# Chapter 52

NEW AIM: How do scientists describe population
growth?
Population Ecology
- The study of changes in population size and the
environmental factors that regulate it.
Population
- group of individuals of a single species that occupy the
same general area
1. Rely on same resources
2. Influenced by the same biotic and abiotic factors
3. High likelihood of interbreeding
NEW AIM: How do scientists describe population
growth?
Population Density
- Number of individuals per unit area or volume
How can we determine this?
1. Count all individuals (not always possible)
2. Count individuals in several random spots and then extrapolate
3. mark-recapture:
- Capture some animals and mark (left)
- Release back to population
- Let animals mix back for days/weeks
- Randomly catch animals again
- Now you get mix of marked and unmarked
Great, but what does that do for me?
NEW AIM: How do scientists describe population
growth?
Population Density
- Number of individuals per unit area or volume
How can we determine this?
1. Count all individuals (not always possible)
2. Count individuals in several random spots and then extrapolate
3. mark-recapture:
- Capture some animals and mark (left)
- Release back to population
- Let animals mix back for days/weeks
- Randomly catch animals again
- Now you get mix of marked and unmarked
- The ratio of marked to unmarked gives population estimate
NEW AIM: How do scientists describe population
growth?
Population Dynamics
- Pop. Density is obviously not going to be static
1. Birth/Death
2. Immigration / Emigration
AIM: What are some of the factors that limit
population growth?
Dispersion
- Pattern of spacing among individuals in a population
What types of patterns are observed?
1. Clumped
- Most common (we do this of course)
- Can be associated with:
1. Resource location like food or water
2. mating
- Improve odds of finding a mate
3. Improve odds of predators
- Pack of wolves more likely than single
wolf to capture a prey animal
4. Improve odds of prey
- Herding behavior = use genetically
unrelated individuals as shield against
predators
AIM: What are some of the factors that limit
population growth?
Dispersion
- Pattern of spacing among individuals in a population
What types of patterns are observed?
2. Uniform
- Can be associated with:
1. Direct interaction of population members
- Some plants secrete chemicals that
inhibit germination of seeds
- Territoriality – defending a space
against encroachment by other
members of population
- Penguins to right will aggressively
defend their space from other penguins
AIM: What are some of the factors that limit
population growth?
Dispersion
- Pattern of spacing among individuals in a population
What types of patterns are observed?
3. Random
- Can be associated with:
1. Absence of strong attraction/repulsion amongst
individuals
2. Key biotic/abiotic factors are
homogenous (equally distributed)
throughout area – food, water, etc…
AIM: What are some of the factors that limit
population growth?
Demography
- Study of statistics of populations and how they change over time.
What influences density and dispersion of populations?
1. Ecological needs of a species (food, water, sunlight, humiditiy, etc…)
2. Structure of the environment (are resources evenly distributed, are
they found in certain areas, are there predators, etc…)
3. Interactions between individuals of population
These factors will be the cause of vital statistics like:
1. Death Rates (Survivorship)
AIM: What are some of the factors that limit
population growth?
Demographers construct Life Tables
Life tables
- Age specific summaries of the survival patterns of a population that allow one to
estimate how long a person of a given age will live – great for life insurance companies
Cohort
- Group of individuals the same age that demographers follow from birth to death for each age group.
Let’s take this life table data and look at it graphical in what are called survivorship curves…
AIM: What are some of the factors that limit
population growth?
Demographers construct Life Tables
Survivorship curves
- Graphical representations of life tables
Survivorship curves for male and female
Belding’s ground squirrels. Be aware that Y-axis
is logarithmic.
Survivorship curves can be classified into three general types as shown on the next slide…
AIM: What are some of the factors that limit
population growth?
Demographers construct Life Tables
Survivorship curves
Three general types:
Type I
- Flat at start meaning low death rate
at early and middle life and then
drops steeply
- Ex. Humans, many other large mammals
with high parental care
Type III
- Drops steeply reflecting high death
rate in young individuals, but then
flattens out for those few individuals
that make it to a critical age.
- Ex. Organisms producing many offspring,
but little parental care like fish, marine
invertebrates like oysters, which release
millions of eggs of which most are eaten by
predators.
Type II
- Between type I and type III with a
fairly constant death rate over a lifetime
- Ex. Annual plants, some rodents, some lizards, etc…
AIM: What are some of the factors that limit
population growth?
Demography
- Study of statistics of populations and how they change over time.
What influences density and dispersion of populations?
