43
Populations
Chapter 43 Populations
Key Concepts
• 43.1 Populations Are Patchy in Space and
Dynamic over Time
• 43.2 Births Increase and Deaths Decrease
Population Size
• 43.3 Life Histories Determine Population
Growth Rates
• 43.4 Populations Grow Multiplicatively, but
Not for Long
Chapter 43 Populations
Key Concepts
• 43.5 Extinction and Recolonization Affect
Population Dynamics
• 43.6 Ecology Provides Tools for Managing
Populations
Chapter 43 Opening Question
How does understanding the population
ecology of disease vectors help us
combat infectious diseases?
Concept 43.1 Populations Are Patchy in Space
and Dynamic over Time
Population—all the individuals of a species that
interact with one another within a given area at
a particular time.
Humans have long been interested in
understanding species abundance:
• to increase populations of species that provide
resources and food and to conserve species for
ethical and aesthetic reasons
• to decrease abundance of crop pests,
pathogens, etc.
Concept 43.1 Populations Are Patchy in Space
and Dynamic over Time
Population density—number of individuals per
unit of area or volume
Population size—total number of individuals in
a population
Counting all individuals is usually not feasible;
ecologists often measure density, then multiply
by the area occupied by the population to get
population size.
Concept 43.1 Populations Are Patchy in Space
and Dynamic over Time
Abundance varies on several spatial scales.
Geographic range—region in which a species is
found
Within the range, species may be restricted to
specific environments or habitats.
Habitat patches are “islands” of suitable habitat
separated by areas of unsuitable habitat.
Figure 43.1 Species Are Patchily Distributed on Several Spatial Scales (Part 1)
Figure 43.1 Species Are Patchily Distributed on Several Spatial Scales (Part 2)
Concept 43.1 Populations Are Patchy in Space
and Dynamic over Time
Population densities are dynamic—they change
over time.
Density of one species population may be
related to density of other species populations.
Figure 43.2 Population Densities Are Dynamic and Interconnected
Concept 43.2 Births Increase and Deaths Decrease
Population Size
Change in population size depends on the
number of births and deaths over a given time.
“Birth–death” or BD model of population
change:
Nt 1  Nt  B  D
Concept 43.2 Births Increase and Deaths Decrease
Population Size
Population growth rate (how fast it is changing):
Nt 1  Nt  N  B  D
N
BD
BD


 BD
T (t  1)  t
1
Concept 43.2 Births Increase and Deaths Decrease
Population Size
Per capita birth rate (b)—number of offspring an
average individual produces
Per capita death rate (d)—average individual’s
chance of dying
Per capita growth rate (r) = (b – d) = average
individual’s contribution to total population
growth rate
N
 rN
T
Concept 43.2 Births Increase and Deaths Decrease
Population Size
If b > d, then r > 0, and the population grows.
If b < d, then r < 0, and the population shrinks.
If b = d, then r = 0, and population size does not
change.
Concept 43.3 Life Histories Determine Population Growth Rates
Life history—time course of growth and
development, reproduction, and death during
an average individual’s life
Life histories are quantitative descriptions of life
cycles.
Example: the life cycle of the black-legged tick.
Figure 43.3 Life History of the Black-Legged Tick
Concept 43.3 Life Histories Determine Population Growth Rates
A life table shows ages at which individuals
make life cycle transitions and how many
individuals do so successfully.
Life tables have two types of information:
• survivorship—fraction of individuals that
survive from birth to different life stages or ages
• fecundity—average number of offspring each
individual produces at those life stages or ages
Table 43.1 Life Table for the 1978 Cohort of Cactus Ground Finch on Isla Daphne
Concept 43.3 Life Histories Determine Population Growth Rates
Life histories vary among species: how many
and what types of developmental stages, age
of first reproduction, frequency of reproduction,
how many offspring they produce, and how
long they live.
Life histories can vary within a species. For
example, different human populations have
different life expectancies and age of sexual
maturity.
Concept 43.3 Life Histories Determine Population Growth Rates
Individual organisms require resources
(materials and energy) and physical conditions
they can tolerate.
