1. Chapter 53
Population Ecology

2. You Must Know:
How density, dispersion, and demographics can describe a population.
The differences between exponential and logistic models of population growth.
How density-dependent and density- independent factors can control population growth.

3. Life takes place in populations
group of individuals of same species in same area at same time
rely on same resources
interact
interbreed
Population Ecology: What factors affect a population?

4. Introduction
Population = group of individuals of a single species living in same general area
Density: # individuals / area
Dispersion: pattern of spacing between individuals

5. Determining population size and density:
Count every individual
Random sampling
Mark-recapture method

6. WHITE BOARDS!

7. # marked / total population = (# recaptured and marked) / # recaptured
WHITE BOARDS!
# marked / total population = (# recaptured and marked) / # recaptured

8. # marked / total population = (# recaptured and marked) / # recaptured
WHITE BOARDS!
# marked / total population = (# recaptured and marked) / # recaptured

9. Patterns of Dispersal:
Clumped – most common; near required resource
Uniform – usually antagonistic interactions
Random – unpredictable spacing, not common in nature

10. Clumped Pattern (most common)
The most common pattern of dispersion is clumped, with the individuals aggregated in patches. Plants or fungi are often clumped where soil conditions and other environmental factors favor germination and growth. For example, mushrooms may be clumped on a rotting log. Many animals spend much of their time in a particular microenvironment that satisfies their requirements. Forest insects and salamanders, for instance, are frequently clumped under logs, where the humidity tends to be higher than in more exposed areas. Clumping of animals may also be associated with mating behavior. For example, mayflies often swarm in great numbers, a behavior that increases mating chances for these insects, which survive only a day or two as reproductive adults. Group living may also increase the effectiveness of certain predators; for example, a wolf pack is more likely than a single wolf to subdue a large prey animal, such as a moose

11. Uniform
May result from direct interactions between individuals in the population
 territoriality
Clumped patterns
A uniform, or evenly spaced, pattern of dispersion may result from direct interactions between individuals in the population. For example, some plants secrete chemicals that inhibit the germination and growth of nearby individuals that could compete for resources. Animals often exhibit uniform dispersion as a result of antagonistic social interactions, such as territoriality —the defense of a bounded physical space against encroachment by other individuals. Uniform patterns are not as common in populations as clumped patterns.

12. Demography: the study of vital statistics that affect population size
Additions occur through birth, and subtractions occur through death.
Life table : age-specific summary of the survival pattern of a population

13. Survivorship Curve: represent # individuals alive at each age
What do these graphs tell about survival & strategy of a species?
Type I: low death rate early in life (humans)
Type II: constant death rate over lifespan (squirrels)
Type III: high death rate early in life, but survivors then live long (oysters)

14. Reproductive strategies
K-selected
late reproduction
few offspring
invest a lot in raising offspring
primates
coconut
r-selected
early reproduction
many offspring
little parental care
insects
many plants
K-selected
r-selected

15. “Of course, long before you mature, most of you will be eaten.”
Trade offs
Number & size of offspring
vs.
Survival of offspring or parent
r-selected
K-selected
“Of course, long before you mature, most of you will be eaten.”

16. Life strategies & survivorship curves25
1000
100
Human
(type I)
Hydra
(type II)
Oyster
(type III) 10 1 50
Percent of maximum life span 75
Survival per thousand
K-selection
A Type I curve is flat at the start, reflecting low death rates during early and middle life, then drops steeply as death rates increase among older age groups. Humans and many other large mammals that produce few offspring but provide them with good care often exhibit this kind of curve. In contrast, a Type III curve drops sharply at the start, reflecting very high death rates for the young, but then flattens out as death rates decline for those few individuals that have survived to a certain critical age. This type of curve is usually associated with organisms that produce very large numbers of offspring but provide little or no care, such as long–lived plants, many fishes, and marine invertebrates. An oyster, for example, may release millions of eggs, but most offspring die as larvae from predation or other causes. Those few that survive long enough to attach to a suitable substrate and begin growing a hard shell will probably survive for a relatively long time. Type II curves are intermediate, with a constant death rate over the organism’s life span. This kind of survivorship occurs in Belding’s ground squirrels and some other rodents, various invertebrates, some lizards, and some annual plants.
r-selection

17. Change in Population Size
dN/dt = B-D
N = population size
t = time
Change in population size during time interval
Births during time interval
Deaths during time interval = -

18. Factors that limit population growth:
Density-Dependent factors: population density matters
i.e. Predation, disease, competition, territoriality, waste accumulation, physiological factors
Density-Independent factors: population density not a factor
i.e. Natural disasters: fire, flood, weather

