Determining Population Size

There are four basic Methods that ecologists
use to determine population size?
 Direct observation
 Indirect observation
 Sampling
 Mark and recapture studies
Determining Population Size

Determining the size of ant population by
knowing the number of ants that inhabit one ant
hill, and then observing how many ant hills are in
one particular area.
Indirect observation
Determining Population Size

Ecologists estimate the size of the rainbow trout
population in a lake by catching a small sample,
tagging them and then releasing them. They come
back three weeks later and catch another sample
noting how many trout have tags, and how many do
not.
Mark and recapture studies
Determining Population Size

Determining the number of frogs in a pond
by counting all of them individually.
Direct Observation
Determining Population Size

Estimating the number of daisies in a 100
square meter meadow by counting the
daisies in a 10 meter by 10 meter plot, and
then multiplying by 100.
Sampling
Putting it all together

Direct observation of a population gives you the most
accurate count of a population while indirect observation
is more of an estimate. Why would an ecologist use
indirect observation instead of direct observation.
• Because it is not always
practical to count all the
individuals in a population.
Changes in Population Size

List 2 ways a population can change in size:
When new members join the population
1. _______________________________
When members leave the population
1. _________________________________
Changes in Population Size

Birth Rate
–

Death Rate
–

Number of deaths in a population over time.
Immigration
–

Number of births in a population over time.
New members move into a population.
Emigration
–
Members leave the population.
Changes in Population Size

Which Two allow a population to increase?
– Birth Rate
– Immigration

Which two allow a population to decrease?
– Death Rate
– Emigration
Changes in Population Size

What does the population statement say?

If birth rate > death rate, population
size increases.

If death rate > birth rate, population
size decreases.
Population Density

The number of individuals in a specific area.
Population Density =
Number of Individuals
Unit Area
Population Density

An ecologist sets out to find out how many red maple
trees are in a forest that is 1000 square meters (m2).
He marks off a 10 square meter plot and counts 5 red
maples in his plot.
– Calculate the population density per square
meter (m2) in the 10 square meter plot.
5 maples
10 square meters (m2)
= 0.5 maples per square meter (m2)
Population Density

What would be a good estimate of the number of red
maples in the entire 1000 square meter forest?
5 maples per 10 square meter x 100 = 500 maples in the forest

What method of study did this scientist use to estimate
the population size (hint look at the front of this
packet)?
– Sampling
Limiting Factors

An environmental factor that causes a
population to decrease.
– Food and Water
– Space
– Weather
Limiting Factors

Pick one of the limiting factors and describe
how it limits population growth.

Food and Water
Space
Weather


Carrying Capacity

The largest population that an area can
support.
– Determined by the limiting factors that
are present.
Putting it all together:

Study the graph on page E17 in your reading
packet and answer the following questions
– Over how many years was the population
10 years
studied? ________
– In what year did the rabbit population reach it's
Year 4
highest point? _________
– What was happening to the population from
year 0 to year 4 of the study? Why?
•
•
The population was increasing because
more rabbits joined the population than left
it.
Resources were not limited.
In hundreds
Analyzing population growth curve of
rabbits in a meadow.
Human Population – Have we reached carrying
capacity?