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CHAPTER
4
Population Ecology
Lesson 4.1 Studying Ecology
Ernst Haeckel defined ecology in 1866 as “the
body of knowledge concerning the economy of
nature—the total relations of the animal to both its
inorganic and organic environment.”
Objectives
• Describe the different levels of organization studied by
ecologists
• Explain the difference between biotic and abiotic
factors
• Discuss how an organism’s habitat relates to its
survival
Lesson 4.1 Studying Ecology
Levels of Ecological Organization
• Everything on Earth is connected, so how do ecologists
study anything?
• Ecology is the study of how organisms interact with each
other and with their environments
• Scientists study ecology at various levels of organization.
Levels of Ecological Organization
• Individual
• Ecology involves describing relationships between individual
organisms and their environment.
• Populations
• Individual organisms are classified into species
• Species is a group of individuals that interbreed and produce
fertile offspring
• Species have similar genetics
• Members of a species that live in the same area at the same
time make up a population.
• Ecology involves how individuals within a population interact
with one another.
Levels of Ecological Organization
• Communities
• All of the populations in a particular area.
• Ecology involves interactions among species.
• Ecosystems
• Include all the livings things and their physical environments
within a particular area
• Living and nonliving things.
• Ecology involves studying the living and nonliving
components of a system together.
Levels of Ecological Organization
• Biosphere
• Includes all parts of Earth that host life, with all of its
organisms and environments.
• Ecology involves how matter and energy cycle through the
biosphere and influence organisms worldwide.
Checkpoint
• What is the difference between a species and a
population? Between a population and a community?
Lesson 4.1 Studying Ecology
Biotic and Abiotic Factors
• Biotic factors: Parts of an
ecosystem that are living or
used to be living
• Ex: living tree, dead tree
• Abiotic factors: Parts of an
ecosystem that have never
been living
• Ex: oxygen, sunlight
Did You Know? Decaying organisms
are biotic factors as long as their
structure remains cellular.
Lesson 4.1 Studying Ecology
Habitat
• The specific environment in which an organism lives
• Habitats consist of biotic and abiotic elements.
• Habitats provide an organism with resources—anything an
organism needs to survive and reproduce, including food,
shelter, and mates.
• The difference between habitat and ecosystem is that a
habitat’s boundaries are defined by the population that lives
there.
• Habitats may be a subset of an ecosystem OR habitats may be a
combination of ecosystems
Objectives Revisited
• Describe the different levels of organization studied by
ecologists
• Explain the difference between biotic and abiotic
factors
• Discuss how an organism’s habitat relates to its
survival
Lesson 4.2 Describing Populations
From 1900 to 2000, the white-tailed deer
population of New York state grew from
about 20,000 to more than 1 million.
Densities of more than 100 deer per sq
mi occur in some metropolitan areas.
Objectives
• Explain the usefulness of tracking population size.
• Define population density.
• Describe the three ways populations can be
distributed.
• Explain what age structure diagrams tell you about a
population.
Lesson 4.2 Describing Populations
Population Size
•
The number of individuals in a population at a given time
•
Sudden and dramatic decreases in population size can
indicate an unhealthy population headed toward
extinction.
•
When population size increases or remains steady, it is
often a sign of a healthy population.
Did You Know? The passenger pigeon
was once North America’s most abundant
bird. Hunting drove them to extinction in
less than 100 years.
Determining Population Size
•It is nearly impossible to count each individual in a
population
•Population is estimated
using sampling
techniques.
•Count individuals in
small sample area
•Use that to estimate
number of individuals
in large overall area
Counting Laysan Albatross Nests
Checkpoint
• When is sampling necessary?
Lesson 4.2 Describing Populations
Population Density
• Measure of how crowded a population is
• Larger organisms
generally have lower
population densities.
• Low population density:
More space, resources;
finding mates can be difficult
• High population density:
Finding mates is easier; tends to be more
competition; more infectious disease; more
vulnerability to predators
Northern pintail ducks
Lesson 4.2 Describing Populations
Population Distribution
• How organisms are arranged within an area:
• Random distribution:
Organisms arranged in
no particular pattern, resource are spread out
• Uniform distribution:
Organisms evenly spaced; individuals hold
territories/compete for space
• Clumped distribution:
Organisms grouped near resources; most
common distribution in nature
Population Distribution
Checkpoint
• What is the difference between population density and
distribution?
