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Chapter 17
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17-1
Population Characteristics


A population is a group of organisms of the
same species located in the same place at
the same time.
A species is the largest possible population.
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17-2
Usually the term population is used to describe
portions of a species.
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Gene Frequency

A measure of how often a specific gene shows up in
the gametes of a population
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17-3
Two populations of the same species can have very
different gene frequencies.
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Gene Flow

The movement of a
gene
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17-4
From one generation to
another
From one population to
another when animals
migrate
Both types of gene flow
usually occur together.
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Age Distribution


The number of organisms in each age in the
population
Organisms are grouped by their reproductive
status.
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17-5
Prereproductive juveniles
Reproductive adults
Postreproductive adults
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Age Distribution

The age categories of a population may not
contain the same number of individuals.
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17-6
The age structure of some species will change
dramatically over the course of a season.
Birds, flowering plants
Age structure impacts how fast the
population will grow.
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Age Distribution in the Human
Population
17-7
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Sex Ratio

The number of males compared to the number
of females
–
Birds and mammals
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Those that have mate-bonding frequently have a 1:1 ratio of
males to females.
Those without mate-bonding usually have more females
than males.
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17-8
Harems
Insects usually have far more females than males.
Sex ratios do not apply to hermaphroditic species.
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Population Distribution


Describes the way
individuals within a
population are arranged
with respect to one
another
Three basic
arrangements
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17-9
Even, random, clumped
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Population Density


The number of organisms of a species per unit area
Population density varies with
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–

Success of the individuals
Resources available
Populations that are highly concentrated will begin to
experience competition.
–
Population pressure describes how the intensity of competition
causes dispersal.
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17-10
Dispersal relieves the pressure and can lead to new populations.
If dispersal doesn’t happen, then individuals will begin to die.
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Reproductive Capacity

The theoretical number of offspring that could
be produced by a population
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17-11
Also called biotic potential
Usually larger than the number of offspring
needed to maintain the population
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Reproductive Capacity

Two strategies for maintaining the species
–
Have a lot of offspring, but not invest a lot of
energy in them after birth

–
Have fewer offspring, but invest a lot of energy in
them after birth
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17-12
The probability of survival to reproductive age is slim.
The probability of survival to reproductive age is quite
good.
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The Population Growth Curve

The growth of a population is dependent on
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The rate at which organisms enter the population
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The rate at which organisms leave the population
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Immigration and birth
Natality = the # of new individuals added by birth/1000
individuals
Emigration and death
Mortality = the # of individuals leaving by death/1000
individuals
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The Population Growth Curve

Population growth curves graph the change
in population size over time.
–
Four phases can be seen in the curve.
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17-14
Lag phase
Exponential growth phase
Deceleration phase
Stable equilibrium phase
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A Typical Population Growth
Curve
17-15
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Lag Phase

A period of time immediately following the
establishment of the population
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–
Population is small
Growth is happening slowly
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17-16
Both natality and mortality are low
Occurs slowly because reproduction takes time
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Exponential Growth Phase

The period of time when the population is
growing rapidly.
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–
As the number of individuals increases, then there are
more individuals available to reproduce.
During this phase, populations increase by the same
percent each year.
Natality is greater than mortality.
Mice population
 Two mice mate and have 4
 Those four mate and have 8
 Those eight mate and produce 16

–
17-17
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Deceleration Phase
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17-18
Occurs when population growth rate begins
to slow
Population continues to grow until birthrate
and deathrate become equal
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Stable Equilibrium Phase

The period of time when the population stops growing
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Exponential growth cannot continue indefinitely.
The availability of resources will limit population growth.
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17-19
Maintains a stable size
When a population becomes large enough, resources will
be in short supply.
Mortality will increase to the point where it equals
natality.
Large organisms that live a long time tend to reach
stable equilibrium.
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Alternate Population Growth
Strategies
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K-strategists are large organisms that a live long
time and reach a population size that can be
sustained (as just described).
r-strategists are small organisms with short lifespans
and tend to have fluctuating population that do not
reach a stable equilibrium phase
–
17-20
Characterized by a “crash” after rapid exponential growth
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Population Growth Curve for
Short-lived Organisms
17-21
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Limits to Population Size

Factors that determine the size at which a
population will reach stable equilibrium
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17-22
Called limiting factors
Collectively known as environmental resistance
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Extrinsic and Intrinsic Limiting
Factors

Extrinsic factors come from outside the population
and limit population growth.
–
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Predators, availability of nutrients, natural disasters
Intrinsic factors come from within the population and
limit population growth.
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Crowded conditions leading to
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17-23
Decreased reproduction
Decreased # of offspring produced per reproductive event
Decrease in maternal care
Increase in death rates
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Density Dependent Limiting
Factors