Rate at which an organism can acquire
resources increases with the availability of the
resources.
Examples: photosynthetic rate increases with
sunlight intensity, or an animal’s rate of food
intake increases with the density of food.
Figure 43.4 Resource Acquisition Increases with Resource Availability (Part 1)
Figure 43.4 Resource Acquisition Increases with Resource Availability (Part 2)
Figure 43.4 Resource Acquisition Increases with Resource Availability (Part 3)
Figure 43.4 Resource Acquisition Increases with Resource Availability (Part 4)
Concept 43.3 Life Histories Determine Population Growth Rates
Principle of allocation—once an organism has
acquired a unit of some resource, it can be
used for only one function at a time:
maintenance, foraging, growth, defense, or
reproduction.
In stressful conditions, more resources go to
maintaining homeostasis.
Once an organism has more resources than it
needs for maintenance, it can allocate the
excess to other functions.
Figure 43.5 The Principle of Allocation
Concept 43.3 Life Histories Determine Population Growth Rates
In general, as average individuals in a population
acquire more resources, the average fecundity,
survivorship, and per capita growth rate
increase.
Concept 43.3 Life Histories Determine Population Growth Rates
Life-history tradeoffs—negative relationships
among growth, reproduction, and survival
Example: investments in reproduction may be at
the expense of adult survivorship or growth.
Environment is also a factor: if high mortality
rates are likely, it makes sense to invest in
early reproduction.
Concept 43.3 Life Histories Determine Population Growth Rates
Species’ distributions reflect the effects of
environment on per capita growth rates.
A study of temperature change in a lizard’s
environment, combined with knowledge of its
physiology and behavior, led to conclusions
about how climate change may affect
survivorship, fecundity, and distribution of these
lizards.
Figure 43.6 Climate Warming Stresses Spiny Lizards (Part 1)
Figure 43.6 Climate Warming Stresses Spiny Lizards (Part 2)
Concept 43.3 Life Histories Determine Population Growth Rates
Laboratory experiments have also shown the
links between environmental conditions, life
histories, and species distributions.
Figure 43.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions
(Part 1)
Figure 43.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions
(Part 2)
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
Population growth is multiplicative—an everlarger number of individuals is added in each
successive time period.
In additive growth, a constant number (rather
than a constant multiple) is added in each time
period.
In-Text Art, Ch. 43, p. 850
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
Charles Darwin was aware of the power of
multiplicative growth:
“As more individuals are produced than can
possibly survive, there must in every case be a
struggle for existence.”
This ecological struggle for existence, fueled by
multiplicative growth, drives natural selection
and adaptation.
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
Multiplicative growth has a constant doubling
time.
The time it takes a population to double in size
can be calculated if r is known.
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
Populations do not grow multiplicatively for very
long. Growth slows and reaches a more or less
steady size:
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
r decreases as the population becomes more
crowded; r is density dependent.
As the population grows and becomes more
crowded, birth rates tend to decrease and
death rates tend to increase.
When r = 0, the population size stops
changing—it reaches an equilibrium size
called carrying capacity, or K.
Figure 43.8 Per Capita Growth Rate Decreases with Population Density (Part 1)
Figure 43.8 Per Capita Growth Rate Decreases with Population Density (Part 2)
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
Spatial variation in environmental factors can
result in variation of carrying capacity.
Temporal variation in environmental conditions
may cause the population to fluctuate above
and below the current carrying capacity.
Example: the rodents and ticks in Millbrook, New
York.
Figure 43.2 Population Densities Are Dynamic and Interconnected
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
The human population is unique. It has grown at
an ever-faster per capita rate, as indicated by
steadily decreasing doubling times.
Technological advances have raised carrying
capacity by increasing food production and
improving health.
Figure 43.9 Human Population Growth (Part 1)
Figure 43.9 Human Population Growth (Part 2)
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
In 1798 Thomas Malthus pointed out that the
human population was growing multiplicatively,
but its food supply was growing additively, and
predicted that food shortages would limit
human population growth.
