1. Chapter 53
Population Ecology

2. Overview
Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size.
A population is a group of individuals of a single species living in the same general area.

3. I. Population density, dispersion, and demographics
Density = number of individuals per unit area
Dispersion = pattern of spacing among individuals within boundaries of population.
Density is result of processes that add individuals and that remove individuals.

4. Immigration = adding new outside individuals
Emigration = movement of individuals out
Sampling techniques used to estimate densities and total population sizes.
extrapolation from small samples, an index of population size, or the Mark-Recapture Method.

5. Population dynamics Births Deaths Births and immigration
add individuals to
a population.
Deaths and emigration
remove individuals
from a population.
Figure 53.3
Immigration
Emigration

6. A. Patterns of Dispersion
Clumped dispersion = individuals aggregate in patches. (influenced by resource availability and behavior)
Uniform dispersion = individuals are evenly distributed. (influenced by social interactions such as territoriality)
Random dispersion = individuals are independent of other individuals. (occurs in the absence of strong attractions or repulsions)

7. Patterns of dispersion within a population’s geographic range
(a) Clumped
(b) Uniform
Figure 53.4 Patterns of dispersion within a population’s geographic range
(c) Random

8. II. Demographics
Demography = study of vital statistics of a population and how they change
Death rates and birth rates

9. A. Life Tables & Survivorship Curves
Life table = age-specific summary of survival pattern of population.
Follows the fate of a cohort (group of individuals of the same age)
Survivorship curve = graph of data in a life table.

10. Survivorship curves for squirrels shows relatively constant death rate
1,000
100
Number of survivors (log scale)
Females 10
Males
Figure 53.5 Survivorship curves for male and female Belding’s ground squirrels 1 2 4 6 8 10
Age (years)

11. Survivorship curves can be classified into three general types:
Type I: low death rates early and middle life, increase among older age (humans)
Type II: the death rate is constant (squirrels)
Type III: high death rates for the young, slower death rate for survivors (Fish)

12. Survivorship Curves Number of survivors (log scale) 1,000 I 100 II 10
Figure 53.6 Idealized survivorship curves: Types I, II, and III
III 1 50
100
Percentage of maximum life span

13. B. Reproductive Rates
Reproductive table (fertility schedule) = age-specific summary of reproductive rates in a population. (describes reproductive patterns of a population)

14. III. Life history
Life history comprises the traits that affect its schedule of reproduction and survival:
age when reproduction begins
How often it reproduces
How many offspring are produced each time

15. IV. “Trade-offs” and Life Histories
Organisms have finite resources, which may lead to trade-offs between survival and reproduction.
Animals small#, dandelions large# and coconuts more stored energy

16. Variation in the size of seed crops in plants
(a) Dandelion
Figure 53.9
(b) Coconut palm

17. V. Exponential model
Study population growth in an idealized situation.
helps understand the capacity of species to increase and the conditions that may facilitate this growth.

18. Zero population growth = birth rate = death rate.
Formula: N t  rN
N = population size, t = time, and r = per capita rate of increase = birth – death

19. VI. Exponential Growth
Exponential population growth is population increase under idealized conditions.
rate of reproduction is at maximum (intrinsic rate of increase)
Results in a J-shaped curve
Exponential Growth is not sustainable.

20. 2,000 = 1.0N 1,500 = 0.5N Population size (N) 1,000 500 5 10 15
Exponential Growth Model
2,000 dN =
1.0N dt
1,500 dN =
0.5N dt
Population size (N)
1,000
500
Figure Population growth predicted by the exponential model 5 10 15
Number of generations

21. The J-shaped curve of exponential growth characterizes some rebounding populations
8,000
6,000
Elephant population
4,000
2,000
Figure Exponential growth in the African elephant population of Kruger National Park, South Africa
1900
1920
1940
1960
1980
Year

22. VII. Logistic model
Exponential growth cannot be sustained for long in any population.
Carrying capacity (K) = max population size the environment can support.
In the logistic population growth model, the rate of increase declines as carrying capacity is reached.
Logistic model of population growth produces a sigmoid (S-shaped) curve.
Some populations overshoot K before settling down to a relatively stable density.

23. Logistic Growth model Exponential growth 2,000 = 1.0N 1,500 K = 1,500dN =
1.0N dt
1,500
K = 1,500
Population size (N)
Logistic growth
1,000 dN
1,500 – N =
1.0N dt
1,500
Figure Population growth predicted by the logistic model
500 5 10 15
Number of generations

24. The growth of laboratory populations fits an S-shaped curve which hovers around the Carrying Capacity of the area.
1,000
180
150
800
120
Number of Paramecium/mL
600
Number of Daphnia/50 mL 90
400 60
200 30 5 10 15 20 40 60 80
100
120
140
160
Figure How well do these populations fit the logistic growth model?
Time (days)
Time (days)
(a) A Paramecium population in the lab
(b) A Daphnia population in the lab

25. VIII. Logistic Model and Life Histories
LH traits favored by natural selection vary with pop density and env connditions.
K-selection = density-dependent selection, selects for traits that are sensitive to population density.
r-selection = or density-independent selection, selects for traits that maximize reproduction.

