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4.2 Populations
Mrs. Jackie
Outline how population size can be affected by
natality, immigration, mortality and emigration
• Factors that affect population size
– Natality ( birth rate)
– Mortality (death rate)
– Immigration (moving in)
– Emigration (moving out)
Outline how population size can be affected by
natality, immigration, mortality and emigration
immigration
+
+
natality
Population abundance
emigration
mortality
Sigmoid shaped population growth
curve
• Exponential growth phase
– All organism have the ability to increase their numbers
rapidly if there are no factors which control them
– A bacterium can undergo division every 20 min. In 2
hours the following happens
Time
0 0:20
0:40
1:00
1:20
1:40
2:00
Number of
bacteria
1 2
4
8
16
32
64
Exponential growth phase or
geometric curve
70
60
50
40
30
20
10
0
0
0:20
0:40
1:00
1:20
1:40
2:00
Example of
the innate
capacity for
increase of a
species
where the
increase in
population
size is as
rapid as its
members can
reproduce
Carrying capacity
• Results will be:
– Competition for food,
shelter, nesting space
etc.
– This leads to
differential mortality
where the individuals
which are best fitted to
the environment
survive and reproduce
P
T
E
The transitional and plateau phases
• The exponential growth only occurs occasionally, for
example when the environment suddenly changes
and only a few organism survive.
• The increase in number will not continue to be
geometric and at certain point the increase will slow
down until it is zero
• The number of individuals in the population has then
reached the maximum which can be supported by
the environment. CARRYING CAPACITY
Factors that set limits to population
increase
• Divide into
– density dependent factors- affect a larger
proportion of the population as the density
increases.
• Examples- mortality due to predation and disease
– Density independent factors- affect a proportion
of the population regardless of its density
• Examples- mortality due to weather conditions or
earthquakes.
• Tend to have a greater effect on a population when the
density of the population is higher.
Another classification system
• Extrinsic population-regulating mechanism– Originate outside the population and include biotic as
well as physical factors.
– Food supplies, natural enemies, disease and weather
• Intrinsic population-regulating mechanism– Originate in an organism´s anatomy, physiology or
behavior.
– Some species like kangaroos and koalas can reabsorb
the developing embryo when conditions are crowded
and resources limited.
– Lemmings will migrate when resources are depleted
Random sample
• If we want to study some individuals to draw
some conclusions about the entire population
we want to take a random sample.
• Random sample: a method to ensure that
every individual in a population has an equal
chance of being selected.
Technique used to estimate the population size
Capture-mark-release-recapture method
• You will capture a number of organisms (random
sample)
• Mark the organism (without harm them or changing
their behavior).
• Release them back into their original population.
• The assumption is that they will mix with the
unmarked individuals in a random way.
• After a suitable time you will again capture a random
sample.
• A certain proportion of your second sample will be
marked
Technique used to estimate the population size
Capture-mark-release-recapture method
• This is the same proportion as the original first
marked sample.
• This technique assumes that natality,
mortality, immigration and emigration are
zero.
Formula
• IN every case
– N1= number in the first sample captured and
marked
– N2= total number in the second sample.
– N3= number of marked individuals in the second
sample
1. Memorize this equation
population size= n1Xn2/n3
Quadrat methods
• Is a wire, shaped into a square
of a known size for example
0.25m2
• Used when you want to know
the population size of one or
more plant species in an
uniform area.
• Throw the quadrat and
investigate the area on the
quadrat.
Calculate the mean of a set of
values
• Mean- average value obtained by dividing the
total of a set of values by the number of
values.
Standard deviation
• State that the term standard deviation is used
to summarized the spread of values around
the mean and that 68% of the values fall +-1
standard deviation of the mean
• Where __
X=mean
x= class value
f- frequency
n=number of values
Standard deviation
• First
– Subtract the mean from each
value
– Then squared these numbers
– Add them and divide them by
the total number of values-1
– The number acquired is the
variance
– The Standard deviation is the
squared root of the variance
Example
• The number of eggs in the nests of a sample
of a species of bird is shown below. Find the
mean and sample standard deviation of these
number of eggs
5 3 5
3
4
2
0
2
1
2
Add the data=27
Find the mean= 27/10=2.7
Solution
Number of eggs
X-mean
(x-mean)2
5
2.3
5.29
3
0.3
0.09
5
2.3
5.29
3
0.3
0.09
4
1.3
1.69
2
-0.7
0.49
0
-2.7
7.29
2
-0.7
0.49
1
-1.7
2.89
2
-0.7
0.49
The sum of the third column gives =24.1
Divide between n-1=10-1=9
Squared root of 24.1/9)=1.636
Solution
• Small standard
deviation indicates that
the data is clustered
closer to the mean
• In the example the sd is
closer to 95% for the
mean plus or minus 2
standard deviation
Explain why the SD is useful…
• Useful for comparing the means and the
spread of ecological data between two or
more populations.
• When comparing to samples from different
populations, the closer the means and the sd,
the more likely the samples are drawn from
similar population
Classwork
• IB Study Guide page 36
• Calculate the standard deviation for this set of
values 27,33, 35,36,41,48,52.
Homework
• Find the mean and standard deviations of the
following data sets. Give your answers correct
to one decimal place.
54
59
54
55
51
55
56
53
57
55
61
54
57
61
52
58
58
52
60
56
57
55
56
60