1. Populations
Chapter 8

2. Chapter 26 begins with the Mystery of Easter Island
Chapter 26 begins with the Mystery of Easter Island. At one time, Easter Island was forested, and supported a large population of humans who carved huge stone statues. Today the island has no forests, is sparsely populated, and the people have no memory of the culture that created the statues. What possible connections are there between the disappearance of the forests and the disappearance of the statue-carving culture? W O R K T G E H

3. Why study populations?
In the field, populations tend to be the unit of study. A population is a natural grouping, so studying populations reflects what is going on in nature.
Even so – it’s not always easy to define a population!

4. Population Science
Studying growth rates of populations helps us understand:
the effects of rapid overpopulation.
how population growth is regulated.
We can derive important lessons for humans from studies of populations in nature.

5. General Principles
A population consists of members of the same species living in the same ecosystem at the same time.
Total population increases or decreases according to the number of births, deaths, immigration, and emigration that occurs.

6. Distribution
Individuals distribute themselves in a population in three general patterns:
Clumped
Uniform
Random

7. Distribution
Clumped distribution is typical of organisms that move in groups (herds, flocks, etc.), or that cluster around resources, such as plants near a water source.

8. Distribution
Uniform distribution is typical where resources are scarce. Individuals compete to claim enough territory to support them and keep a distance from others.

9. Distribution
Random distribution is rare. Organisms may distribute randomly if resources are abundant and the organisms do not form social groups. Trees in a diverse forest may distribute randomly.

10. They live in social groups.
Male marine iguanas are highly territorial. They also compete for females. Male iguanas tend to be distributed uniformly throughout their territory. Why?
They live in social groups.
Each male has its own distinct breeding territory.
The iguana’s resources are localized.

11. Which pattern of distribution do human populations tend to show?
Clumped
Uniform
Random

12. Growth Rate
To determine the actual change in numbers of a population in a given unit of time, we look at the difference between losses (deaths and emigration) and additions (births and immigration)
(births - deaths) + (immigrants - emigrants) = change in population size.

13. What is the change in a population over a ten-year period if in that time there are 9,000 births, 2,000 deaths, 800 immigrants, and 400 emigrants?
1220
6600
7000
7400

14. Falklands Conservation
Growth Rate
If we want to know the rate (r) at which a population is increasing, we need to know:
Birth rate (b) = number of births in a population during a certain time period.
Example: 150 births in a gull population of 1000 = 150/1000 = 0.15 per year.
Falklands Conservation

15. Falklands Conservation
Growth Rate
We also need to know:
Death rate (d) = number of deaths in the same time period.
Example: 50 deaths in a gull population of 1000 = 50/1000 = per year.
Falklands Conservation

16. Growth Rate Growth rate (r) = birth rate – death rate r = b – d
Ex: 0.15 – 0.05 = 0.1 (10% per year)
minus =
percent increase

17. Growth Rate
If we want to know the actual number of individuals by which the population increased, we use this formula:
G = r x N
G = 0.1 x 1000 = an increase of 100 individuals per year.

18. Try this:
You are studying a population of 30 ferns. This year you saw six new fern plants become established, and 3 fern plants died. Calculate the growth rate of the population.
Remember: r = b – d G = r x N W O R K T G E H

19. What is the annual growth rate of a population of 10,000 sea turtles if there are 500 deaths and 1,500 births per year? 5%
10%
15%
20%

20. Choice A: $1,000,000 on your birthday.
Suppose your eccentric uncle says that for your birthday, he will give you your choice between two presents:
Choice A: $1,000,000 on your birthday.
Choice B: A penny on your birthday, two pennies the next day, four the next, and so on for 30 days.
Which would you take? Why? W O R K T G E H

21. What happened? Why did Choice B give you so much more money?
Choice A yields $ 1,000,000
Choice B yields $10,737, (Why? See: pennies.html)
What happened? Why did Choice B give you so much more money?
What does this have to do with population growth? W O R K T G E H

22. What happens to r and G when b gets big and d gets little?
Exponential Growth
Biotic potential = Maximum growth possible. This assumes a maximized birth and minimized death rate Calculated as: r = b - d G = r x N
What happens to r and G when b gets big and d gets little?

23. Exponential growth produces a J-shaped population graph.

24. Age of first reproduction affects the rate of population growth. Why?
Exponential Growth
Age of first reproduction affects the rate of population growth. Why?

25. Death rates and average lifespan also affects growth rate. Why?
Exponential Growth
Death rates and average lifespan also affects growth rate. Why?

26. Under what conditions can exponential growth occur in nature?
You’ve probably guessed that exponential growth can’t go on forever. What factors limit population growth? W O R K T G E H

27. Environmental resistance
Population Limits
Two opposing forces act on population growth.
Environmental resistance
Biotic potential

28. Environmental Resistance
Species introduced to a new environment may experience exponential growth.
Environmental resistance will eventually limit growth.
Some populations experience “boom and bust” cycles.
Others stabilize and show logistic growth.

29. Population Limits K = Carrying Capacity: # births = # deaths
The upper limit for population growth is determined by the carrying capacity of the environment.

30. Available space limits barnacle populations.
Population Limits
Available space limits barnacle populations.
Where there are many natural controls, populations tend to demonstrate logistic growth.

31. Population Limits
If a population overshoots the carrying capacity of the environment, the result is a population crash.

