is the least frequently found distribution in nature. Useful measures of population density must take into account both patchiness of habitat and dispersion of individual organisms within the population’s boundaries.