Principles of Life, 2e

Populations

42

Chapter 42 Populations

Key Concepts

42.1 Populations Are Patchy in Space and

Dynamic over Time

42.2 Births Increase and Deaths Decrease

Population Size

42.3 Life Histories Determine Population Growth

Rates

Chapter 42 Populations

Key Concepts

42.4 Populations Grow Multiplicatively, but the

Multiplier Can Change

42.5 Immigration and Emigration Affect

Population Dynamics

42.6 Ecology Provides Tools for Conserving and

Managing Populations

Chapter 42 Opening Question

How does understanding the population ecology of disease vectors help us combat infectious diseases?

Concept 42.1 Populations Are Patchy in Space and

Dynamic over Time

Populations: groups of individuals of the same

species

Humans have long been interested in

understanding species abundance:

• To increase populations of species that

provide resources and food

• To decrease abundance of crop pests,

pathogens, etc.


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.


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 42.1 Species Are Patchily Distributed on Several Spatial Scales


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 42.2 Population Densities Are Dynamic

Concept 42.2 Births Increase and Deaths Decrease

Population Size

Change in population size depends on the

number of births and deaths over a given

length of time.

“Birth–death” or BD model of population

change:

DBNN tt 1


Population Size

Population growth rate (change in size over one

time interval):

Nt1 Nt N (Nt Nt) BD BD

DBDB

tt

DB

T

N

1)1(


Population Size

Change in population size can be measured

only for very small populations that can be

counted, such as zoo animals.

To estimate growth rates, ecologists keep track

of a sample of individuals over time.


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

rNT

N


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 42.3 Life Histories Determine Population Growth Rates

Demography: study of processes influencing

birth, death, and population growth rates

Life history: timing of key events such as

growth and development, reproduction, and

death during an average individual’s life

• Example: Life cycle of the black-legged tick

Figure 42.3 Life History of the Black-Legged Tick


A life history shows the ages at which

individuals make life cycle transitions and how

many individuals do so successfully:

• Survivorship—fraction of individuals that

survive from birth to different life stages or

ages

• Fecundity—average number of offspring

each individual produces at different life

stages or ages

Table 42.1


Survivorship can also be expressed as

mortality: the fraction of individuals that do not

survive from birth to a given stage or age.

Mortality = 1 – survivorship


Survivorship and fecundity affect r. The higher

the fecundity rate and survivorship, the higher r

will be.

If reproduction shifts to earlier ages, r will

increase as well.


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.


Individual organisms require resources

(materials and energy) and physical conditions

they can tolerate.

The rate at which an organism can acquire a

resource increases with the availability of the

resource.

• Examples: Photosynthetic rate increases

with sunlight intensity; an animal’s rate of

food intake increases with the density of

food

Figure 42.4 Resource Acquisition Increases with Resource Availability—Up to a Point


Principle of allocation

Once an organism has acquired a unit of some

resource, it can be used for only one function

at a time, such as maintenance, 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 42.5 The Principle of Allocation


In general, as average individuals in a

population acquire more resources, the

average fecundity, survivorship, and per capita

growth rate increase.


Life-history tradeoffs—negative relationships

among growth, reproduction, and survival

• Example: A species that invests heavily in

growth early in life cannot simultaneously

invest heavily in defense.

Environment is also a factor: if high mortality

rates are likely, it makes sense to invest in

early reproduction.


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 42.6 Climate Warming Stresses Spiny Lizards (Part 1)




Laboratory experiments have also shown the

links between environmental conditions, life

histories, and species distributions.

• Example: Quantifying life history traits of

two species of grain beetles in different

temperature and humidity conditions

explained distributions of these species in

Australia.

Figure 42.7 Environmental Conditions Affect Per Capita Growth Rates and Species Distributions

Concept 42.4 Populations Grow Multiplicatively, but the Multiplier

Can Change

Population growth is multiplicative—an ever-

larger 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, Chapter 42, p. 873 (2)


Can Change

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.


