1. Chapter 53
Population Ecology

2. Population Ecology A population
Is a group of individuals of a single species living in the same general area
Population ecology is the study of populations in relation to environment
Including environmental influences on population density and distribution, age structure, and variations in population size
Northern Fur Seals

3. Density and Dispersion
Is the number of individuals per unit area or volume
Dispersion
Is the pattern of spacing among individuals within the boundaries of the population

4. Population Dynamics
Density is the result of a dynamic interplay between processes that add individuals to a population and those that remove individuals from it
Births and immigration add individuals to a population.
Births
Immigration
PopuIation size
Emigration
Deaths
Deaths and emigration remove individuals from a population.
Ins
Outs

5. Patterns of Dispersion
Environmental and social factors
Influence the spacing of individuals in a population

6. A clumped dispersion Is one in which individuals aggregate in patches
May be influenced by resource availability and behavior
(a) 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.

7. A uniform dispersion
Is one in which individuals are evenly distributed
May be influenced by social interactions such as territoriality
(b) 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.

8. A random dispersion
Is one in which the position of each individual is independent of other individuals
(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

9. 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

10. Life Tables A life table
Is an age-specific summary of the survival pattern of a population

11. Life table of Belding’s ground squirrels

12. Survivorship Curves A survivorship curve
Is a graphic way of representing the data in a life table
Shows a pattern of survival

13. Survivorship curve for Belding’s ground squirrels
1000
100 10 1
Number of survivors (log scale) 2 4 6 8
Age (years)
Males
Females
This survivorship curve shows a fairly constant death rate

14. Survivorship curves (types I, II, and III)50
100 1 10
1,000
Percentage of maximum life span
Number of survivors (log scale)

15. Reproductive Rates A reproductive table, or fertility schedule
Is an age-specific summary of the reproductive rates in a population

16. Life History Life history traits
Traits that affect an organism’s schedule of reproduction and survival
They are Trade-Offs between survival and reproduction (parental care, # of offspring, etc.)
They are very diverse
They are evolutionary outcomes that are reflected in the development, physiology, and behavior of an organism

17. Life History Life history entails 3 basic variables
Age at which reproduction begins
Frequency of reproduction
# of offspring produced per reproductive cycle

18. Semelparity, or “big-bang” reproduction
Reproduce a single time and die
Iteroparity, or repeated reproduction
Produce offspring repeatedly over time
Agave
Rabbit

19. Life History trade-off
Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)
EXPERIMENT
The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.
CONCLUSION
100 80 60 40 20
Reduced brood size
Normal brood size
Enlarged brood size
Parents surviving the following winter (%)
Male
Female
RESULTS

20. Some plants produce a large number of small seeds ensuring that at least some of them will grow and eventually reproduce
(a) 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.

21. Some plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

22. Population Growth A Couple Questions for Population Ecologists.
What environmental factors stop a population from growing?
Why do some populations show radical fluctuations in size over time, while others remain stable?

23. Population Change and Population Density
In density-independent populations
Birth rate and death rate do not change with population density
In density-dependent populations
Birth rates fall and death rates rise with population density

24. Population-Limiting Factors
Competition
Territoriality
Health
Predation
Waste accumulation
Intrinsic factors (factors related to the biology/physiology of the particular species)

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

26. Territoriality
In many vertebrates and some invertebrates territoriality may limit density
Cheetahs are highly territorial
Chemical communication can warn other cheetahs of their boundaries

27. Territoriality Territories can be small
Gannets exhibit territoriality in nesting behavior

28. Health
Population density can influence the health and survival of organisms
In dense populations pathogens can spread more rapidly

29. Predation As a prey population builds up
Predators may feed preferentially on that species

30. Toxic Wastes The accumulation of toxic wastes
Can contribute to density-dependent regulation of population size
The Wine Example
Alcohol content of wine generally peaks out at about 13%. Beyond that level the yeast die off from ethanol toxicity

31. Intrinsic Factors For some populations
Intrinsic (physiological) factors appear to regulate population size
(Such as an increase in aggressive behavior in a crowded colony of animals leading to a decrease in reproduction rate) Stress has a negative impact on the reproductive system.

32. Population Dynamics Population dynamics
Focuses on the interactions between biotic and abiotic factors that cause variation in population size

33. Commercial catch (kg) of male crabs (log scale)
Extreme fluctuations in population size are typically more common in invertebrates than in large mammals
1950
1960
1970
1980
Year
1990
10,000
100,000
730,000
Commercial catch (kg) of male crabs (log scale)

34. Metapopulations and Immigration
Groups of populations
linked by immigration and emigration
This butterfly exists in many small populations. Individuals migrate between patches (populations) and can also colonize unoccupied patches.

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

36. Human Populations
Human population growth has slowed after centuries of exponential increase
No population can grow indefinitely
humans are no exception

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

38. The rate of growth began to slow approximately 40 years ago
1950
1975
2000
2025
2050
Year
2003
Percent increase
2.2 2
1.6
1.4
1.2 1
0.8
0.6
0.4
0.2
1.8
The rate of growth began to slow approximately 40 years ago

39. Regional Patterns of Population Change
To maintain population stability (zero population growth)
A regional human population can exist in one of two configurations
Zero population growth = High birth rates – High death rates
Zero population growth = Low birth rates – Low death rates

40. The demographic transition is the move from the first toward the second state.
Is associated with various factors in developed and developing countries
The transition in Sweden was slow. In Mexico the change was abrupt early and has since slowed. 50 40 20 30 10
1750
1800
1850
1900
1950
2000
2050
Birth rate
Death rate
Year
Sweden
Mexico
Birth or death rate per 1,000 people

41. Age Structure The relative number of individuals at each age
Affects present and future growth trends

42. Age structure represented in pyramids
Rapid growth Afghanistan
Slow growth United States
Decrease Italy
Male
Female
Age 8 6 4 2
Percent of population
80–84
85
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
20–24
25–29
10–14
5–9
0–4
15–19

43. Global Carrying Capacity
Just how many humans can the biosphere support?
…no agreement in the scientific community on a number.
But, estimates of global population in 2050 are between 8 and 11 billion (currently about 6 billion)
Will technological advances continue to increase the Earth’s capacity for humans.

44. Ecological Footprint Concept
The aggregate land and water area needed to sustain the people of a nation. What area is needed to produce the resources consumed and absorb wastes generated.
Ecological capacity
the actual resource base of a country
Looking at the ecological footprint is one measure of how close we are to the carrying capacity of Earth

45. Ecological footprints for 13 countries
Show that the countries vary greatly in their footprint size and their available ecological capacity 16 14 12 10 8 6 4 2
New Zealand
Australia
Canada
Sweden
World
China
India
Available ecological capacity (ha per person)
Spain UK
Japan
Germany
Netherlands
Norway
USA
Ecological footprint (ha per person)
Red dots are countries that were in ecological deficit in 1997
At more than 6 billion people the world is already in ecological deficit