CHAPTER
4
Population EcologyContinued
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.
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
Survivorship Curves
Birth and Death Rates
• Age Structure and Population Growth:
• For a population following a type I
survivorship curve (higher mortality at older
ages):
• If population is made up of more young
people, there is likely to be more births
than deaths
• If population is made up of more old
people, 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
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
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.