Characteristics of
Populations
Characteristics of Populations
• A population is a number of individuals
belonging to the same species that reside
in a given area/ecosystem.
• You know better know what a species is –
you studied this as part of your evolution
unit – review this concept if needed.
• The place where an organism normally
lives is its habitat. The habitat is selected
because it provides the organism with all
of its required resources.
Population Size & Density
• Population density is the number of individuals
of the same species that occur per unit of area
or volume.
• The formula for population density is…
D =N
S
• In this…
• D is the population density.
• N is the number of organisms counted.
• S is the space occupied by the population.
Don’t Be Crude!
• Population density can be expressed as a crude
density or an ecological density.
• Crude density is the number of organisms of
the same species within the entire habitat.
• Ecological density is the number of individuals
of the same species per unit area or volume that
is used by the species.
• Ecological density is a more accurate descriptor
of the population density of a species.
Population Dispersion Patterns
• The population dispersion for a species is the
pattern in which individuals in a population are
distributed through an area.
• Clumped dispersion – The population is highly
concentrated in certain parts of the habitat.
• Uniform dispersion – The population is spaced
evenly throughout the habitat.
• Random dispersion – The population is spread
out in an unpredictable and patternless manner.
Measuring Population
Characteristics
• You can’t just go out and count every single member of
some populations – it could be next to impossible.
• So how do you get an accurate estimate of the
population you are studying?
• Quadrat Sampling – Divide larger area up into smaller
sections (quadrats)…count within a few of these smaller
sections…average it out…multiply the average of the
counted sections by the number of total sections in your
area.
• Mark-recapture Method – capture as many organisms
in the area as you can…mark them and leave…go back
and capture more of the organisms…compare the
number caught this time with the number that were
marked.
Mark-recapture Sampling
• There is a ratio for the mark-recapture sampling
procedure that allows you to estimate the
population.
M═m
N n
• In this…
•
•
•
•
M = Total # marked
N = Total population
m = # of recaptures
n = Size of second sample
• Try the practice problems on pages 656 & 657!
Measuring & Modeling
Population Change
Take it to the Limit
• How big can a population get?
• The answer involves the idea of a carrying
capacity for the given area where the population
resides – this is the maximum number of organisms
that can be sustained by an ecosystem over time.
• The word “sustained” is used because the
population must be able to live there year after year
for the ecosystem to be considered
healthy/successful.
• Organisms will multiply as long as there are
resources available to support them. When one or
more of these resources begins to run out – the
population will stabilize or even decline.
Factors That Affect Population Growth
•
There are a variety of factors that affect the
growth of a population:
Natality – Birth rate.
Mortality – Death rate.
Immigration – Move into area.
Emigration – Move away from area.
Fecundity – The potential number of offspring that
could be produced in a lifetime.
6. Survivorship - The ability to make it to sexual
maturity (AKA – adulthood).
1.
2.
3.
4.
5.
•
Look at the survivorship curves on page 661.
There are three types (I, II, III) which are all
based on some of the factors above.
Calculating Population Changes
• The formula for population change is:
Pop. Change = [(b + i) – (d + e)] x 100%
n
• In this…
–
–
–
–
–
“b” is the number of births.
“i” is the number of immigrants.
“d” is the number so deaths.
“e” is the number of emigrants.
“n” is the initial population size
• This formula will yield a percent change in the
population for the given time in which the data
was collected.
Open & Closed Populations
• An open population is one in which birth,
death, immigration and emigration are all adding
to or subtracting from the population numbers.
– An open population would resemble the population of
people in Canada.
• A closed population is one in which only birth
and death are contributing factors to the
population growth.
– A closed population may include the populations of
wild game within a closed reserve in Africa.
• Biotic potential – the maximum number a
population can attain if resources were
unlimited. (Ideal conditions)
Population Growth Models
•
There are three different models which display
population growth patterns.
1. Geometric Growth – Population growth pattern
where organisms reproduce at fixed intervals at a
constant rate (seasonal – like deer).
2. Exponential Growth – Population growth pattern
where organisms reproduce continuously at a
constant rate (all the time – like humans).
3. Logistic Growth – Population growth pattern where
population grows and then eventually levels off as
the carrying capacity is reached. (All populations will
follow this pattern eventually.)
FIN!