His essay provided Charles Darwin with a
mechanism for natural selection.
Malthus could not predict the effects of
technology such as medical advances and the
Green Revolution.
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
Many believe that the human population has
now overshot its carrying capacity for two
reasons:
• Technological advances and agriculture have
depended on fossil fuels, a finite resource.
• Climate change and ecosystem degradation
have been a consequence of 20th century
population expansion.
Concept 43.4 Populations Grow Multiplicatively, but Not for Long
If the human population has indeed exceeded
carrying capacity, ultimately it will decrease.
We can bring this about voluntarily if we continue
to reduce per capita birth rate.
Concept 43.5 Extinction and Recolonization Affect
Population Dynamics
Regional populations (metapopulations) are
made up of subpopulations in habitat patches.
Individuals may move in or out of
subpopulations.
Concept 43.5 Extinction and Recolonization Affect
Population Dynamics
The BIDE model of popultion growth adds the
number of immigrants (I) and emigrants (E) to
the BD growth model.
Nt 1  Nt  B  I  D  E
Figure 43.10 A Metapopulation Has Many Subpopulations
Concept 43.5 Extinction and Recolonization Affect
Population Dynamics
Small subpopulations in habitat patches are
vulnerable to environmental disturbances and
chance events and may go extinct.
Individuals from other subpopulations can
recolonize the patch if dispersal is possible.
Concept 43.6 Ecology Provides Tools for Managing Populations
Understanding life history strategies can be
useful in managing other species.
Fisheries:
Black rockfish grow throughout their life. The
number of eggs a female produces is
proportional to her size. Older, larger females
also produce eggs with oil droplets that give the
larvae a head start on growth.
Concept 43.6 Ecology Provides Tools for Managing Populations
Fishermen prefer to catch big fish. Intense
fishing reduced the average age of female
rockfish from 9.5 to 6.5 years.
These younger females were smaller, produced
fewer eggs, and larvae didn’t survive as well.
Management may require no-fishing zones
where some females can mature and
reproduce.
Concept 43.6 Ecology Provides Tools for Managing Populations
Reducing disease risk:
The black-legged tick’s life history indicates that
success of larvae in obtaining a blood meal has
greatest impact on the abundance of nymphs.
Thus, controlling the abundance of rodents that
are hosts for the larvae is more effective in
reducing tick populations than controlling the
abundance of deer, the hosts for adults.
Concept 43.6 Ecology Provides Tools for Managing Populations
Conserving endangered species:
Larvae of the endangered Edith’s checkerspot
butterfly feed on two plant species found only
on serpentine soils.
The two plant species are being suppressed by
invasive non-native grasses.
Grazing by cattle can control the invasive
grasses.
Concept 43.6 Ecology Provides Tools for Managing Populations
Conservation plans begin with inventories of
habitat and potential risks to the habitat.
Largest patches can potentially have the largest
populations and are given priorty.
Quality (carrying capacity) of the patches is
evaluated; ways to restore or maintain quality
are developed.
Ability of the organism to disperse between
patches is evaluated.
Concept 43.6 Ecology Provides Tools for Managing Populations
For some species, a continuous corridor of
habitat is needed to connect subpopulations
and allow dispersal.
Dispersal corridors can be created by
maintaining vegetation along roadsides, fence
lines, or streams, or building bridges or
underpasses that allow individuals to avoid
roads or other barriers.
Figure 43.11 Corridors Can Rescue Some Populations (Part 1)
Figure 43.11 Corridors Can Rescue Some Populations (Part 2)
Figure 43.12 A Corridor for Large Mammals
Answer to Opening Question
By understanding the factors that control
abundance and distribution of pathogens and
their vectors, we can devise ways to control
their abundance or avoid contact.
Black-legged ticks are vectors for the bacterium
that causes Lyme disease.
For these ticks, abundance of hosts for larvae
(rodents) determines tick abundance.
Answer to Opening Question
Rodent abundance depends on acorn
availability.
Acorn production can be used to predict areas
that are likely to become infested with ticks,
and measures can be taken to minimize
human contact.