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Lesson Overview
How Populations Grow
Chapter 5
Populations
Lesson Overview
How Populations Grow
Describing Populations
How do ecologists study populations?
Lesson Overview
How Populations Grow
Describing Populations
Researchers study populations’
 geographic range,
 density and distribution,
 growth rate,
 age structure
Lesson Overview
How Populations Grow
Geographic Range
The area inhabited
by a population is
called its geographic
range.
A population’s range can vary
enormously in size, depending
on the species.
Lesson Overview
How Populations Grow
Density and Distribution
Population density refers to the number of
individuals per unit area.
Populations of different species often have very different densities, even
in the same environment.
A population of ducks in a pond may have a low density, while fish and
other animals in the same pond community may have higher densities.
Lesson Overview
How Populations Grow
Density and Distribution
Distribution refers to how individuals in a
population are spaced out across the range of
the population—randomly, uniformly, or mostly
concentrated in clumps.
Lesson Overview
How Populations Grow
Density and Distribution
Random
Uniform
Clumped
Lesson Overview
How Populations Grow
Growth Rate
A population’s growth rate determines
whether the population size increases,
decreases, or stays the same.
Lesson Overview
How Populations Grow
Age Structure
To fully understand a plant or animal population, researchers need to
know the population’s age structure—the number of males and females
of each age a population contains.
Lesson Overview
How Populations Grow
Organisms That Reproduce Rapidly
In exponential growth, the larger a population
gets, the faster it grows.
The size of each generation of offspring will be larger than the generation
before it.
If nothing were to stop this kind of growth, the population would become
larger and larger, faster and faster, until it approached an infinitely large
size.
Lesson Overview
How Populations Grow
Natural populations don’t grow exponentially for
long.
Sooner or later, something stops exponential
growth. What happens?
Lesson Overview
How Populations Grow
The Logistic Growth Curve
This curve has an S-shape that represents what is called logistic
growth.
Logistic growth occurs when a population’s
growth slows and then stops, following a period
of exponential growth.
Lesson Overview
How Populations Grow
Carrying Capacity
Carrying capacity is the maximum number of
individuals of a particular species that a
particular environment can support.
Lesson Overview
How Populations Grow
Limiting Factors
What factors determine carrying capacity?
Acting separately or together, limiting
factors determine
the carrying capacity of an environment for a
species.
Lesson Overview
How Populations Grow
Limiting Factors
A limiting factor is a factor
that controls the growth of a
population.
There are several kinds of limiting factors.
Some—such as competition, predation,
parasitism, and disease —depend
on
population density.
Others—including natural disasters and
do not depend
on population density.
unusual weather—
Lesson Overview
How Populations Grow
Historical Overview
How has human population size changed over time?
The human population, like populations of other organisms, tends to
increase. The rate of that increase has changed dramatically over time.
Lesson Overview
How Populations Grow
Historical Overview
For most of human existence, the population grew slowly because life was
harsh. Food was hard to find. Predators and diseases were common and
life-threatening.
Lesson Overview
How Populations Grow
Historical Overview
These limiting factors kept human death rates very high. Until fairly
recently, only half the children in the world survived to adulthood.
Because death rates were so high, families had many children, just to
make sure that some would survive.
Lesson Overview
How Populations Grow
Exponential Human Population Growth
As civilization advanced, life became easier, and the human population
began to grow more rapidly. That trend continued through the Industrial
Revolution in the 1800s.
Lesson Overview
How Populations Grow
Exponential Human Population Growth
Several factors, including improved nutrition, sanitation, medicine, and
healthcare, dramatically reduced death rates. Yet, birthrates in most
parts of the world remained high.
The combination of lower death rates and high birthrates led to
exponential growth.
Lesson Overview
How Populations Grow
The Predictions of Malthus
This kind of exponential growth could not continue forever.
Two centuries ago, English economist Thomas Malthus suggested that
only war, famine, and disease could limit human population growth.
Malthus thought that human populations would be
regulated by competition (war), limiting resources
(famine), parasitism (disease), and other densitydependent factors.
Malthus’s work was vitally important to the thinking of Charles Darwin.
Lesson Overview
How Populations Grow
Future Population Growth
To predict how the world’s human
population will grow, demographers
consider many factors, including the age
structure of each country and the effects
of diseases on death rates—especially
AIDS in Africa and parts of Asia.
Current projections suggest that by 2050
the world population will reach 9 billion
people.