1. Chapter 52
Population Ecology

2. 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. Density and Dispersion
Density is the number of individuals per unit area or volume
Dispersion is the pattern of spacing among individuals within the boundaries of the population
Immigration
Births
Determining the density of natural populations is difficult (mark & recature)
In most cases, it is impractical or impossible to count all individuals in a population
Density is the result of an interplay between processes that add individuals to a population and those that remove individuals
Population
size
Deaths
Emigration

4. Patterns of Dispersion
Environmental and social factors influence spacing of individuals in a population
In a clumped dispersion, individuals aggregate in patches
A clumped dispersion may be influenced by resource availability and behavior
Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.
A uniform dispersion is one in which individuals are evenly distributed
It may be influenced by social interactions such as territoriality
Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.
In a random dispersion, the position of each individual is independent of other individuals
Random. Dandelions grow from windblown seeds that land at random and later germinate.

5. Demography
Demography is the study of the vital statistics of a population and how they change over time
Death rates and birth rates are of particular interest to demographers
Life tables, survivorship curves and reproductive rates help demographers show change in populations over time

6. Life Tables
A life table is an age-specific summary of the survival pattern of a population
It is best made by following the fate of a cohort
The life table of Belding’s ground squirrels reveals many things about this population

7. Number of survivors (log scale)
Survivorship Curves
A survivorship curve is a graphic way of representing the data in a life table
The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate
1,000
Females
Plots of the individuals that are alive at the start of each year
100
Males
Number of survivors (log scale) 10 1 2 4 6 8 10
Age (years)

8. Number of survivors (log scale) Percentage of maximum life span
Survivorship curves can be classified into three general types: Type I, Type II, and Type III
1,000 I
100
Number of survivors (log scale) II 10
III 1 50
100
Percentage of maximum life span

9. Reproductive Rates
A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population
It describes reproductive patterns of a population

10. Concept 52.2: Life history traits are products of natural selection
Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

11. Life History Diversity
Life histories are very diverse (reproductive age varies)
Allocation of limited resources
Number of reproductive episodes per lifetime
Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die
Agave and salmon
Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly
Lizzards
IF survival rate is LOW (unpredictable environments) semelparity is favored (increases survivorship)
IF survival rate is HIGH (more stable environments) Iteroparity is favored

12. LE 52-8a
Selective pressures mandate trade-offs between investment in reproduction and survival
Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.
In animals, parental care of smaller broods may facilitate survival of offspring

13. It is useful to study population growth in an idealized situation
Concept 52.3: The exponential model describes population growth in an idealized, unlimited environment
It is useful to study population growth in an idealized situation
Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth

14. Per Capita Rate of Increase
If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
Zero population growth (ZPG) occurs when the birth rate equals the death rate
Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:
N = population size
r = increase in growth rate
K = carrying capacity
t = time interval dN
d t  rN

15. Exponential Growth dN  rmaxN dt N = population size
Exponential population growth is population increase under idealized conditions
Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase
Equation of exponential population growth:
N = population size
rmax = intrinsic rate of increase
K = carrying capacity
t = time interval dN dt 
rmaxN
Exponential population growth results in a J-shaped curve
When r is greater than 0, populations are growing exponenetially

16. 2,000 dN
= 1.0N dt
1,500 dN
= 0.5N dt
Population size (N)
1,000
500 5 10 15
Number of generations

17. The J-shaped curve of exponential growth characterizes some rebounding populations
8,000
6,000
Elephant population
4,000
2,000
1900
1920
1940
1960
1980
Year

18. Concept 52.4: The logistic growth model includes the concept of carrying capacity
Exponential growth cannot be sustained for long in any population
A more realistic population model limits growth by incorporating carrying capacity

19. The Logistic Growth Model
Carrying capacity (K) is the maximum population size the environment can support
The Logistic Growth Model
In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached
We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N increases
The logistic growth equation includes K, the carrying capacity dN dt 
(K  N) K
rmax N
When r is equal/less than 0, populations are growing logistically

20. The logistic model of population growth produces a sigmoid (S-shaped) curve
2,000 dN
= 1.0N
Exponential
growth dt
1,500
K = 1,500
Logistic growth
Population size (N)
1,000 dN
1,500 – N
= 1.0N dt
1,500
500 5 10 15
Number of generations

21. Number of Paramecium/mL
The Logistic Model and Real Populations
1,000
The growth of laboratory populations of paramecia fits an S-shaped curve
800
600
Number of Paramecium/mL
400
200 5 10 15
Time (days)
A Paramecium population in the lab

