1. Population
Ecology
Chapter 52

2. Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size.
Demographics refers to the study of vital statistics especially birth and death rates.

3. Fig
Figure White rhinoceros mother and calf

4. Population, Density and Dispersion
A population is a group of individuals of a single species living in the same general area
Density is the number of individuals per unit area or volume
Ex: the number of squirrels per square kilometer or
The number of esterichia coli bacteria per milliliter in a test tube.

5. Determining Density
In most cases, it is impractical or impossible to count all individuals in a population
Researchers use different methods to estimate:
One way is to count the number of individuals in a series of randomly located plots, calculate the average density in the samples, and extrapolate to estimate the population size in the entire area.
Such estimates are accurate when there are many sample plots and a homogeneous (evenly dispersed) habitat.

6. The mark-recapture method
Commonly used to estimate wildlife populations. Individuals are trapped and captured, marked with a tag, recorded, and then released.
After a period of time has elapsed, traps are set again, and individuals are captured and identified.
The second capture yields both marked and unmarked individuals.
From this data, researchers estimate the total number of individuals in the population.

7. Density in a population depends on:
Fig. 53-3
Density in a population depends on:
Immigration is the influx of new individuals from other areas
Emigration is the movement of individuals out of a population
Births
Deaths
Births and immigration
add individuals to
a population.
Deaths and emigration
remove individuals
from a population.
Figure 53.3 Population dynamics
Immigration
Emigration

8. Dispersion is the pattern of spacing among individuals within the boundaries of the population.. They may be
Clumped
Uniform
random

9. Patterns of Dispersion
Dispersion is clumped when individuals aggregate in patches.
Plants and fungi are often clumped where soil conditions favor germination and growth.
Animals may clump in favorable microenvironments (such as isopods under a fallen log) or to facilitate mating interactions.
Group living may increase the effectiveness of certain predators, such as a wolf pack.

10. Video: Flapping Geese (Clumped)
Fig. 53-4a
Figure 53.4a Patterns of dispersion within a population’s geographic range
(a) Clumped dispersion in the intertidal zone
Video: Flapping Geese (Clumped)

11. CLUMPED DISPERSION in a forest

12. Dispersion is uniform when individuals are evenly spaced.
For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors.
Animals often exhibit uniform dispersion as a result of territoriality, the defense of a bounded space against encroachment by others.

13. Fig. 53-4b
Figure 53.4b Patterns of dispersion within a population’s geographic range
(b) Uniform

14. In random dispersion, the position of each individual is independent of the others, and spacing is unpredictable.
Random dispersion occurs where there is neither a strong attraction or repulsion among individuals in a population, or when key physical or chemical environmental factors are relatively homogeneously (evenly) distributed.
For example, plants may grow where windblown seeds land.

15. Video: Prokaryotic Flagella (Salmonella typhimurium) (Random)
Fig. 53-4c
Figure 53.4c Patterns of dispersion within a population’s geographic range
(c) Random
Video: Prokaryotic Flagella (Salmonella typhimurium) (Random)

16. Demography
Demography is the study of the vital statistics of populations and how they change over time.
Of particular interest are
birth rates and how they vary among individuals (specifically females)
death rates.
A life table is an age-specific summary of the survival pattern of a population.

17. A graphic way of representing the data
Fig. 53-5
A graphic way of representing the data
in a life table is a survivorship curve.
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)

18. Survivorship curves can be classified into three general types:
A graphic way of representing the data in a life table is a survivorship curve.
Survivorship curves can be classified into three general types:
A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle life, and drops steeply as death rates increase among older age groups.
Humans and many other large mammals exhibit Type I survivorship curves.

19. The Type II curve is intermediate, with constant mortality over an organism’s life span.
Many species of rodent, various invertebrates, and some annual plants show Type II survivorship curves.
Prey species that are subject to predation.

20. A Type III curve drops off at the start, reflecting very high death rates early in life, then flattens out as death rates decline for the few individuals that survive to a critical age.
Type III are usually organisms that produce large numbers of offspring but provide little or no parental care.
Examples are many fishes, long-lived plants, and marine invertebrates.
Also young that are subject to predation and severe environmental conditions

21. Idealized survivorship curves: Types I, II, and III
Fig. 53-6
Idealized survivorship curves:
Types I, II, and III
1,000 I
100 II
Number of survivors (log scale) 10
Figure 53.6 Idealized survivorship curves: Types I, II, and III
III 1 50
100
Percentage of maximum life span

22. Life history traits
Natural selection favors traits that improve an organism’s chances of survival and reproductive success.
In every species, there are trade-offs between survival and traits such as
The age at which reproduction begins
How often the organism reproduces
How many offspring are produced during each reproductive cycle
The traits that affect an organism’s schedule of reproduction and survival make up its life history.

