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Populations
Definition of a Population
Population Biology
• Studies the interaction between
populations and their environment
• The three most important characteristics of
a population are…
– Its geographic distribution
– Its population density
– Its growth rate
Population Density
• Population density refers to the number of individuals of
the same species within a defined area
Population Dispersion
Population Growth
• Measures the size of a population, which
can be affected by three factors
– The number of births
– The number of deaths
– The number of individuals that enter or leave
the population
• Remember, population doesn’t count the number
of individuals within an entire species, just the
number in a given area!
Population Growth
• Populations simply grow when the birth
rate exceeds the death rate.
• If the death rate is greater than the
birthrate, the population shrinks.
• External factors such as immigration and
emigration can also affect population
– Immigration is the movement of individuals into an
area, a factor that can cause populations to grow
– Emigration is the movement of individuals out of an
area, a factor that can cause populations to decline
Population Growth
• If a population has abundant space and
food and is protected from predators and
disease, then that population will increase.
• For example, bacteria reproduce by
splitting in half.
– Suppose there was one bacterial cell with an
abundance of food, resources, and unlimited
living space.
– If this bacteria splits every hour, how many
bacteria would I have after 1 day?
This type of growth is known as
exponential growth
Exponential growth occurs when
individual in a population
reproduce at a constant rate.
Hour
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
# of Bacteria
2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
131072
262144
524288
1048576
2097152
4194304
8388608
16777216
Exponential Growth
• Typically occurs during the first phase of
growth.
• High availability of resources to individuals
of a population
• Little competition and predation
Logistic Growth
• Of course, animals do not cover the entire
plant.
• Therefore, exponential growth is not
sustained, and eventually during into
logistic growth
Logistic Growth
• Logistic growth occurs
when resources
become less available,
slowing or stopping the
growth of a population
near its carrying
capacity
• Carrying capacity is
defined as the number
of individuals a given
environment can
support
Factors Affecting Growth
• Remember limiting factors…
– These are factors that causes population
growth to decrease
– These are subdivided into density-dependent
and density-independent limiting factors
Factors Affecting Growth
• Density-dependent factors include
competition, predation, parasitism, and
disease.
• These factors occur when populations
reach a specific level
• When populations become crowded,
organisms compete for resources.
– The more individuals there are, the quicker
resources are used up.
Predator and Prey
Relationships
• Populations in nature are often controlled
by predation.
• Predators kill their prey for food.
• Conversely, the number of prey control the
number of predators in a habitat
Predator and Prey
Relationships
Moose
Increase
“Balanced”
Wolves
Increase
Wolves
Decline
Moose
Decline
Wolves
Die Off
Moose
Increase
Rapidly
Predator and Prey
Relationships
• Parasites and diseases
can also limit the grwoth
of a population.
• Like predators, diseases
and parasites hurt their
host, which can weaken
them or outright kill them.
Factors Affecting Population
Growth
• Density independent factors
– These are limiting factors that are not dependent on
a population’s size
– Example, the climate change, human activities
(dams, deforestation), and a lack of resources