Transcript Chapter 11. Diversification of the Eukaryotes: Animals

Chapter 14: Population Ecology
Planet at capacity: patterns
of population growth
Lectures by Mark Manteuffel, St. Louis Community College
Learning Objectives
• Describe the sub-discipline of population
ecology
• Discuss what a life history is.
• Explain how ecology influences the
evolution of aging in a population.
• Describe how the human population is
growing.
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 environments…
 …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,
we need 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?
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.
As population size increases,
organisms experience:
• reduced food supplies due to competition
• diminished accessibility to places to live
and breed due to competition
• increased incidence of parasites and
disease
• increased predation risk
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
xN
 Multiply by [(K – N)/K]
• varies between 0 and 1
the new term, [(K – N)/K], is close to
1, population growth is essentially
unchanged.
 If
How the Carrying Capacity of an
Environment Influences a Population’s
Growth
r
xN
 Multiply by [(K – N)/K]
 varies between 0 and 1
the new term, [(K – N)/K], is close to
0, the environment is nearly full to
capacity, and population growths reduces
to almost zero
 If
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 constrained
by density-dependent factors: as density
increases, a population reaches the
carrying capacity of its environment, and
limited resources put a ceiling on growth.
• It can also be reduced by densityindependent factors such as natural or
human-caused environmental calamities.
14.5 Some populations cycle
between large and small.
Do lemmings jump off cliffs,
committing suicide when their
populations get too big?
Take-home message 14.5
 Although
the logistic growth pattern is
better than any other model for describing
the general growth pattern of populations,
some populations cycle between periods
of rapid growth and rapid shrinkage.
14.6 “Maximum sustainable
yield” is useful but nearly
impossible to implement.
Maximum Sustainable Yield
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
might seem easy to utilize natural
resources efficiently and sustainably.
 In
practice, however, difficulties such as
estimating population size and carrying
capacity complicate the implementation of
such strategies.
A life history is like a species
summary.
14.7 Life histories are
shaped by natural selection.
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, and
• 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.
Q
Why do
humans defer
reproducing so
much longer
than cats or
mice?
Take-home message 14.7
 An
organism’s investment pattern in
growth, reproduction, and survival is
described by its life history.
 Very
different strategies can achieve the
same outcome in which a mating pair of
individuals produces at least two surviving
offspring.
14.8 There are tradeoffs
between growth,
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 14.8
 Because
constraints limit evolution, life
histories are characterized by tradeoffs
between investments in growth,
reproduction, and survival.
14.9 THIS IS HOW WE DO IT
Life history trade-offs:
rapid growth comes at a cost.
Confounding Factors
1. poor nutrition
2. access to better nutrition
3. correlations between growth rate, adult
body size, and longevity
Why is it useful to
randomize subjects to
experimental treatments?
What happened to fish
growth when their water
was warmer or cooler than
usual?
Is there a cost to growing
more quickly?
What must an organism
give up in exchange?
What can we conclude from
these results?
• Groups that spent Period 2 in catch-up
growth had a decreased median life
span—14.5% lower than that of the
normal-temperature group.
• Groups that spent Period 2 in sloweddown growth had an increased life span—
30.6% higher than that of the normaltemperature fish.
Take-home message 14.9
• Three-spined sticklebacks exposed to
relatively cold or warm temperatures early
in life have, respectively, reduced or
increased growth rates.
• Returned to normal temperature, they
exhibit catch-up or slowed-down growth.
Take-home message 14.9
• Catch-up growth reduces longevity, while
slowed-growth increases longevity,
reflecting a trade-off between growth and
life span.
14.10 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 14.10
 Life
tables and survivorship curves
summarize the survival and reproduction
patterns of the individuals in a population.
Take-home message 14.10
 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.
Ecology
influences
the evolution
of aging in a
population.
14.11 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.
So what does this mean?
Many genetic diseases kill old
people, but almost none kill
children.
Why?
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 14.11

Natural selection cannot weed out harmful
alleles that do not diminish an individual’s
reproductive output.

Consequently, these mutant alleles
accumulate in the genomes of individuals of
nearly all species.

This leads to the physiological breakdowns
that we experience as we age.
14.12 What determines the
average longevity in
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 14.12
 The
rate of aging and pattern of
mortality are determined by the
hazard factor of the organism’s
environment.
Take-home message 14.12
 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.
14.13 Can we slow down
the process of aging?
Life extension is possible.
Take-home message 14.13
 By
increasing the strength of natural
selection later in life, it is possible to
increase the mean and maximum
longevity of individuals in a population.
 This
occurs in nature and has also been
achieved under controlled laboratory
conditions.
The human
population is
growing
rapidly.
14.14 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 14.14
 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.
14.15 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 14.15
 The
demographic transition tends to occur
with the industrialization of countries.
 It
is characterized by an initial reduction in
the death rate, followed later by a
reduction in the birth rate.
14.16 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 14.16
 The
world’s human population is currently
growing at a very high rate, but limited
resources will eventually limit this growth,
most likely at a population size between 7
and 11 billion.