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Chapter 14: Population Ecology
Planet at capacity: patterns of population growth
Lectures by Mark Manteuffel, St. Louis Community College
14.1–14.6
Population
ecology is the
study of how
populations
interact with their
environments.
14.1 What is ecology?
Take-home message 14.1
 Population
ecology is the study of the
interaction between populations of
organisms and their environment,
particularly their patterns of growth and
how they are influenced by other species
and by environmental factors.
14.2 A population perspective
is necessary in ecology.
Take-home message 14.2
 Most
ecological processes cannot be
observed or studied within an individual.
 Rather,
when studying them it is necessary
to consider the entire group of individuals
that regularly exchange genes in a
particular locale.
14.3 Populations can grow
quickly for a while, but not
forever.
There is no exception to the rule that every organic
being naturally increases at so high a rate that, if not
destroyed, the Earth would soon be covered by the
progeny of a single pair.
—Charles Darwin, The Origin of Species
In stable populations,
 How
many of the five million eggs that a
female cod might lay over the course of
her life will, on average, survive and grow
to adulthood?
 Who
leaves more surviving offspring, a
pair of elephants or a pair of rabbits?
Figuring Out How a Population
Grows (or Shrinks)
 Two
pieces of information are needed:
• Growth rate, abbreviated as “r”
• Number of individuals in the population (N)
•rN
Population Growth Rate Calculation

500 individuals in a population.

Over the course of the year 125 offspring are
born.
• Birth rate is 125/500 or .25 births per person.

If 25 out of the 500 individuals die during the
course of the same year,
• the death rate is 25/500 or .05 deaths per
person.

The growth rate is .25  .05 or .20 individuals
per person.
Take-home message 14.3
 Populations
tend to grow exponentially,
but this growth is eventually limited.
14.4 A population’s growth
is limited by its environment.
Density-dependent Factors
 The
limitations on a population’s growth
that are a consequence of population
density
 This
ceiling on growth is the carrying
capacity, K, of the environment.
How the Carrying Capacity of an
Environment Influences a
Population’s Growth
r
*N
 Multiply by [(K – N)/K]
• varies between 0 and 1
 If
the new term, [(K – N)/K], is close to 1,
population growth is essentially
unchanged.
How the Carrying Capacity of an
Environment Influences a
Population’s Growth
r
*N
 Multiply by [(K – N)/K]
 varies between 0 and 1
 If
the new term, [(K – N)/K], is close to 0,
the environment is nearly full to capacity,
and population growth reduces to almost
zero.
Density-independent Forces
 Factors
that strike populations without
regard for the size of the population
 Mostly
weather-based
How many people can earth
support?
Why does the answer keep
increasing?
Take-home message 14.4
A
population’s growth can be reduced
both by density-dependent factors related
to crowding and density-independent
factors such as natural or human-caused
environmental calamities.
14.5 Some populations cycle
between large and small.
Take-home message 14.5
 Although
the logistic growth pattern
describes the general growth pattern of
populations better than any other model,
some populations cycle between periods
of rapid growth and rapid shrinkage.
14.6 “Maximum sustainable
yield” is a useful but impossibleto-implement concept.
Almost all natural resource
managers working for the U.S.
government fail to do their job
exactly as mandated.
Why?
What We Often Do Not Know…
 Population
carrying capacity
 Number
of individuals alive
 Stability
of carrying capacity from year to
year
 Which
individuals to harvest
Take-home message 14.6
 Based
on models of population growth, it
seems easy to efficiently and sustainably
utilize natural resources.
 In
practice, however, difficulties such as
estimating population size and carrying
capacity complicate the implementation of
such strategies.
14.7–14.9
A life history
is like a
species
summary.
14.7 Life histories are
shaped by natural selection.
Do any animals mate
themselves to death?
Why?
Why all the variation?
 Is
one strategy better than others,
evolutionarily?
 There
are many possible responses to the
challenge of:
• when to reproduce
• how often to reproduce
• how much to reproduce
Life History
 The
vital statistics of the species
 Includes:
age at first reproduction,
probabilities of survival and reproduction
at each age, litter size and frequency, and
longevity
Reproductive Investment
 The
material and energetic contribution
that an individual will make to its offspring
 Single
episode of reproduction
 Repeated
episodes of reproduction
Which life history strategy is best?
1. What is the cost of reproductive
investment during any reproductive
episode?
2. What is an individual’s likelihood of
surviving to have future reproductive
episodes?
Natural selection favors lifetime reproductive
success.
Why do humans put off mating so
much longer than cats or mice?
Take-home message 14.7
 An
organism’s investment pattern in
growth, reproduction, and survival is its
life history.
14.8 Populations can be
described quantitatively in life
tables and survivorship curves.
Life Tables and Survivorship Curves
 Life
table
• Allow biologists to predict an individual’s
likelihood of either dying within a particular
age interval or surviving the interval.
Life Tables and Survivorship Curves
 Survivorship
curves
• graphs of the proportion of individuals of a
particular age that are alive in a population
Take-home message 14.8
 Life
tables and survivorship curves
summarize the survival and reproduction
patterns of the individuals of a population.
Take-home message 14.8
 Species
vary greatly in these patterns: the
highest risk of mortality may occur among
the oldest individuals or among juveniles
or mortality may strike evenly at all ages.
14.9 There are tradeoffs
between reproduction and
longevity.
Designing an Organism
To structure its life history for maximum fitness,
create one that could:





