Inspire Biology

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Prior to the winter of 1947–48 there were apparently no wolves on Isle Royale. During

that winter, a single pair of wolves crossed the ice on Lake Superior, reaching the

island. During the next ten years the population of wolves reached about twenty indi-

viduals. Notice that the graph in

Figure 5

on the previous page

shows that the rise and

fall of the numbers of each group was dependent on the other group. For example,

follow the wolves’ line on the graph. As the number of wolves decreased, the number of

moose increased.

Disease

Another density-dependent factor is disease. Outbreaks of disease tend to

occur when population size has increased and population density is high. When

population density is high, disease is transmitted easily from one individual to another

because contact between individuals is more frequent. Therefore, the disease spreads

easily and quickly through a population. This is just as true for human populations as it

is for populations of protists, plants, and other species of animals.

Competition

Competition between

organisms also increases when density

increases. When the population increases to a

size where resources such as food or space

become limited, individuals in the population

must compete for the available resources.

Competition can occur within a species or

between two different species that use the

same resources. For example, the foxes fighting

over the squirrel in

Figure 6

also compete with

other species, such as coyotes, for the same

food source.

Competition for insufficient resources might

result in a decrease in population density in an

area due to starvation or to individuals leaving the area in search of additional

resources. As the population size decreases, competition becomes less severe.

Parasites

Like disease, parasitic organisms can place limits on a population. The

presence of parasites is a density-dependent factor that can negatively affect population

growth at higher densities.

Population growth rate

An important characteristic of any population is its growth rate. The

population growth

rate

(PGR) explains how fast a given population grows. One of the characteristics of the

population ecologists must know, or at least estimate, is natality. The natality of a

population is the birthrate, or the number of individuals born in a given time period.

Ecologists also must know the mortality—the number of deaths that occur in the popu-

lation during a given time period.

The number of individuals emigrating or immigrating also is important.

Emigration

(em

uh GRAY shun) is the term ecologists use to describe the number of individuals moving

away from a population.

Figure 6

A decrease in the food supply can trigger

competition between members of the same species.

Module 4 • Population Ecology