1. Ecological needs of a species (food, water, sunlight, humiditiy, etc…)
2. Structure of the environment (are resources evenly distributed, are
they found in certain areas, are there predators, etc…)
3. Interactions between individuals of population
These factors will be the cause of vital statistics like:
1. Death Rates (Survivorship)
2. Reproductive Rates
AIM: What are some of the factors that limit
population growth?
Demographers construct Reproductive Tables
Reproductive tables
- Fertility schedule, age-specific summary of reproductive rates in a population
Constructed by measuring reproductive
output of a cohort from birth to death – the
number of offspring a group has in a
AIM: What are some of the factors that limit
population growth?
Review Demography
AIM: What are some of the factors that limit
population growth?
Life History Traits
Life History
Traits of a population that affect reproduction and survival
What are the three general traits affecting life history?
1. Age at which reproduction begins
2. How often organism reproduces
3. Number of offspring produced during
each reproductive episode
***Excluding humans, organisms do not choose consciously when to reproduce or how many offspring to have…
AIM: What are some of the factors that limit
population growth?
Life History Traits
Life History
Traits of a population that affect reproduction and survival
1. Semelparity (big-bang reproduction)
- “one-shot” pattern of reproduction
- Ex. Pacific salmon will produce
thousands of eggs one time and then die.
Under what environmental conditions would this be a good strategy to evolve?
- When survival rate of offspring is low due to highly variable /
unpredictable environment.
AIM: What are some of the factors that limit
population growth?
Life History Traits
Life History
Traits of a population that affect reproduction and survival
2. Iteroparity (repeated reproduction)
- Individuals will have smaller numbers or
offspring, but consistently over a lifetime
- We obviously display iteroparity along
with birds, most reptiles, all mammals, and
most fish.
Under what environmental conditions would this be a good strategy to evolve?
- When environment is more dependable and competition against other
individuals is critical
- A few, relatively large, well-provisioned individuals will stand a better
chance of survival and reproduction
AIM: What are some of the factors that limit
population growth?
Life History Traits
Life History
Traits of a population that affect reproduction and survival
Why are there not species that combine the two and have tons of offspring
regularly?
Evolutionary trade-offs, there is just not enough time, energy and
nutrients available to do this.
AIM: How do scientists describe population
growth?
Let’s look at population growth of bacteria starting from a single bacterium
- binary fission every 20 minutes under ideal conditions
Q: How many will there be after 36 hours?
AIM: How do scientists describe population
growth?
n
2
Where n = # of divisions
Let’s look at population growth of bacteria starting from a single bacterium
- binary fission every 20 minutes under ideal conditions
Q: How many will there be after 36 hours?
A: 2108 (enough to cover the planet one foot deep)
Graph it:
AIM: How do scientists describe population
growth?
1. Exponential Growth Model
(J-shaped curve)
What does the rate of growth depend on in this model?
The number of individuals ONLY
So why is Earth not covered in bacteria?
AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
Ex. Fur seals on St. Paul Island
AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
Ex. Fur seals on St. Paul Island
1. Uncontrolled hunting until 1925
2. Population jumped until 1935
- Almost exponential
3. Population stabilized
- Some hunting
- limited breeding grounds
AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
(S - shaped curve)
Q: How does the logistic model
differ from the exponential model?
AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
(S - shaped curve)
Q: How does the logistic model
differ from the exponential model?
A: It accounts for limiting factors
in the environment
AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
(S - shaped curve)
Carrying capacity (K)
- max. population the environment can
Determined by:
The species itself and the environment
(resources available, predation, etc…)
**Carrying capacity if obviously not fixed. It depends
on abundance of limiting resources at a particular
space and time. Figure out way to make or get more
food…carrying capacity increases.
AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
When is growth rate the lowest?
1. When population is small or large
Limited number of individuals
and then limited resources
When is it the highest?
1. Population at intermediate level
relative to carrying capacity.
2. Plenty of resources
NO POPULATION FITS EITHER
MODEL PERFECTLY
AIM: How do scientists describe population
growth?
Population limiting factors
- environmental factors that restrict population growth
Human Population
We have a major problem here. What is
our future? The number one problem on
this planet for humans and many, many
other species is our own
overpopulation.
What is the solution?
AIM: How do scientists describe population
growth?
Review
Population ecology
Exponential growth model
- J-shaped
- NO limiting factors
- Growth rate determined by population
size and type of organism
Logistic Growth Model
- S-shaped
- Population limiting factors
FEW POPULATIONS FIT EITHER
MODEL PERFECTLY AS YOU WILL
SEE ON THE NEXT SLIDE…
AIM: How do scientists describe population
growth?