26. IX. Population Change and Population Density
Density-independent populations, birth and death rates do not change with population density.
Density-dependent populations, birth rates fall and death rates rise with population density.

27. X. Density-Dependent Population Regulation
Density-dependent rates are an example of negative feedback that regulates population growth.
They are affected by many factors (competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors)

28. Percentage of juveniles producing lambs
Decreased reproduction at high population densities
100 80 60
Percentage of juveniles producing lambs 40
Figure 53.16 20
200
300
400
500
600
Population size

29. A. Territoriality
Competition for territory may limit density.

30. Territoriality (a) Cheetah marking its territory (b) Gannets
Figure 53.17
(b) Gannets

31. B. Other Factors Dense populations, pathogens can spread more rapidly
As prey builds up, predators feed preferentially on that species.
Accumulation of toxic wastes can contribute to density-dependent regulation of population size.

32. XI. Population Dynamics
Population dynamics = interactions between biotic and abiotic factors that cause variation in population size.

33. 50 2,500 Wolves Moose 40 2,000 30 1,500 Number of wolves
Changes in predation pressure can drive population fluctuations 50
2,500
Wolves
Moose 40
2,000 30
1,500
Number of wolves
Number of moose 20
1,000 10
500
Figure Fluctuations in moose and wolf populations on Isle Royale, 1959–2006
1955
1965
1975
1985
1995
2005
Year

34. Number of hares (thousands) Number of lynx (thousands)
Snowshoe hare
160
120 9
Figure Population cycles in the snowshoe hare and lynx
Lynx
Number of hares
(thousands)
Number of lynx
(thousands) 80 6 40 3
1850
1875
1900
1925
Year

35. XII. Human population Humans can not grow indefinitely
Global population is still growing, the rate of growth began to slow during the 1960s.
Most of the current global population growth is concentrated in developing countries.

36. Human population growth7 6 5 4
Human population (billions) 3 2
The Plague
Figure Human population growth (data as of 2006) 1
8000
B.C.E.
4000
B.C.E.
3000
B.C.E.
2000
B.C.E.
1000
B.C.E.
1000
C.E.
2000
C.E.

37. Regional human population can exist in one of two configurations:
A. Regional Patterns
Regional human population can exist in one of two configurations:
Zero population growth = High birth rate – High death rate
Zero population growth = Low birth rate – Low death rate
The demographic transition is the move from the first state toward the second state.

38. B. Age Structure
Relative number of individuals at each age.
Age structure diagrams can predict a population’s growth trends.

39. Age-structure pyramids for the human population of three countries
Rapid growth
Slow growth
No growth
Afghanistan
United States
Italy
Male
Female
Age
Male
Female
Age
Male
Female
85+
85+
80–84
80–84
75–79
75–79
70–74
70–74
65–69
65–69
60–64
60–64
55–59
55–59
50–54
50–54
45–49
45–49
40–44
40–44
35–39
35–39
30–34
30–34
25–29
25–29
20–24
20–24
Figure Age-structure pyramids for the human population of three countries (data as of 2005)
15–19
15–19
10–14
10–14
5–9
5–9
0–4
0–4
10  8 6 4 2 2 4 6 8
10  8 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8
Percent of population
Percent of population
Percent of population

40. C. Limits on Human Population
Ecological footprint = summarizes the land and water area needed to sustain the people of a nation.
Countries vary greatly in footprint size and available ecological capacity.
Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes.
Estimated CC of humans is billion

41. K = carrying capacity Population size (N) dN K – N = rmax N dt K
Review: Population Growth Curve
K = carrying capacity
Population size (N) dN
K – N =
rmax N dt K
Number of generations

42. You should now be able to:
Define and distinguish between the following sets of terms: density and dispersion; clumped dispersion, uniform dispersion, and random dispersion; life table and reproductive table; Type I, Type II, and Type III survivorship curves; semelparity and iteroparity; r-selected populations and K-selected populations.
Explain how ecologists may estimate the density of a species.

43. Explain how limited resources and trade-offs may affect life histories.
Compare the exponential and logistic models of population growth.
Explain how density-dependent and density-independent factors may affect population growth.
Explain how biotic and abiotic factors may work together to control a population’s growth.