32. Population Limits
Where there are few natural controls, a population may rise rapidly, exceed carrying capacity, then crash as most of the population starves.

33. Which population is most likely to experience exponential growth?
Algae introduced into a small pond in North Dakota.
A migrating herd of pronghorn antelope in Eastern Oregon.
Chinook salmon in the Columbia River.

34. Cyanobacteria population boom How can this be represented graphically?
In July growth conditions for cyanobacteria become favorable
population boom
The population grows rapidly
By early September the nutrient supply has been depleted and competition for what is left is fierce. Most cyanobacteria can’t get enough and die.
How can this be represented graphically?

35. How can the cyanobacteria example be represented graphically?_
number
time

36. What caused the cyanobacteria to crash was environmental resistance that was:
Dependent on the density of the population.
Independent of the density of the population.

37. This graph shows human population over the last 14,000 years
This graph shows human population over the last 14,000 years. What kind of curve is this? What implications does this have for humans? W O R K T G E H
2012*
2006
Date
Billions
Time to add
each billion
(years)
1999
1804 1
All of human
history
1987
1927 2
123
1960 3 33
1975
1974 4 14
billions of people
1987 5 13
1999 6 12
bubonic plague
1960
2012 7* 13
1930
*projected
1830
Technical and
cultural advances
Agricultural advances
Industrial and
medical
advances

38. A growing population may become self-limiting.
Population Limits
A growing population may become self-limiting.
In fruit flies, reproduction rate drops in response to crowded conditions.
lifespan
offspring per day
days
population density

39. Seasonal weather changes
Population Limits
Density-independent factors limit populations regardless of how large or small the population is. Usually abiotic.
Examples:
Seasonal weather changes
Natural disasters
Pollution

40. Population Limits
Density-dependent factors affect a population more strongly the larger it grows. Usually biotic.
Examples:
Predation
Parasites
Disease
Resource competition

41. Which of these is a density-dependent factor?
Harsh, cold winters with lots of snow and ice.
A sudden tornado.
An outbreak of cholera in a refugee camp.
A violent earthquake.

42. Density-dependent factor Density-independent factor
A population of Bluebirds is displaced when a new housing development destroys the meadow where they nested. They move to another meadow where other nine male bluebirds live. The males compete intensely for nesting sites. At the end of the season, there are still only nine successful males. Competition for nesting sites is a:
Density-dependent factor
Density-independent factor

43. Density-dependent factor. Density-independent factor.
A squirrel population is isolated on the Capitol grounds in Salem. Heavy traffic on all sides makes it hard for squirrels to leave the grounds. Squirrel fatalities happen as squirrels try to cross the streets. Is traffic a density-dependent or density-independent factor for these squirrels?
Density-dependent factor.
Density-independent factor.

44. For each of these scenarios, list both density-independent and density- dependent factors that could be involved.
During a drought, a thick stand of young pine trees is attacked by pine bark beetles.
A large herd of deer is caught by a winter storm that buries much of their food supply. Several of the deer, suffering from parasites as well as lack of food, are caught and killed by wolves. W O R K T G E H

45. Survivorship
Populations show different patterns in survival at different ages, which in turn can affect population growth.
Early loss – many young die
Constant loss – equal loss at all ages
Late loss – high survivor of young, most deaths in old age

46. Survivorship curves very for different species, depending on their reproductive strategy.

47. (a) Developed countries
2006
2025
2050
75 and older
male
female
postreproductive (45–79 yr)
age
reproductive (15–44 yr)
0 - 14
prereproductive (0–14 yr)
millions of people
(b) Developing countries
75 and older
male
female
age
0 - 14
millions of people
Different survivorship curves can have different consequences for populations, even of the same species.

48. Birth rates do slow down as nations become more industrialized
Birth rates do slow down as nations become more industrialized. However, the world population is not evenly developed, and in developed nations, resource consumption per capita is high.

49. Based on what you have learned in Chapter 26, how can you explain the disappearance of the ancient, statue- carving culture on Easter Island? W O R K T G E H

50. Deer and Wolves
Using the worksheets provided, calculate the population change in the deer population for each year.
Population changes = births – deaths
In this case, deaths are due to both starvation and predation.

51. Deer population change
Year
Wolves
Deer
Deer offspring
Predation
Starvation
Deer population change
1997 10
2,000
800
400
100
300
1998 12
2,300
920
480
240
200
1999 16
2,500
1,000
640
500
-140
2000 22
2,360
944
880
180
-116
2001 28
2,224
996
1,120 26
-150
2002 24
2,094
836
960 2
-126
2003 21
1,968
788
840
-52
2004 18
1,916
766
720 46
2005 19
1,952
780
760 20
2006
1,972
790 30

53. Without wolves:
exponential
growth
population
crash

54. braconid wasp (predator)
How does this graph relate to your deer/wolf graph?
bean weevils (prey)
braconid wasp (predator)
A high predator
population
reduces the prey
The prey population
peaks when the
predator population
is low

55. (a) Predators often kill weakened prey

56. On the back of your graph:
Describe what happened to deer and wolf populations between 1997 and 2006.
What might have happened if wolves had NOT been introduced to the island?
Some people think it was cruel to introduce wolves. Some think it would have been cruel NOT to. Is there another management plan that would have been as good or better?

57. Recap
Population size changes through birth, death, immigration, and emigration.
Population size is regulated by environmental restraints that increase deaths or decrease births.
Populations are distributed in various patterns for social reasons or because of resource availability.