Can Change

Multiplicative growth with a constant r has a

constant doubling time.

The time it takes a population to double in size

can be calculated if r is known.


Can Change

Populations do not grow multiplicatively for very

long. Growth slows and reaches a more or less

steady size:


Can Change

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.


Can Change

K can be thought of as the number of individuals

that a given environment can support

indefinitely.

When population density reaches K, an average

individual has just the amount of resources it

needs to exactly replace itself.

When density <K, an average individual can

more than replace itself; when density >K, the

average individual has fewer resources than it

needs to replace itself.

Figure 42.8 Per Capita Growth Rate Decreases with Population Density


Can Change

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 42.2 Population Densities Are Dynamic


Can Change

Environmental changes affected fecundity of the

Galápagos cactus ground finches:

• When females were 7 and 8 years old, they

produced no surviving young, and

survivorship dropped.

• Low food availability during these years

resulted from a severe drought in 1985.

• When the females were 5, a wet year

produced abundant food and high fecundity.

Table 42.1


Can Change

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 42.9 Human Population Growth


Can Change

In 1798 Thomas Malthus pointed out that the

human population was growing multiplicatively,

but food supply was growing additively, and

predicted that food shortages would limit

human population growth.

His essay provided Charles Darwin with a

critical insight for the mechanism of natural

selection.

Malthus could not have predicted the effects of

technology such as medical advances and the

Green Revolution.


Can Change

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.


Can Change

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 42.5 Immigration and Emigration Affect

Population Dynamics

Many species occupy habitat patches separated

from other patches by unsuitable

environments.

Each patch is occupied by a subpopulation,

the set of subpopulations in a region is a

metapopulation.

Individuals may move in or out of

subpopulations.

Figure 42.10 A Metapopulation Has Many Subpopulations


Population Dynamics

The BIDE model of population growth adds the

number of immigrants (I) and emigrants (E) to

the BD growth model.

EDIBNN tt 1


Population Dynamics

In the BD model, populations are considered

closed systems—no immigration or

emigration.

In the BIDE model, subpopulations are

considered open systems—individuals can

move among them.


Population Dynamics

Small subpopulations in habitat patches are

vulnerable to environmental disturbances and

chance events and may go extinct.

If dispersal is possible, individuals from other

subpopulations can recolonize the patch and

“rescue” the subpopulation from extinction.


Population Dynamics

Immigrants also contribute to genetic diversity

within subpopulations.

This gene flow combats the genetic drift that can

occur in a small population that reduces a

species’ evolutionary potential.


Population Dynamics

In the metapopulation of Edith’s checkerspot

butterfly, all but the largest subpopulation went

extinct during a severe drought between 1975

and 1977.

In 1986, nine habitat patches were recolonized

from the Morgan Hill subpopulation.

Patches closest to Morgan Hill were most likely

to be recolonized because adult butterflies do

not fly very far.

Figure 42.1 Species Are Patchily Distributed on Several Spatial Scales

Concept 42.6 Ecology Provides Tools for Conserving and


Understanding life history strategies can be

useful in managing other species.

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.



Fisheries

• Black rockfish grow throughout their life.

• Older, larger females produce more eggs,

and the eggs have larger oil droplets, which

give the larvae a head start on growth.

• Because 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 the larvae did not

survive as well.

• Population density rapidly declined.

• Management may require no-fishing zones

where some females can mature and

reproduce.



Reducing disease risk

• The black-legged tick’s life history indicates

that success of larvae in getting 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.



Conservation plans begin with inventories of

habitat and potential risks to the habitat.

Largest patches can potentially have the largest

populations and genetic diversity and are given

priority.

Quality of the patches is evaluated; ways to

restore or maintain quality are developed.

Ability of the organism to disperse between

patches is evaluated.

Figure 42.11 Habitat Corridors Can “Rescue” Subpopulations from Extinction (Part 1)





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 42.12 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.

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Principles of Life, 2e