FEW POPULATIONS FIT EITHER
MODEL PERFECTLY:
AIM: How do scientists describe population
growth?
K-selection (quality) vs r-selection (quantity)
K-selection
- Population selected for fewer offspring and higher parental care.
- Typically found when environments are stable resulting in high
population density and therefore intense competition – fighting other
individuals, not the environment
- iteroparity
- Traits typically large body size, long life expectancy, fewer offspring,
higher parental care.
r-selection
- Population selected for having many offspring with low parental
care.
- Typically found when environments are unstable/unpredictable
resulting in lower population densities – have many and hope
something makes it – fighting the environment , not other individuals.
- semelparity
- Traits typically large body size, long life expectancy, fewer offspring,
higher parental care.
AIM: How do scientists describe population
growth?
Let’s look at some specific population limiting factors and examples.
NEW AIM: What are some of the factors that
limit population growth?
Population limiting factors
1. Biotic factors
a. Competition among members (intraspecific)
- food supply, water, space, energy
(oil), mates, etc…
Song Sparrow Population Experiment
(small island in British Columbia)
- Clutch size increased
Fig. 35.4A
AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
a. Competition among members (intraspecific)
- food supply, water, space, energy
(oil), mates, etc…
Competition for space…
AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
a. Competition among members (intraspecific)
i. food supply
ii. space (territory)
A. Dispersion patterns
1. clumped
2. uniform
3. random
AIM: What are some of the factors that limit
population growth?
Dispersion patterns
AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
b. Health
- increased spread of disease with increased pop. density
- accumulation of waste
AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
c. Predation
As the hare population increases, so
does the Lynx population, which will
cause the hare population to decrease
and in turn the lynx population
decreases, etc…
“boom-and-bust” cycles
Fig. 35.5
AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
d. Physiological factors
White-footed mouse experiment
1. Enclose in a small field
2. Reproduce quickly to 30 or 40
3. Reproduction declines and pop. stabilizes
around 40
4. Add more food and water
- no change
RESULT: high population induces stress syndrome (hormonal changes)
- sexual maturity delayed, reproductive organs shrink, depressed immune system
AIM: What are some of the factors that limit
population growth?
Population limiting factors
2. Abiotic factors
a. Climate and weather
Fig. 35.4B
Ex. Aphids
- insects that feed on phloem sap of plants
1. Exponential growth in spring
2. Rapid die out in hot, dry summer
3. A few individuals survive and reproduce
when conditions improve
What happens to mosquitoes and grasshoppers during winter?
- die out completely, leaving only fertilized eggs
AIM: What are some of the factors that limit
population growth?
Population limiting factors
2. Abiotic factors
b. Fire, flood, etc…
AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
a. Competition among members
- food supply
Fig. 35.4C
- space
b. Health
c. Predation
d. Physiological factors
2. Abiotic factors
a. Climate and weather
b. Natural disasters
(25-year study of the song sparrow)
Almost all populations are regulated by a mixture of these and other factors
AIM: What are some of the factors that limit
population growth?
Age Structure of HUMAN populations
Age structure of a given population can indicate if the population is growing or declining as well
as current social structures…
Indicate if the above human populations are growing or declining.
AIM: What are some of the factors that limit
population growth?
Age Structure of HUMAN populations
What do these allow us to predict about social structure?
Ex. Afghanistan is going to have issues with opportunities for jobs and education with the huge influx of young people. In the
US, a smaller population of working people will be supporting a larger population of retired people…
AIM: What are some of the factors that limit
population growth?
Ecological footprint
The amount of land required to sustain an individual or a population/nation in terms of food/water and absorbing generated
waste in hectares (ha; 1 hectare = 100m x 100m)
Ecological capacity
How much land a country actually has per person
(US has 6.2 ha/person, but uses 8.4 ha/person on average)
AIM: What are some of the factors that limit
population growth?
“Milestone QUESTIONS”
1. One species of forest bird is highly territorial, while the second lives in
flocks. What is each species’ likely pattern of dispersion? Explain.
2. Each female of a particular fish species produces millions of eggs per
year. What is its likely survivorship pattern? Explain.
3. Consider two rivers: One is spring fed and is constant in water volume
and temperature year-round; the other drains a desert landscape and floods
and dries out at unpredictable intervals. Which is more likely to support
many species of iteroparous animals and why? Would you guess this to be
an r-selection or K-selection species and why?
4. Where is exponential growth by a plant population more likely – on a
newly formed island or in a mature, undisturbed rain forest?
5. Identify three factors that limit population size.
AIM: What are some of the factors that limit
population growth?
“Milestone QUESTIONS” on next slide…study, study…