19. Biotic & abiotic factors  Population fluctuations
: peak in wolf numbers
1995: harsh winter weather (deep snow)

20. What do you notice about the population cycles of the showshoe hare and lynx?

21. Boom-and-bust cycles Predator-prey interactions
Eg. lynx and snowshoe hare on 10-year cycle

22. Introduced species Non-native species kudzu
transplanted populations grow exponentially in new area
out-compete native species
loss of natural controls
lack of predators, parasites, competitors
reduce diversity
examples
African honeybee
gypsy moth
zebra mussel
purple loosestrife
gypsy moth
kudzu

23. Population Growth Models

24. Exponential population growth: ideal conditions, population grows rapidly

25. Exponential growth rate
Characteristic of populations without limiting factors
introduced to a new environment or rebounding from a catastrophe
Whooping crane
coming back from near extinction
African elephant
protected from hunting
The J–shaped curve of exponential growth is characteristic of some populations that are introduced into a new or unfilled environment or whose numbers have been drastically reduced by a catastrophic event and are rebounding. The graph illustrates the exponential population growth that occurred in the population of elephants in Kruger National Park, South Africa, after they were protected from hunting. After approximately 60 years of exponential growth, the large number of elephants had caused enough damage to the park vegetation that a collapse in the elephant food supply was likely, leading to an end to population growth through starvation. To protect other species and the park ecosystem before that happened, park managers began limiting the elephant population by using birth control and exporting elephants to other countries.

26. Exponential Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size

27. Exponential Growth Problem
Sample Problem:
A certain population of mice is growing exponentially. The growth rate of the population (r) is 1.3 and the current population size (N) is 2,500 individuals. How many mice are added to the population each year?

28. Zebra mussel reduces diversity
~2 months
ecological & economic damage
reduces diversity
loss of food & nesting sites for animals
economic damage

29. Logistic rate of growth
Can populations continue to grow exponentially?
Of course not!
no natural controls
K = carrying capacity
Decrease rate of growth as N reaches K
effect of natural controls
What happens as N approaches K?

30. Carrying capacity
Time (years)
1915
1925
1935
1945 10 8 6 4 2
Number of breeding male
fur seals (thousands)
Maximum population size that environment can support with no degradation of habitat
varies with changes in resources
500
400
300
200
100 20 10 30 50 40 60
Time (days)
Number of cladocerans
(per 200 ml)
What’s going on with the plankton?

31. Changes in Carrying Capacity
Population cycles
predator – prey interactions
At what population level is the carrying capacity? K K

32. Unlimited resources are rare
Logistic model: incorporates carrying capacity (K)
K = maximum stable population which can be sustained by environment

33. Logistic Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size
K = carrying capacity

34. Logistic Growth Problem
Sample Problem:
If a population has a carrying capacity (K) of 900, and the growth rate (r) is 1.1, what is the population growth when the population (N) is 425?

35. WHITE BOARDS!
r is increasing
r = 0.045

36. WHITE BOARDS!
172 frogs

37. WHITE BOARDS! dN/dt = (b-d)N (1134-1492)1 = (b – 0.395) 1492
So b = 0.155
b = 0.155

38. WHITE BOARDS!
8.5 turkeys/acre
110 turkeys/year
890 turkeys

39. WHITE BOARDS!
3200 dandelions

40. WHITE BOARDS! Last one
1371 dandelions

41. Life History: traits that affect an organism’s schedule of reproduction and survival
3 Variables:
Age of sexual maturation
How often organism reproduces
# offspring during each event
Note: These traits are evolutionary outcomes, not conscious decisions by organisms

42. Semelparity Big-bang reproduction Many offspring produced at once
Individual often dies afterwards
Less stable environments
Agave Plant

43. Iteroparity Repeated reproduction Few, but large offspring
More stable environments
Lizard
Critical factors: survival rate of offspring and repeated reproduction when resources are limited

44. K-selection r-selection
K-selection: pop. close to carrying capacity
r-selection: maximize reproductive success
K-selection
r-selection
Live around K
Exponential growth
High prenatal care
Little or no care
Low birth numbers
High birth numbers
Good survival of young
Poor survival of young
Density-dependent
Density independent
ie. Humans
ie. cockroaches

45. Zero Population Growth

46. Human Population Growth
2 configurations for a stable human population (zero population growth):
High birth / high death
Low birth / low death
Demographic transition: occurs when population goes from A  B

47. Age-Structure Diagrams

48. Global Carrying Capacity
Current world population (2015) = 7.3 billion
Estimated carrying capacity = billion?
Ecological footprint: total land + water area needed for all the resources a person consumes in a pop.
1.7 hectares (ha)/person is sustainable
Typical person in U.S. = 10 ha footprint
Limitations? Consequences? Solutions?

50. Map of ecological footprint of countries in the world (proportional sizes shown)