Lesson 4.2 Describing Populations
Age Structure
• Age structure describe the relative number of organisms of
each age group within population
• Can be used to predict future population growth of a
population
• Individuals capable of having offspring make up the
reproductive group
• Young individuals who have not yet reached the age where
they can reproduce are called pre-reproductive
• Older individuals past the age of having offspring are called
post-reproductive
Age Structure
• Population with even age distribution will likely remain
stable (births = deaths).
• Population made up of mostly post-reproductive
individuals will likely decline over time.
• Population made up of mostly pre-reproductive
individuals will likely increase over time.
Age 
Age Structure Diagram
How will each population change over time?
Lesson 4.2 Describing Populations
Sex Ratios
• Proportion of males to females
• Age structure diagrams give information about sex ratios.
• For a monogamous species, the ideal sex ratio is 50:50.
Objectives Revisited
• Explain the usefulness of tracking population size.
• Define population density.
• Describe the three ways populations can be
distributed.
• Explain what age structure diagrams tell you about a
population.
Lesson 4.3 Population Growth
From 1800 to today, the human
population has grown from about
1 billion to more than 6.8 billion—an
exponential rate of increase.
Objectives
• Describe the factors that influence a population’s
growth rate.
• Explain exponential growth and logistic growth.
• Explain how limiting factors and biotic potential affect
population growth.
Lesson 4.3 Population Growth
Birth and Death Rates
• A population’s relative birth and death rates (natality and
mortality) affect how it grows.
• When birthrate is greater than death rate, population size
increases.
• When death rate is greater than birth rate, population size
decreases.
• Survivorship curves show how the likelihood of death varies
with age.
Birth and Death Rates
• Survivorship curves:
•
•
•
•
Graphs that show how the likelihood of death varies with age
Type I curve has higher mortality at older ages
Type II curve has equal mortality rates at all ages
Type III curve has higher mortality rate at younger ages
Checkpoint
• Which type of survivorship curve describes
populations whose mortality is highest at young ages?
Birth and Death Rates
• Age Structure and Population Growth:
• Consider a population following a type I survivorship curve
(higher mortality at older ages)
• If population is made up of more young people, then there is
likely to be more births than deaths
• If population is made up of more old people, then there is likely
to be more deaths than births.
Lesson 4.3 Population Growth
Immigration and Emigration
• In addition to births and deaths, population growth is
affected by immigration and emigration—individuals
moving into and out of a population.
• Migration, seasonal movement into and out of an area,
can temporarily affect
population size.
Lesson 4.3 Population Growth
Calculating Population Growth
• Determined by the following equation:
(birthrate + immigration rate) – (death rate + emigration rate)
• Growing populations have a positive growth rate; shrinking
populations have a negative growth rate.
• Usually expressed in terms of individuals per 1000
Did You Know?
Immigration contributes
more than 1 million people
to the U.S. population
per year.
Lesson 4.3 Population Growth
Exponential Growth
• Population increases by
a fixed percentage
every year.
• Normally occurs only
when small populations
are introduced to an
area with ideal
environmental conditions
• Rarely lasts long
• Ex: Mold growing on
bread
Lesson 4.3 Population Growth
Logistic Growth and Limiting
Factors
• Growth almost always slows and stops due to limiting
factors.
• Limiting factors:
Environmental characteristics slow population growth and
determine carrying capacity (largest population size a given
environment can support).
• Density-dependent:
Influence changes with population density; examples are competition
for resources, predation, disease
• Density-independent:
Influence does not change with population density; Examples are
floods, fires, landslides
Logistic Growth and Carrying
Capacity
Population Growth In Nature
Fluctuating:
•Cycle indefinitely above
and below carrying capacity
Rise and Crash
•Rise very quickly, overshoot the carrying capacity, and
then crash
Checkpoint
• Why is severe weather considered a densityindependent factor?
Lesson 4.3 Population Growth
Biotic Potential
• An organism’s maximum ability
to produce offspring in ideal conditions
• Many factors influence biotic potential,
including gestation
time (time to “be born”) and generation
time (span from time of organism’s birth
until time it has its own offspring).
• Organisms with high biotic potential can
recover more
quickly from population declines than
organisms with low biotic potential.
Objectives
• Describe the factors that influence a population’s
growth rate.
• Explain exponential growth and logistic growth.
• Explain how limiting factors and biotic potential affect
population growth.