14.1 Characteristics of Populations
466 MHR • Unit 5 Population Dynamics
Demographyis the study of populations —
particularly the characteristics of populations that
can be quantified, such as size, density, and growth
rate. Demographers (those who study demography)
are interested in what are sometimes called the
vital statistics of a population. These include the
characteristics mentioned above as well as others,
such as the ratio of males to females and the ratio
of old to young individuals in a population. These
are among the factors that determine if, and by how
much, the population is going to grow. Demographers
generally start their study by defining the
population of interest.
Recall from the previous chapter that a
population consists of a group of individuals, all
of the same species, simultaneously occupying a
particular area. The boundaries of this area, which
partly define the population, may be natural. For
example, the shores of Lake Ontario define the
limits of the population of lake whitefish that live
in the lake. Similarly, the shores of Isle Royale in
Lake Superior define that island’s moose (Alces
alces) population.
Many populations, however, have arbitrary
boundaries set by a researcher or a government
agency for management or study purposes (see
the example in Figure 14.1). White-tailed deer
(Odocoileus virginianus) in Algonquin Park often
travel beyond the park’s boundaries, and there is
not much to distinguish between a maple tree
inside the park and one just outside. Nonetheless,
it is often useful to talk about the park’s deer or
tree populations, with the understanding that
the geographic limits of these populations are
somewhat arbitrary.
Regardless of how the boundaries of a population
are determined, all populations can be described
in terms of two important characteristics: density
and dispersion. The densityof a population is the
number of individuals per unit of volume or area
(for example, the number of oak trees per square
kilometre in Prince Edward County). Dispersion
relates to how the individuals in a population are
spread out within its geographical boundaries. (Arethey spaced uniformly throughout the habitat, or
are they clumped together in groups?) Both of these
features can affect how a population grows and what
impact the population has on the environment.Figure 14.1Although we might refer to Ontario’s blue jay
(Cyanositta cristata) population, the birds themselves are
not restricted by provincial boundaries. Individuals fly in
and out of the other provinces and across the border into
the United States daily.Measuring Population Density
The density of a population depends on its size —
that is, how many individuals it contains. In rare
cases, it is possible to count all the members of a
given population. Such a complete count is
referred to as a census. The governments of many
countries regularly conduct censuses of their
human populations.
Often, however, a complete census is impossible
due to time, energy, or financial constraints — a
population may simply be too large to count.
Therefore, ecologists must frequently estimate the
size or density of the population of interest. There
are a variety of ways to do this. In most cases, the
number of individuals in a number of samples
(small portions or subsets of the entire population)
are counted or estimated and then averaged. TheEXPECTATIONSDescribe characteristics of a population, such as size and density.
Estimate the size and density of populations using various sampling methods.