Biology 12

Chapter 14 Population Ecology • MHR 467

results are then extrapolated to the entire area
occupied by the population, as shown in Figure 14.2.

Estimating Numbers by Using

Transects or Quadrats

In some cases, organisms are sampled along a
transect, which is essentially a very long rectangle.
In transect sampling, a starting point and direction
are randomly chosen and a line of a certain length
(for example, 100 m) is marked out. The occurrence
of any individual within a certain distance of this
line is recorded. This distance might be 1 m if
plants are being sampled, or perhaps 50 m if more
mobile organisms (such as birds or mammals) are
being studied. A sample transect is shown in
Figure 14.3. Transects are particularly useful when
the density of a species is low, or when organisms
are very large (such as trees in a forest).
For plants and other types of organisms that tend
to stay in one spot all their lives, ecologists generally
use quadratsto sample a given population. First,
several locations are randomly chosen within the
area, and at each site a quadrat of known size (for
example, 1 m^2 ) is marked out. Next, the number
of individuals of a species within the quadrat is

counted (as illustrated in Figure 14.4 on the next

page), or the number of individuals of each of

several species are counted if more than one

population is being studied. The density of the

population is determined by calculating the

average number of individuals per quadrat, and

dividing by the size of the quadrat (for example,

2.5 individuals per m^2 ). The size of the population

can then be estimated by extrapolating from the

density figure. For example, if the geographical

area occupied by a population is 1000 m^2 and on

average there are 2.5 individuals per m^2 , then the

population size is roughly 2500 individuals.

Figure 14.3What is the area sampled along this transect?

100 m

5 m

Figure 14.2Examine a random sample of 5 of these 25 quadrats. Based on your
sample, what is the average number of birds per quadrat? How many birds do you
estimate there are in this flock?