22. Number of Daphnia/50 mL 20 40 60 80 100 120 140 160 Time (days)
Some populations overshoot K before settling down to a relatively stable density
180
150
120
Number of Daphnia/50 mL 90 60 30 20 40 60 80
100
120
140
160
Time (days)
A Daphnia population in the lab

23. The Logistic Model and Life Histories
The logistic model fits few real populations but is useful for estimating possible growth
Life history traits favored by natural selection may vary with population density and environmental conditions
K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density
Sickness/disease, competition, territoriality, health
birth rates fall and death rates rise with population density
Density-dependent birth and death rates are an example of negative feedback that regulates population growth
r-selection, or density-independent selection, selects for life history traits that maximize reproduction
Natural disasters
birth rate and death rate do not change with population density

24. Competition for Resources
LE 52-15
Competition for Resources
In crowded populations, increasing population density intensifies intraspecific competition for resources
4.0
10,000
3.8
3.6
Average number of seeds
per reproducing individual
(log scale)
1,000
Average clutch size
3.4
3.2
3.0
100
2.8 1 10
100 10 20 30 40 50 60 70 80
Plants per m2 (log scale)
Females per unit area
Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.
Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

25. Territoriality (density dependent, k-selection)
Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries
In many vertebrates and some invertebrates, territoriality may limit density
Oceanic birds exhibit territoriality in nesting behavior

26. Health (density dependent, k-selection)
Population density can influence the health and survival of organisms
In dense populations, pathogens can spread more rapidly
Predation (density dependent, k-selection)
As a prey population builds up, predators may feed preferentially on that species
Toxic Wastes (density dependent, k-selection)
Accumulation of toxic wastes can contribute to density-dependent regulation of population size
Intrinsic Factors (density dependent, k-selection)
For some populations, intrinsic (physiological) factors appear to regulate population size

27. Population Dynamics
The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

28. Metapopulations and Immigration
Metapopulations are groups of populations linked by immigration and emigration
High levels of immigration combined with higher survival can result in greater stability in populations
Song sparrow populations on a cluster of small islands make up a metapopulation. Immigration keeps the linked populations more stable than the isolated population on the larger island. 60 50
Mandarte
Island 40 30
Number of breeding females 20
Small
islands 10
1988
1989
1990
1991
Year

29. Many populations undergo boom-and-bust cycles
Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors
Snowshoe hare
160
120
Lynx 9
Hare population size
(thousands)
Lynx population size
(thousands) 80 6 40 3
1850
1875
1900
1925
Year

30. Concept 52.6: Human population growth has slowed after centuries of exponential increase
No population can grow indefinitely, and humans are no exception

31. Human population (billions)
The Global Human Population
The human population increased relatively slowly until about 1650 and then began to grow exponentially 6 5 4
Human population (billions) 3 2
The Plague 1
8000
B.C.
4000
B.C.
3000
B.C.
2000
B.C.
1000
B.C.
1000
A.D.
2000
A.D.

32. Annual percent increase
LE 52-23
Though the global population is still growing, the rate of growth began to slow about 40 years ago
2.2 2
1.8
1.6
2003
1.4
Annual percent increase
1.2 1
0.8
0.6
0.4
0.2
1950
1975
2000
2025
2050
Year

33. Regional Patterns of Population Change
To maintain population stability, a 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

34. Birth or death rate per 1,000 people50 40 30
Birth or death rate per 1,000 people 20 10
Sweden
Mexico
Birth rate
Birth rate
Death rate
Death rate
1750
1800
1850
1900
1950
2000
2050
Year

35. The demographic transition is associated with various factors in developed and developing countries
Family planning
Volunteer contraception
Delayed marriage and reproduction
Education

36. Age Structure
One important demographic factor in present and future growth trends is a country’s age structure
Age structure is the relative number of individuals at each age
It is commonly represented in pyramids
Age structure diagrams can predict a population’s growth trends
They can illuminate social conditions and help us plan for the future

37. Infant Mortality and Life Expectancy
Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition

38. Estimates of Carrying Capacity
The carrying capacity of Earth for humans is uncertain
At more than 6 billion people, the world is already in ecological deficit

39. Ecological Footprint
The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation
It is one measure of how close we are to the carrying capacity of Earth
Countries vary greatly in footprint size and available ecological capacity

40. Ecological footprint (ha per person) Available ecological capacity16
Ecological deficit 14 12
New Zealand 10
USA
Germany
Ecological footprint (ha per person) 8
Australia
Netherlands
Japan
Canada
Norway 6
Sweden UK 4
Spain
World 2
China
India 2 4 6 8 10 12 14 16
Available ecological capacity
(ha per person)