How many are there?

Collection of
all the
organisms in
an area AND
the non-living
parts of the
environment
 The
total
number of
one species
living in a
defined
region

Several
populatio
ns in the
same
area that
interact
with each
other
Populations


How can we figure out how
many individuals are in each
population?
A census is a count of a
population
True census – actual count
 Sample census – estimate of
population
 A sample census is made by
marking off small areas or plots
and counting in the areas

Name of
Population
Actual
Count
Sample
Count
Ducks
Cattails
Water Lilies

Each member of a
population is
counted one by one
Name of
Population
Ducks
Cattails
Water Lilies
Actual
Count
Sample
Census
6
20
4
Why?
•Mark off a small
area or plot.
•Count the
population in one
plot.
•Multiply by the
total number of
plots to get an
estimate for the
whole area.
Name of
Population
Ducks
Cattails
Water Lilies
Actual
Count
6
20
4
Sample
Count
1 x10 = 10
3 x 10 = 30
1 x 10 = 10
•Why was the last census not very
accurate?
•What could we do to make it more
accurate?
•Where have you already done a
sample census?
Examples:
 There are 270 people per square mile in
Pennsylvania
 There are 25 deer per square mile in PA
Why is a sample a good way to
count a population?
 What are some disadvantages
to using a sample?
 What could you do to sample a
population of animals that do
not stay in one place?

A way to count
organisms that
move or are
hard to see.
 Catch, mark, let
go, catch again.
 Use a formula
to estimate the
number.



Is Mark and Recapture a true
census or a sample census?
For the following animals, what
method would be most
appropriate for getting a census?