Ch. 36 Population Ecology
POPULATION STRUCTURE AND DYNAMICS
36.2 Define population density and describe different types of dispersion
patterns.
Population Ecology: The study of how members of a population interact with
their environment, focusing on factors that influence population density and
population growth.
Population Density: the number of individuals of a species per unit area or
volume. # red oak / km2 or # of earthworms / m3.
Density is determined by sampling and extrapolating.
Dispersion patterns: the way individuals are spaced.
see fig. 36.2a-c
clumped: most common-due to clumping of resources
examples:
fungus growing on a rotting log
schools of fish
uniform: evenly spaced
examples:
shore birds (gannets and king penguins)
plants secrete chemicals to decrease competition
random: unpredictable spacing, no pattern
example:
dandelions due to windblown seed distr.
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Explain how life tables are used to track mortality and survivorship in
populations. Compare Type I, Type II, and Type III survivorship curves.
Life Tables track survivorship in populations. Insurance
companies use them for human life expectancy.
see Table 36.3
Type I: Characterized by high parental care
Type II: Individuals are no more vulnerable at any age.
example: squirrels preyed upon by hawks
Type III: Very little or no parental care
example: insects, mollusks, fish, frogs
36.3 Describe and compare the exponential and logistic
population growth models, illustrating both with
examples. Explain the concept of carrying capacity.
Population growth is figured by taking the number of births
and subtracting the number of deaths (assuming immigration
and emigration are equal.)
Exponential Growth Model: The rate of population increase
under ideal conditions. see fig. 36.4a
Logistic Growth Model: Due to limiting factors (resources),
population growth rate will level off (possible crash). see fig
36.4b
Carrying Capacity: is the maximum population size that a
particular environment can sustain (“carry”) with available
resources. Carrying Capacity = K.
36.4 Describe the factors that regulate growth in natural
populations.
Competition for limiting resources:

Food or nutrients:
o As resources are divided between more and
more individuals, birth rate decreases.
o Example: song birds fig. 36.5a female density
vs. clutch size results in an indirect relationship.

Nesting sites

Shelter

Mates
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Influences on health and survival

Crowded plants tend to be smaller with fewer flowers,
fruits, seeds.

Increased disease transmission

accumulation of toxic waste products
Predation

Search image established for most common prey
Physiological factors

High popul. densities in mice appear to induce a stress
syndrome. Hormones change, delay sexual maturation

Cause reprod. organs to shrink

Depress the immune system
Abiotic factors

First hard freeze kills all adult insect, eggs hatch in spring

Aphids show exponential growth with rapid decrease with
high heat and low humidity (abiotic). A few individuals
remain to perpetuate the species.

Fig. 36.5b

Complex interaction of both density-dependant and abiotic
factors (density-independent). see fig. 36.5c
36.5 Define boom-and-bust cycles, explain why they
occur, and provide examples.
“Booms” : rapid exponential growth followed by “busts,”
population falls back to a minimal level.
Example:

lemmings in artic tundra boom every 3-4 yr cycle

Natural changes in the lemmings food supply
Example:

Lynx and Snowshoe Hare N.forests Alaska/Canada 10 yr
cycle.

Hare cycle caused by?:
a. winter food shortage although providing food did not
stop cycle.
b. predator/prey interactions: 3 predators
c. food resource limitation and excessive predation (both)

Lynx cycle caused by?:
a. Single species of prey depleted
b. predators can turn on each other accelerating
decline
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Explain how life-history traits vary with environmental
conditions and with population density. Compare rselection and K-selection and indicate examples of each.
The traits that affect an organism’s schedule of reproduction
and death make up its life history. The combination of life
history traits in a popul. represents tradeoffs that balance the
demands of reproduction and survival.
Life History Traits

age at first reproduction

frequency of reproduction

number of offspring

amount of parental care
Natural selection can NOT optimize all of these traits
simultaneously because an organism has limited time, energy
and nutrients.
r-selection (r = per capita rate of increase)
Organisms characterized by:

small bodied

short lived

reach sexual maturity rapidly

large number of offspring

little or no parental care

Examples: insects, rodents, dandelions
Environments characterized by:

resources are abundant

exponential growth

unpredictable disturbances: fire, flood, hurricanes,
droughts or cold weather

abiotic factors that drastically reduce numbers but

create new opportunities
human activity is a major cause of disturbance so r-select

colonize road cuts

freshly cleared fields

woodlots

poorly maintained lawns.
K-selection (K = per capita rate of increase)
Organisms characterized by:

large bodied

long-lived

reach sexual maturity slowly

few offspring

well cared for

Examples: bears and elephants, coconut palms
Environments characterized by:

population near carrying capacity (K)

density dependant factors limit growth (logistic growth
curve)

Competition for resources is keen

long-life: allocation of energy to own survival and survival
of descendants is an advantage

stable climate
Oversimplification?
Most organisms are somewhere between the extremes.
Evolution of life histories as a new sub-field of ecol. research
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THE HUMAN POPULATION
36.10 Explain how the age structure of a population can
be used to predict changes in population size and
social conditions.
Age Structure: the number of individuals in different agegroups. Tool used to predict future population growth.
Example:

After WWII, there was a baby-boom. Schools needed to
be built to accommodate them. Babies born near the end
of the two-decade trend, graduated college to find stiff
competition for jobs.

The boomers created their own “boomlet.”

Boomers are now retiring and their large numbers threaten
social security funds.

What would “zero population growth” look like on an age
structure diagram?
Age structure diagrams can be used to predict world
population in 2025 to be 8 billion.

Food production will need to double although
agricultural lands are under pressure today.

Overgrazing growing heads of livestock is turning
grassland into desert

Water use has risen sixfold over the past 70 years

Space requirements will cause the extinction of many
thousands of other species
The global human population has grown almost continuously
throughout history, it increased with the agricultural revolution, but it
skyrocketed after the Industrial Revolution. After the 1940’s, health
care improvements including the discovery of antibiotics like
penicillin (the 1st), greatly improved life expectancy.
Annual percent increase in the global
human population (data as of 2003).
The dashed portion of the curve
indicates projected data. The sharp
dip in the 1960’s is due mainly to a
famine in China in which about 60
million people died.
What is Earth’s carrying capacity?
36.11 Explain the concept of an ecological footprint. Describe the uneven
use of natural resources in the world.
An Ecological Footprint is an estimate of the amount of land required to provide
the raw materials an individual or a nation consumes, including food, fuel, water,
housing and waste disposal.
Total area of productive land on Earth = about 2 hectare (2.47 acres)
global population
Reserving some land for parks and conservation reduces this to 1.7 ha / person
2003 = 2.23 ha / person
We have already overshot the Earth’s carrying capacity.
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Americans

The U.S. = 8.4 ha / person

U.S. can support 6.2 ha / person

This is a large ecological deficit
Overpopulation and over-consumption are the main problems. See fig. 36.11a
20% of global population uses 86% of the world’s resources leaving 14% of global resources for the other 80% of people.
See figure 36.11B
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