Biology 12

MINI LAB

What Is the Population

of Your School?

The size of a population of mobile organisms is often
estimated using one of a number of mark–recapture
sampling techniques (sometimes referred to as capture–
recapture sampling). Small, lightweight leg bands are often
used to mark birds, fin or gill tags are used for fish, and ear
tags are sometimes used to mark captured mammals. It is
important that the markers do not interfere with the ability
of the subject to forage and perform other activities related
to survival or reproduction.

In this MiniLab, you will estimate the population of your
school. To do so will require careful planning, your principal’s
permission, and co-operation from other students. “Capture”
a random sample of 25 or 50 students (for example, capture
the first 50 students passing you in the hallway of your
school) and “mark” them in some way. You might use pieces
of string or ribbon as armbands to mark your captured
individuals. Release them and let them again become
randomly mixed with the general population. You will have to
decide in advance how long it might take them to become
re-mixed (an hour? a day?).

Take a second random sample of students (perhaps the
same size as your first sample) after re-mixing has occurred,
and record how many of them are marked. The proportion

of individuals in the second sample that are marked can be

used to estimate the size of the entire population by using

the formula:

For example, if 100 students were caught initially, marked,

and released, and 10 of these individuals were recaptured

when a second random sample of 100 was taken, an

estimate of the population size (N) could be obtained by

re-arranging the formula given above:

100

N

=^10

100

therefore,

N=^100 ×^100

10

=1000 students

Analyze

1.Compare your estimate of the size of the school’s

population with the true size. How close were you?

2. (a)What problems do you think might affect the
   accuracy of your estimate?
   (b)Do you think any of these problems might also be
   faced by ecologists studying non-human animal
   populations?
   3.How might you improve your sampling design?

number of individuals caught

and marked in the first sample

total population size

=

number of marked

individuals recaptured

total number

in second sample

Chapter 14 Population Ecology • MHR 469

resources, the types of interactions that typically
occur among members of a population, and the
distance offspring generally disperse from their
parents. In general, how the members of a population
are spaced throughout an area depends on a complex
interplay among these factors (each will be discussed
in more detail below). As always, ecology and
evolution are intertwined. The dispersion pattern
typically seen in a population is produced by

behaviours and other features that increase the

ability of individuals to survive and reproduce.

Dispersion patterns, which occur in ecological

time, are therefore the result of characteristics that

have been selected for over evolutionary time.

In some cases, it is primarily the distribution

of food, water, or other needed resources that

determines the dispersion of individuals. If these

resources are not distributed evenly but rather are

clumped uniform random

Figure 14.5Think of examples of populations that fit each of these
dispersion patterns.