23. Evolution and Life History Diversity
Life histories are very diverse
Species that exhibit semelparity, or big-bang reproduction, reproduce once and die
Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly
Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction

24. Fig. 53-7
Century Plants grow in arid climates with unpredictable rainfall. After several years it sends up a large flowering stalk, reproduces and then dies.
semelparity
Figure 53.7 An agave (Agave americana), an example of big-bang reproduction

25. Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
Dandelion

26. Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
Coconut palm

27. how much does an individual gain in reproductive success through one pattern versus the other?
The critical factor is survival rate of the offspring.
In highly variable or unpredictable environments, when the survival of offspring is low, semelparity (big-bang) reproduction is favored.
In dependable environments where competition for resources is intense, iteroparity (repeated reproduction ) is favored.

28. 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

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

30. Population Growth Exponential population growth is a
population increase under idealized
conditions
rate of reproduction is at its maximum, called the intrinsic rate of increase.
Any species regardless of life history is capable of exponential growth if resources are unlimited

31. 2,000 = 1.0N 1,500 = 0.5N Population size (N) 1,000 500 5 10 15
Fig
2,000
Exponential curve dN =
1.0N dt
1,500 dN =
0.5N dt
J shaped curve
Population size (N)
1,000
500
Figure Population growth predicted by the exponential model 5 10 15
Number of generations

32. Fig
The J-shaped curve of exponential growth characterizes some rebounding populations such as the African Elephant
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

33. The Logistic Growth Model (logical)
Exponential growth cannot be sustained for long in any population.
Limiting resources, food, water, space eventually limit population growth.
Carrying capacity (K) is the maximum population size the environment can support.

34. In the logistic growth model, per capita rate of increase slows down or declines as carrying capacity is reached.

35. Fig
Figure How well do these populations fit the logistic growth model?
1,000
180
150
800
120
Number of Paramecium/mL
600
Number of Daphnia/50 mL 90
400 60
200 30
Figure How well do these populations fit the logistic growth model? 5 10 15 20 40 60 80
100
120
140
160
Time (days)
Time (days)
(a) A Paramecium population in the lab
(b) A Daphnia population in the lab

36. S shaped curve Exponential growth 2,000 = 1.0N 1,500 K = 1,500
Fig
J shaped curve
Exponential
growth
2,000 dN =
1.0N dt
1,500
K = 1,500
S shaped curve
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

37. The Logistic Model and Life Histories
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
r-selection, or density-independent selection, selects for life history traits that maximize reproduction

38. Density- Dependent factors that regulate population growth
Factors that reduce birth rate or increase death rates are density dependent.
Competition for resources such as food, space, water or essential nutrients intensifies as populations increase.
Territoriality- available space for territory or nesting may be limited.

39. Disease- Increasing densities allow for easier transmission of disease.
Swine flu is a perfect example
Predation- As prey populations increase, predators may find prey more easily.
Density-dependent regulation provides a negative feedback system that helps reduce birth rates and increase death rates or a population would grow exponentially!

40. Density Independent Factors
When a birth or death rate does not change with regard to population density it is said to be density independent.
Natural Disasters will cause an increase in death rate regardless of density.
Weather and climate such as floods or
Drought are density independent factors.

41. The human population
Global human populations have grown almost continuously throughout history.
But skyrocketed after the industrial revolution.
Note the dip resulting from the plague in Europe during the 1300 s

42. 7 6 5 4 3 2 1 Human population (billions) The Plague 8000 B.C.E. 4000
Fig 7 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.

43. The Global Human Population
The human population increased relatively slowly until about 1650 and then began to grow exponentially
Though the global population is still growing, the rate of growth began to slow during the 1960s

44. Famine in china 1960s about 60 million died
Fig
2.2
2.0
1.8
1.6
1.4
2005
Annual percent increase
1.2
Famine in china 1960s about 60 million died
Projected
data
1.0
0.8
0.6
Figure Annual percent increase in the global human population (data as of 2005)
0.4
0.2
1950
1975
2000
2025
2050
Year

45. Age Structure Pyramids
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
Each horizontal bar represents a specific age group dividing male and female sides

46. Wide bottom small top Developing nations rapid growth
Fig
Wide bottom small top Developing nations rapid growth
Columnar structure industrialized nation slow growth
Small bottom wider in the middle stable population
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

47. Estimates of Carrying Capacity
The carrying capacity of Earth for humans is uncertain
The average estimate is 10–15 billion people
A concept termed ecological footprint examines the total land and water area needed for all resources a person consumes in a population.
Currently 1.7 hectares (app 4.2 acres) per person is considered sustainable.
A typical person in the United States has a footprint of 10 hectares (almost 25 acres)