These factors become more effective at limiting
population size as the population gets bigger.
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Predators are more likely to capture individuals in a large
population.
Disease epidemics are more common in large populations.
Competition for resources increases as population size
increases.
Intrinsic factors such as decreased reproductive rates occur
as population size increases.
Populations of large, long-lived organisms are
usually controlled by these factors.
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Density Independent Limiting
Factors
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These factors control population growth no
matter how large the population is.
Usually accidental extrinsic factors
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Weather patterns
Natural disasters
Drought/excessive rainfall
Populations of small, short-lived organisms
are often controlled by these factors.
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Categories of Limiting Factors

Factors that limit population growth can be
categorized as
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Availability of raw materials
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For plants
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For animals
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Water, minerals, materials/sites for housing/nesting, food
(as a source of carbon and nitrogen)
Availability of energy
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17-26
Nitrogen, water, magnesium
Availability of light to plants
Availability of food to animals
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Categories of Limiting Factors
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Production and disposal of waste products
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Usually not relevant for plant species
Animal species generate more waste.
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Waste can be toxic.
As waste builds up, reproduction slows, death rates
increase.
Accounts for the death phase in bacteria
– Follows stable equilibrium phase
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Bacterial Population Growth
Curve
17-28
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Categories of Limiting Factors

Interactions with other organisms
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Predation, parasitism and competition limit
population growth.
These interactions usually
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17-29
Involve more than one population
Limit the size of both populations involved
Generate population cycles
Lemming example
Mutualistic interactions allow populations to grow
at a faster rate than they would have alone.
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Population Cycles
17-30
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Carrying Capacity
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17-31
Populations at stable equilibrium phase have
reached their maximum size for that environment.
– This maximum, sustainable population size is
called carrying capacity.
– Determined by a set of limiting factors
Environmental changes, succession, disease, and
natural disasters can change carrying capacity for a
given population.
The size of the organism and the size of its habitat
can determine carrying capacity.
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Carrying Capacity
17-32
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The Effect of Biomass
on Carrying Capacity
17-33
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Limiting Factors to Human
Population Growth

Human populations follow the same pattern of
growth.
–
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The human population growth curve
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17-34
Shows the lag and exponential phases
Although the human population has approached
carrying capacity for some time,
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They are limited by the same kinds of factors.
Technology continues to increase carrying capacity.
Nevertheless, limiting factors will cause human
population growth to level off.
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Human Population Growth Curve
17-35
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Available Raw Materials
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Include
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–
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Drinking water
Irrigation water
Iron ore
Silicon
Food
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Many portions of the global human population do not have
enough food.
However, the world can produce enough food to feed everyone.
Political and economic barriers prevent equal distribution of food.
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Availability of Energy
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Ultimately, humans depend on sunlight for energy.
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However, limited reserves of petroleum, natural gas
and coal exist.
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Solar energy can be stored in fossil fuels.
It is hard to predict how long these will last.
Rate of use of the reserves increases as population size
increases.
It took millions of years to build up these reserves, but we
are using them at a rate that will deplete them in a matter of
hundreds of years.
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Production of Wastes
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Human biological wastes can be
decomposed by detrivores.
Technological wastes cannot be
decomposed by organisms.
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17-38
Excess technological wastes cause pollution.
Most of this waste is toxic and is damaging the
environment.
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Interactions with Other
Organisms
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Humans interact with other species in many
ways.
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We are hosts for a variety of parasites.
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We are predators of a number of species.
We have mutualistic relationships with
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17-39
Bacteria and viruses
Domesticated and wild animals
A number of plant species
A variety of bacterial species
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Interactions with Other
Organisms
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We compete with other species for resources.
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Because we win, we frequently displace other
species.
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We compete with one another for resources.
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17-40
This has caused a number of extinctions.
Evident in an increase in infant mortality
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Control of Human Population
is a Social Problem

Humans are able to predict the outcome of a
specific course of action.
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However, we do not always make the best
unbiased decisions.
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17-41
We have the information to control the human
population and improve health and quality of life.
We tend not to consider what is best for the
population.
We tend to consider what is best for us,
individually.
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Control of Human Population
is a Social Problem

Continued population growth will present more
social, political and economic problems than
biological problems.
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Will likely result in increased famine and starvation
One major factor seems to be the educational status of
women.
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Women with higher levels of education have fewer children.
Improving the education of women may be one social strategy
for limiting population growth.
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