produce many offspring,
beginning just after birth,
continuing every year,
while growing tremendously large, to reduce
the predation risk
and living forever.
Evolutionary Constraints
 These
traits are not all possible because
selection that changes one feature tends
to adversely affect others.
 Evolutionary
tradeoffs
Three areas to which an organism
can allocate its resources:
 Growth
 Reproduction
 Survival
Take-home message 14.9
 Because
constraints limit evolution, life
histories are characterized by tradeoffs
between investment in growth,
reproduction, and survival.
14.10–14.12
Ecology
influences the
evolution of
aging in a
population.
14.10 Things fall apart: What is
aging and why does it occur?
Physiological Deterioration over Time
Aging: an increased risk of dying with
increasing age.
Why do organisms age?
The force of natural selection
lessens with advancing age.
Many genetic diseases kill old
people, but almost none kill
children.
Why not?
Mutations That Arise and Cause Their
Carrier to Be More Likely to Die Later in
Life

Such mutations include those that increase
the risk from cancers or heart disease or
other types of ailments.

Do not affect reproductive output.

Consequently, these mutants are never
cleaned out of a population.
A cure for cancer may be
discovered but not a cure for
aging.
Why the difference?
Take-home message 14.10
 Natural
selection cannot weed out bad
alleles that do not diminish an individual’s
relative reproductive success.
 Consequently,
they accumulate in the
genomes of nearly all species.
 This
leads to multiple physiological
breakdowns that we see as aging.
14.11 What determines the
longevity of different species?
Hazard Factors
 High-risk
worlds
• Death from external sources
• Reproduce early
 Low-risk
worlds
• Death from external sources low
Age at Time of Reproduction
A
key factor determining longevity.
 Early
reproduction will also favor early
aging.
 Later
reproduction will also favor later
aging.
Take-home message 14.11

The rate of aging and pattern of mortality in a
species is determined by the hazard factor of
that organism's environment.

In environments characterized by low mortality
risk, populations of slowly aging individuals with
long life spans evolve.

In environments characterized by high mortality
risk, populations of early-aging, short-lived
individuals evolve.
14.12 Can we slow down
the process of aging?
Life extension is possible.
Take-home message 14.12
 By
increasing the strength of natural
selection later in life, it is possible to
increase the mean and maximum
longevity of the individuals within a
population.
 This
occurs in nature and has also been
done under controlled laboratory
conditions.
14.13–14.15
The human
population is
growing
rapidly.
14.13 Age pyramids reveal much
about a population.
What is the baby boom?
Why is it bad news for young
people today?
Describing Populations
 In
terms of the proportion of individuals
from each age group
 The
population age distribution
 Age
groupings called cohorts
Take-home message 14.13
 Age
pyramids show the number of
individuals in a population within any age
group.
 They
allow us to estimate birth and death
rates over multi-year periods.
14.14 As less-developed
countries become more
developed, a demographic
transition often occurs.
Population growth is alarmingly
slow in Sweden and alarmingly
fast in Mexico.
Why is there a difference?
Take-home message 14.14
 The
demographic transition tends to occur
with the industrialization of countries.
 It
is characterized by an initial reduction in
the death rate, later followed by a
reduction in the birth rate.
14.15 Human population
growth: How high can it go?
How high can it go?!
 Very
difficult to assess just how many
resources each person needs.
 Ecological
footprints
• Evaluating how much land, how much food
and water, and how much fuel, among other
things, are necessary.
Take-home message 14.15
 The
world population is currently growing
at a very high rate, but limited resources
will eventually limit it, most likely at a
population size between 7 and 11 billion.