Population Graphs
See p. 40 – 42 of your
textbook.
What is a population?
A population is a group of members of the
same species that live in the same area.
The number of individuals in a population often
changes. Factors such as natality,
mortality, immigration and emigration
change the number of individuals.
Population Changes
To calculate a change in a population:
New population # =
(original population + natality + immigration)
(factors that increase populations)
– (mortality + emigration)
(factors that decrease populations)
Population Changes
Natality is the number of live births in a
population over a period of time.
Mortality is the number of deaths that occur in
a population over a period of time.
Population Changes
Immigration is the number of individuals that
move into a population over a period of time.
Emigration is the number of individuals that
move out of a population over a period of
time.
Population Changes
In an existing population of Canada Geese
there are 21 individuals. Over a period of
time, 12 goslings are born, 10 geese die, 4
geese join the flock from a different
population and 3 are lost from the population.
How many geese will be in this flock now?
New Population =
(original pop + natality + immigration) –
(mortality + emigration)
= (21 + 12 + 4) – (10 + 3)
= 24
Factors Affecting Population Size
Populations would grow and grow and grow if
there were unlimited amounts of food, water
and space. Therefore, population size is
determined by the forces of:
a) biotic potential
b) limiting factors
What things allow for a population size
to grow?
An organisms ability to make many more of itself
(through reproduction) is called biotic potential.
Biotic potential is the maximum number of offspring
that a species can produce with unlimited resources.
Factors that Determine Biotic Potential
a) Birth potential = the maximum number of
offspring per birth (e.g. a whooping crane will
lay 2 eggs per year, but only one survives).
b) Capacity for survival = the number of
offspring that reach reproductive age
(e.g. sea turtles lay many eggs (150-200), but
very few reach maturity (about 0.01%)).
Factors that Determine Biotic Potential
c)
d)
Procreation = The number of times the
organism reproduces per year (e.g. elk
reproduce once per year).
Length of reproductive life = The amount
of time the individual can reproduce (e.g.
elephants can reproduce from about age 15
to 90 (75 years)).
Limiting Factors
There are factors in an ecosystem that prevent
populations from becoming too large. These
are known as limiting factors.
Limiting factor is a factor that prevent
increases in the number of organisms or
prevents them from moving into a new
population.
Limiting Factors
There are 2 types of limiting factors:
a) Abiotic – factors such as availability of light,
temperature, and chemicals.
Increase
Decrease
Much light
Warm temperatures
fertilizers
Low light
Cold temperatures
pesticides
Limiting Factors
b) Biotic factors – factors such as the
availability of food, predators, diseases
parasites, competition for resources
Increase
Decrease
Lots of food
Few predators
Few diseases
Few parasites
Little competition
Little food
Many predators
Many diseases
Many parasites
Much competition
Population Graphs
Population graphs show the growth of a
population in an area, and can often be used
to determine the carrying capacity of the
population for that environment.
Carrying capacity is the maximum number of
individuals of a species that can be
supported by an ecosystem.
Population Graphs
P
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p
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a
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n
n
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m
b
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A
B
C
Time
Section A
Section A represents an area of slow growth.
Growth of a population is often slow because
the initial number of organisms in that
population is low. There may also be other
factors in the environment that are limiting
growth of the population, such as:
Lack of resources, predators, competition,
disease
Section B
Section B represent an area of rapid growth. In
this type of growth, there are few limiting
factors restricting the reproduction of species.
The rate of growth can be exponential.
Section C
Section C represents an area where carrying
capacity has been met. The abiotic and biotic
factors in an environment will regulate the
number of organisms of a species that can
survive in an environment. The population will
fluctuate (temporarily rise and fall repeatedly)
slightly around that number, but at this point the
population is said to be in equilibrium – the
number of individuals remains relatively the
same.
In a sustainable ecosystem, none of the
populations in a community can exceed their
carrying capacities for very long. Why?
A population that exceeds its carrying capacity
will soon run out of resources and will strain the
environment in which they live.
When this happens, competition will cause the
population to decrease.