1. 5-2 What Limits the Growth of Populations?
Concept 5-2 No population can continue to grow indefinitely because of limitations on resources and because of competition among species for those resources. 1

2. Most Populations Live Together in Clumps or Patches (1)
Population: group of interbreeding individuals of the same species
Population distribution
Clumping
Uniform dispersion
Random dispersion 2

3. Most Populations Live Together in Clumps or Patches (2)
Why clumping?
Species tend to cluster where resources are available
Groups have a better chance of finding clumped resources
Protects some animals from predators
Packs allow some to get prey 3

4. Population of Snow Geese
Figure 5.11: This large group of birds is a population, or flock, of snow geese.
Fig. 5-11, p. 112 4

5. Generalized Dispersion Patterns
Figure 5.12: This diagram illustrates three general dispersion patterns for populations. Clumps (a) are the most common dispersion pattern, mostly because resources such as grass and water are usually found in patches. Where such resources are scarce, uniform dispersion (b) is more common. Where they are plentiful, a random dispersion (c) is more likely. Question: Why do you think elephants live in clumps or groups?
Fig. 5-12, p. 112 5

6. (a) Clumped (elephants)
Figure 5.12: This diagram illustrates three general dispersion patterns for populations. Clumps (a) are the most common dispersion pattern, mostly because resources such as grass and water are usually found in patches. Where such resources are scarce, uniform dispersion (b) is more common. Where they are plentiful, a random dispersion (c) is more likely. Question: Why do you think elephants live in clumps or groups?
(a) Clumped (elephants)
Fig. 5-12a, p. 112

7. (b) Uniform (creosote bush)
Figure 5.12: This diagram illustrates three general dispersion patterns for populations. Clumps (a) are the most common dispersion pattern, mostly because resources such as grass and water are usually found in patches. Where such resources are scarce, uniform dispersion (b) is more common. Where they are plentiful, a random dispersion (c) is more likely. Question: Why do you think elephants live in clumps or groups?
(b) Uniform (creosote bush)
Fig. 5-12b, p. 112

8. (c) Random (dandelions)
Figure 5.12: This diagram illustrates three general dispersion patterns for populations. Clumps (a) are the most common dispersion pattern, mostly because resources such as grass and water are usually found in patches. Where such resources are scarce, uniform dispersion (b) is more common. Where they are plentiful, a random dispersion (c) is more likely. Question: Why do you think elephants live in clumps or groups?
(c) Random (dandelions)
Fig. 5-12c, p. 112

9. Populations Can Grow, Shrink, or Remain Stable (1)
Population size governed by
Births
Deaths
Immigration
Emigration
Population change =
(births + immigration) – (deaths + emigration) 9

10. Populations Can Grow, Shrink, or Remain Stable (2)
Age structure
Pre-reproductive age
Reproductive age
Post-reproductive age 10

11. Some Factors Can Limit Population Size
Range of tolerance
Variations in physical and chemical environment
Limiting factor principle
Too much or too little of any physical or chemical factor can limit or prevent growth of a population, even if all other factors are at or near the optimal range of tolerance
Precipitation
Nutrients
Sunlight, etc

12. Trout Tolerance of Temperature
Figure 5.13: This diagram illustrates the range of tolerance for a population of organisms, such as trout, to a physical environmental factor—in this case, water temperature. Range of tolerance restrictions prevent particular species from taking over an ecosystem by keeping their population size in check. Question: For humans, what is an example of a range of tolerance for a physical environmental factor?
Fig. 5-13, p. 113

13. Zone of physiological stress Zone of physiological stress
Lower limit
of tolerance
Higher limit
of tolerance
No organisms
Few organisms
Few organisms
No organisms
Abundance of organisms
Population size
Figure 5.13: This diagram illustrates the range of tolerance for a population of organisms, such as trout, to a physical environmental factor—in this case, water temperature. Range of tolerance restrictions prevent particular species from taking over an ecosystem by keeping their population size in check. Question: For humans, what is an example of a range of tolerance for a physical environmental factor?
Zone of intolerance
Zone of physiological stress
Zone of physiological stress
Zone of intolerance
Optimum range
Low
Temperature
High
Fig. 5-13, p. 113

14. No Population Can Grow Indefinitely: J-Curves and S-Curves (1)
Size of populations controlled by limiting factors:
Light
Water
Space
Nutrients
Exposure to too many competitors, predators or infectious diseases 14

15. No Population Can Grow Indefinitely: J-Curves and S-Curves (2)
Environmental resistance
All factors that act to limit the growth of a population
Carrying capacity (K)
Maximum population a given habitat can sustain 15

16. No Population Can Grow Indefinitely: J-Curves and S-Curves (3)
Exponential growth
Starts slowly, then accelerates to carrying capacity when meets environmental resistance
Logistic growth
Decreased population growth rate as population size reaches carrying capacity 16

17. Logistic Growth of Sheep in Tasmania
Figure 5.15: This graph tracks the logistic growth of a sheep population on the island of Tasmania between 1800 and After sheep were introduced in 1800, their population grew exponentially, thanks to an ample food supply and few predators. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep.
Fig. 5-15, p. 115

