CHAPTER
4
Population Ecology
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 - species
• Species = group of individuals that
interbreed and produce fertile offspring
• Members of a species that live in the same
area at the same time = a population.
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.
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
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.
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.
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.
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
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
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
• Young individuals who have not yet reached the
age where they can reproduce are called prereproductive
• 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 postreproductive individuals will likely decline
over time.
•Population made up of mostly prereproductive individuals will likely
increase over time.
Age 
Age Structure Diagram
How will each population change over time?
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.
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.