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Chapter 4.1
Population Ecology
Population Characteristics
• Population Density: The number of living
entities per unit of area
Population Characteristics
• Spatial Dispersion: the pattern of spacing of
a population within an area
– Uniform
– Clumped (groups or herds)
– Random
• Dispersion patterns depend on the
availability of resources
Limiting Factors
• Limiting Factors: any abiotic or biotic factor
that restricts the number, reproduction, or
distribution of organisms
Limiting Factors
• Two types of limiting factors:
– Density independent
– Density dependent
Chapter 4
Population Ecology
4.1 Population Dynamics
 Density-independent factor: Any
factor (that limits population size) in an
environment that does not depend on
the number of members in a population
per unit area.
• Examples?
Density-Independent Factors
 Weather events
 Fire
 Human alterations of the landscape
 Air, land, and water pollution
• Can wipe out populations of species
regardless if there are a few or a lot in a
given area
Chapter 4
Population Ecology
4.1 Population Dynamics
 Density-dependent factor: Any factor, that
limits population size, in an environment that
depends on the number of members in a
population per unit area
 The greater the number of species, the more
that are wiped out
• Examples?
Density-Dependent Factors
 Usually biotic factors and may include:
 Disease
 Parasites
 Competition for resources
 Predation
Density-Dependent Factors
• Example:
• If one deer in a heavily populated community
contracts tuberculosis, a communicable
disease, what will happen to the deer
population?
• Parasite?
Examples of Density-Dependent
Factors:
• Competition: for food, space, mates
• Causes a lower birth rate (natality),
increases death rate (mortality), or both
Density-Dependent Factors
• Predation: interaction between species on
different trophic levels
• Almost every species is food for another species
• Predators (hunters) and prey (the hunted) have, in
most cases, existed together for years
Predator-Prey Graph
• What do you notice about the populations?
Chapter 4
Population Ecology
4.1 Population Dynamics
Population Growth Rate
 Population growth rate (PGR) explains
how fast a given population grows.
 What are some factors that would affect
how quickly a populations grow?
Population Growth Rate
• Depends on several factors:
• Natality: birthrate (during a given time)
• Mortality: death rate (during the same given
time)
• These two factors affect PGR the most
• Emigration: the number of individuals
moving away from a population
• Immigration: the number of individuals
moving into a population
• These two factors are usually about the
same
• Two mathematical models that represent
population growth rate:
• 1. Exponential Growth Model
• 2. Logistic Growth Model
Chapter 4
Population Ecology
4.1 Population Dynamics
Exponential Growth Model
 Exponential growth :
the growth rate is
proportional to the
size of the population.
 All populations grow
exponentially until
some limiting factor slows the population’s
growth.
• If all offspring survive and reproduce, the
population grows slowly at first
• Lag phase: slow growth period
• The rate of population growth begins to
increase rapidly
• Exponential growth phase: rapid
population growth due to total number of
organisms without any limiting factors
• Exponential growth creates a J-shaped curve
• Second Model:
Chapter 4
Population Ecology
4.1 Population Dynamics
Logistic Growth Model
 The population’s
growth slows or
stops following
exponential growth,
at the population’s
carrying capacity.
• Carrying capacity: the maximum number of
organisms in a population that an
environment can support for the long term
 Carrying capacity is limited by the energy,
water, oxygen, and nutrients available
(resources).
Logistic Growth Model
• Similar to the exponential growth model, but
with an S-shaped curve
• Occurs when the population’s growth slows
or stops after exponential growth at the
population’s carrying capacity
Chapter 4
Population Ecology
4.1 Population Dynamics
 Exceeding the carrying capacity of an
area results in death to the organisms
living there
Boom and Bust Graph
• A = “old” carrying capacity
• B = exponential growth
• C = massive death
• D = “new” carrying capacity
Chapter 4
Population Ecology
Chapter 4
Population Ecology
4.1 Population Dynamics
Reproductive Patterns
 Species vary in the number of births per
reproduction cycle, in the age that
reproduction begins, and in the life span of
the organism.
 Two main patterns:
R-strategy
K-strategy
Chapter 4
Population Ecology
4.1 Population Dynamics
 R-strategy: (Rate Strategy) is a population
pattern seen where fluctuations in biotic or
abiotic factors occur in the environment.
 An r-strategist is generally a small organism.
 Short life span
 Produces many offspring
• Examples of r-strategists:
• Fruit fly
• Mice
• Locusts
Chapter 4
Population Ecology
4.1 Population Dynamics
 k-strategy : (carrying-capacity strategy) a
population reproduction pattern that is
adapted for living in stable environments.
 A k-strategist is generally a larger organism.
 Long life span
 Produces few offspring
• Examples:
• Bears
• Elephants
Factors that Contribute to Biotic
Potential
High (lots of babies)
r-strategists
Maternal age
Low (few babies)
k-strategists
older
Number of offspring
1-2 (low)
1,000s (high)
Maternal care
months/years
short/none
Gestation period
long
short
Organism’s size
large
small
younger
Human Population 4.2
Chapter 4
Population Ecology
4.2 Human Population
Human Population Growth
 Demography: The study of human population size,
density, distribution, movement, and birth and death
rates.
Chapter 4
Population Ecology
4.2 Human Population
Technological Advances
 For thousands of years, the size of the human
population remained relatively constant and
below the environment’s carrying capacity.
 Why is it increasing in population now?
Populations are Increasing Because:
• Advances in technology:
• Farming and domestication of animals
• Medicine—antibiotics and vaccines
• Better shelter
Chapter 4
Population Ecology
4.2 Human Population
Human Population
Growth Rate
 Although the
human population
is still growing, the
rate of its growth
has slowed.
• What might have happened in the 1960s?
• What might be responsible for the decline in
percent increase in human population (world
wide)?
Chapter 4
Population Ecology
4.2 Human Population
Trends in Human Population Growth
 Population
trends can
be altered by
events such
as disease
and war.
 Human
population
growth is not the same in all countries.
Why not?
Demographic Transition
• Demographic Transition: a change in
growth rate resulting from high birth and
death rates to low birth and death rates
• Consists of 3 stages
• Most countries have not yet undergone a
complete demographic transition
– 80% of the world’s population lives in these areas
• Stage 1: high birth rate and a high death
rate
– Families have lots of children
• Stage 2: Improvements in living conditions
– Increased food production
– Medical advances
– Improved sanitation
• Stage 3: The birth rate decreases
– Families have fewer children
– Population growth slows and may stabilize
– Undergone an industrial revolution
• Future population growth depends somewhat
on how many people of different ages are
living in that country today.
Chapter 4
Population Ecology
4.2 Human Population
Zero Population Growth
 Zero population growth (ZPG) occurs when
the birthrate and immigration rate equal the
deathrate and emigration rate.
Chapter 4
Population Ecology
4.2 Human Population
Age Structure
 Age structure:
the number of
males and
females in
each of three
age groups:
pre-reproductive stage, reproductive stage, and postreproductive stage.
• At zero population growth, an age structure
graph should appear more balanced with
numbers at pre-reproductive, reproductive,
and post-reproductive ages being
approximately equal.
Chapter 4
Population Ecology
4.2 Human Population
Human Carrying Capacity
 Scientists are
concerned about the
human population
reaching or exceeding
the carrying capacity.
 An important factor is the
amount of resources from
the biosphere that are
used by each person.