18. Population overshoots carrying capacity Carrying capacity
2.0
Population overshoots carrying capacity
Carrying capacity
1.5
Population recovers and stabilizes
Population runs out of resources
and crashes
Number of sheep (millions)
1.0
Exponential growth .5
Figure 5.15: This graph tracks the logistic growth of a sheep population on the island of Tasmania between 1800 and After sheep were introduced in 1800, their population grew exponentially, thanks to an ample food supply and few predators. By 1855, they had overshot the land’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep.
1800
1825
1850
1875
1900
1925
Year
Fig. 5-15, p. 115

19. Science Focus: Why Do California’s Sea Otters Face an Uncertain Future?
Low biotic potential
Prey for orcas
Cat parasites
Thorny-headed worms
Toxic algae blooms
PCBs and other toxins
Oil spills 19

20. Population Size of Southern Sea Otters Off the Coast of So
Population Size of Southern Sea Otters Off the Coast of So. California (U.S.)
Figure 5.B: This graph tracks the population size of southern sea otters off the coast of the U.S. state of California, 1983–2009. According to the U.S. Geological Survey, the California southern sea otter population would have to reach at least 3,090 animals for 3 years in a row before it could be considered for removal from the endangered species list. (Data from U.S. Geological Survey)
Fig. 5-B, p. 114 20

21. Case Study: Exploding White-Tailed Deer Population in the U.S.
1900: deer habitat destruction and uncontrolled hunting
1920s–1930s: laws to protect the deer
Current population explosion for deer
Spread Lyme disease
Deer-vehicle accidents
Eating garden plants and shrubs
Ways to control the deer population 21

22. Mature Male White-Tailed Deer
Figure 5.16: This is a mature male (buck) white-tailed deer.
Fig. 5-16, p. 115

23. When a Population Exceeds Its Habitat’s Carrying Capacity, Its Population Can Crash
A population exceeds the area’s carrying capacity
Reproductive time lag may lead to overshoot
Population crash
Damage may reduce area’s carrying capacity 23

24. Exponential Growth, Overshoot, and Population Crash of a Reindeer
Figure 5.17: This graph tracks the exponential growth, overshoot, and population crash of reindeer introduced onto the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, when the herd size plummeted to only 8 reindeer by Question: Why do you think the sizes of some populations level off while others such as the reindeer in this example exceed their carrying capacities and crash?
Fig. 5-17, p. 116 24

25. Population overshoots carrying capacity 2,000
1,500
Population crashes
Number of reindeer
1,000
Figure 5.17: This graph tracks the exponential growth, overshoot, and population crash of reindeer introduced onto the small Bering Sea island of St. Paul. When 26 reindeer (24 of them female) were introduced in 1910, lichens, mosses, and other food sources were plentiful. By 1935, the herd size had soared to 2,000, overshooting the island’s carrying capacity. This led to a population crash, when the herd size plummeted to only 8 reindeer by Question: Why do you think the sizes of some populations level off while others such as the reindeer in this example exceed their carrying capacities and crash?
Carrying capacity
500
1910
1920
1930
1940
1950
Year
Fig. 5-17, p. 116

26. Species Have Different Reproductive Patterns (1)
Some species
Many, usually small, offspring
Little or no parental care
Massive deaths of offspring
Insects, bacteria, algae 26

27. Species Have Different Reproductive Patterns (2)
Other species
Reproduce later in life
Small number of offspring with long life spans
Young offspring grow inside mother
Long time to maturity
Protected by parents, and potentially groups
Humans
Elephants

28. Under Some Circumstances Population Density Affects Population Size
Density-dependent population controls
Predation
Parasitism
Infectious disease
Competition for resources 28

29. Several Different Types of Population Change Occur in Nature
Stable
Irruptive
Population surge, followed by crash
Cyclic fluctuations, boom-and-bust cycles
Top-down population regulation
Bottom-up population regulation
Irregular 29

30. Population Cycles for the Snowshoe Hare and Canada Lynx
Figure 5.18: This graph represents the population cycles for the snowshoe hare and the Canadian lynx. At one time, scientists believed these curves provided evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—through predation by lynx and other predators—and bottom-up population control, in which changes in the availability of the food supply for hares help to determine their population size, which in turn helps to determine the lynx population size. (Data from D. A. MacLulich)
Fig. 5-18, p. 118 30

31. Population size (thousands)
160
Hare
Lynx
140
120
100 80
Population size (thousands) 60 40 20
Figure 5.18: This graph represents the population cycles for the snowshoe hare and the Canadian lynx. At one time, scientists believed these curves provided evidence that these predator and prey populations regulated one another. More recent research suggests that the periodic swings in the hare population are caused by a combination of top-down population control—through predation by lynx and other predators—and bottom-up population control, in which changes in the availability of the food supply for hares help to determine their population size, which in turn helps to determine the lynx population size. (Data from D. A. MacLulich)
1845
1855
1865
1875
1885
1895
1905
1915
1925
1935
Year
Fig. 5-18, p. 118

32. Humans Are Not Exempt from Nature’s Population Controls
Ireland
Potato crop in 1845
Bubonic plague
Fourteenth century
AIDS
Global epidemic 32