2. Objectives – Population Ecology
1. Define: population, population ecology.
2. Distinguish among the different forms of population
distribution (dispersion patterns).
3. Compare and contrast the three types of survivorship curves
and give examples of organisms that exhibit those types of
curves.
4. Describe the factors that affect the size of a population.
5. Describe how birth and death rates influence a population.
6. Distinguish between exponential and logistic growth equations.
7. Define carrying capacity and understand how it can affect
population size.
8. Compare and contrast density-dependent and density
independent factors.
9. Compare and contrast r-selected and K-selected life history
adaptations.
8. Estimating population size
Estimating population size
also interests many
ecologists. Consider this
population of wildebeest.
How can the number of
individuals be
determined?
• Aerial photos
• Sampling small subset
• Mark-recapture
11. Three types of survivorship curves
Section 37.2
Survivorship curves
follow predictable
patterns.
Figure 37.5
12. Type I survivorship curve
Section 37.2
A type I survivorship
curve is representative
of species that invest
much energy caring for
young and have low
death rates early in life.
Most individuals survive
to reproduce.
Figure 37.5
13. Type II survivorship curve
Section 37.2
Type II species have an
approximately equal
probability of dying at
any age.
Figure 37.5
14. Type III survivorship curve
Section 37.2
A type III survivorship
curve is representative of
species that invest little
energy raising their
young and have high
death rates among
offspring. Few individuals
survive to reproductive
age.
Figure 37.5
16. Factors that increase population size
Section 37.2
Birth rate is the number of
individuals produced per
unit of time.
Immigration is the
movement of individuals
into a population.
Table 37.1
T A B L E 37.1 Factors Affecting Population Growth: A Summary
Factor Affected by…
Additions
Births • Number of reproductive episodes per lifetime
• Number of offspring per reproductive episode
• Age at first reproduction
• Population age structure (proportion at
reproductive age)
Immigration • Availability of dispersal mechanism
• Availability of suitable habitat
Subtractions
Deaths • Availability of nutrients
• Predation
• Accidents
• Genetic/infectious disease
Emigration • Availability of dispersal mechanism
17. Factors that decrease population size
Section 37.2
Death rate is the number of
deaths per unit of time.
Emigration is the movement
of individuals out of a
population.
Table 37.1
T A B L E 37.1 Factors Affecting Population Growth: A Summary
Factor Affected by…
Additions
Births • Number of reproductive episodes per lifetime
• Number of offspring per reproductive episode
• Age at first reproduction
• Population age structure (proportion at
reproductive age)
Immigration • Availability of dispersal mechanism
• Availability of suitable habitat
Subtractions
Deaths • Availability of nutrients
• Predation
• Accidents
• Genetic/infectious disease
Emigration • Availability of dispersal mechanism
21. A population growth experiment
Section 37.3
A population is growing exponentially if the number of new individuals is
proportional to the size of the population.
Researchers put 100 rotifers in an environment with abundant
resources and no predators. The population grew exponentially.
Figure 37.6
22. Day 5 of the experiment
Section 37.4
By day 5, the growth rate was 49 individuals per day.
Time
(days)
r N G = rN
Total Population at
End of Day
1 0.22 100 22 122
2 0.22 122 27 149
3 0.22 149 33 182
4 0.22 182 40 222
5 0.22 222 49 271
6 0.22 271 60 331
7 0.22 331 73 404
8 0.22 404 89 493
9 0.22 493 108 601
10 0.22 601 132 733
11 0.22 733 161 894
12 0.22 894 197 1091
13 0.22 1091 240 1331
14 0.22 1331 293 1624
15 0.22 1624 357 1981
16 0.22 1981 436 2417
17 0.22 2417 532 2949
18 0.22 2949 649 3598
19 0.22 3598 792 4390
20 0.22 4390 966 5356
r = Per capita rate of increase
N = Number of individuals at start of time interval
G = Growth rate (number of individuals added per unit time)
Figure 37.6
23. Days 15 and 20 of the experiment
After 15 days, the growth
rate had increased to 357
individuals per day.
At the end of the monitoring
period, the growth rate was
966 individuals per day!
But exponential growth
cannot continue indefinitely.
Time
(days)
r N G = rN
Total Population at End
of Day
1 0.22 100 22 122
2 0.22 122 27 149
3 0.22 149 33 182
4 0.22 182 40 222
5 0.22 222 49 271
6 0.22 271 60 331
7 0.22 331 73 404
8 0.22 404 89 493
9 0.22 493 108 601
10 0.22 601 132 733
11 0.22 733 161 894
12 0.22 894 197 1091
13 0.22 1091 240 1331
14 0.22 1331 293 1624
15 0.22 1624 357 1981
16 0.22 1981 436 2417
17 0.22 2417 532 2949
18 0.22 2949 649 3598
19 0.22 3598 792 4390
20 0.22 4390 966 5356
r = Per capita rate of increase
N = Number of individuals at start of time interval
G = Growth rate (number of individuals added per unit time)
24. Population growth eventually slows
When resources become limited (or when any factor happens
that increases death rate or reduces birth rate), population
growth levels off.
26. Day 26 of the experiment
Section 37.4
At 26 days, the population size is near carrying capacity.
The growth rate has slowed to 30 individuals per day.
Figure 37.8
27. The logistic growth equation
Section 37.4
The following equation is used to calculate the growth rate at any population
size.
Notice that this equation is similar to the one for exponential growth, except
for the following addition.
𝐺 = 𝑟𝑁
𝐾 − 𝑁
𝐾
The boxed portion of
the equation
approaches zero as
population size gets
close to the carrying
capacity. Growth rate
therefore also
approaches zero.
G is the growth rate, r is the per capita rate of increase, N is the initial
population size, and K is the carrying capacity.
33. A summary of life histories
Opportunistic life history
• High reproduction rate
• Many offspring
• Each offspring receives little
parental care
• Low survival rate for juveniles
• Early reproductive maturity
• Type III survivorship curve:
Equilibrium life history
• Low reproduction rate
• Few offspring
• Each offspring receives
extensive parental care
• High survival rate for juveniles
• Late reproductive maturity
• Type I survivorship curve:
37. Human population growth
The human population cannot
continue to grow exponentially.
Humans overexploit resources.
This diagram shows how much
land area each country needs
to support its current lifestyle.
Many countries have a large
ecological footprint, which
means they require more land
than is available.
