1. Chapter 8
Population Ecology

2. Chapter Overview Questions
What are the major characteristics of populations?
How do populations respond to changes in environmental conditions?
How do species differ in their reproductive patterns?

3. Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction?
They were over-hunted to the brink of extinction by the early 1900’s and are now making a comeback.
Figure 8-1

4. Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction?
Sea otters are an important keystone species for sea urchins and other kelp-eating organisms.
Figure 8-1

5. POPULATION DYNAMICS AND CARRYING CAPACITY
Most populations live in clumps although other patterns occur based on resource distribution.
Figure 8-2

6. (a) Clumped (elephants)
Figure 8.2
Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches.
(a) Clumped (elephants)
Fig. 8-2a, p. 162

7. (b) Uniform (creosote bush)
Figure 8.2
Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches.
(b) Uniform (creosote bush)
Fig. 8-2b, p. 162

8. (c) Random (dandelions)
Figure 8.2
Natural capital: generalized dispersion patterns for individuals in a population throughout their habitat. The most common pattern is clumps of members of a population throughout their habitat, mostly because resources are usually found in patches.
(c) Random (dandelions)
Fig. 8-2c, p. 162

9. Changes in Population Size: Entrances and Exits
Populations increase through births and immigration
Populations decrease through deaths and emigration

10. Age Structure: Young Populations Can Grow Fast
How fast a population grows or declines depends on its age structure.
Prereproductive age: not mature enough to reproduce.
Reproductive age: those capable of reproduction.
Postreproductive age: those too old to reproduce.

11. Limits on Population Growth: Biotic Potential vs
Limits on Population Growth: Biotic Potential vs. Environmental Resistance
No population can increase its size indefinitely.
The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources.
Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat.

12. Exponential and Logistic Population Growth: J-Curves and S-Curves
Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off.
Figure 8-4

13. Environmental Resistance
Carrying capacity (K)
Population size (N)
Biotic
Potential
Exponential
Growth
Figure 8.3
Natural capital: no population can continue to increase in size indefinitely. Exponential growth (lower part of the curve) occurs when resources are not limited and a population can grow at its intrinsic rate of increase (r) or biotic potential. Such exponential growth is converted to logistic growth, in which the growth rate decreases as the population becomes larger and faces environmental resistance. Over time, the population size stabilizes at or near the carrying capacity (K) of its environment, which results in a sigmoid (S-shaped) population growth curve. Depending on resource availability, the size of a population often fluctuates around its carrying capacity, although a population may temporarily exceed its carrying capacity and suffer a sharp decline or crash in its numbers.
Time (t)
Fig. 8-3, p. 163

14. Exponential and Logistic Population Growth: J-Curves and S-Curves
As a population levels off, it often fluctuates slightly above and below the carrying capacity.
Figure 8-4

15. Number of sheep (millions)
Overshoot
Carrying capacity
Number of sheep (millions)
Figure 8.4
Boom and bust: 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. 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.
Year
Fig. 8-4, p. 164

16. Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size
Members of populations which exceed their resources will die unless they adapt or move to an area with more resources.
Figure 8-6

17. Population overshoots carrying capacity Number of reindeer Population
Crashes
Number of reindeer
Carrying
capacity
Figure 8.6
Exponential growth, overshoot, and population crash of reindeer introduced to 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, with the herd size plummeting to only 8 reindeer by 1950.
Year
Fig. 8-6, p. 165

18. Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size
Over time species may increase their carrying capacity by developing adaptations.
Some species maintain their carrying capacity by migrating to other areas.
So far, technological, social, and other cultural changes have extended the earth’s carrying capacity for humans.

19. Population Density and Population Change: Effects of Crowding
Population density: the number of individuals in a population found in a particular area or volume.
A population’s density can affect how rapidly it can grow or decline.
e.g. biotic factors like disease
Some population control factors are not affected by population density.
e.g. abiotic factors like weather

20. Types of Population Change Curves in Nature
Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically.
Stable: fluctuates slightly above and below carrying capacity.
Irruptive: populations explode and then crash to a more stable level.
Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles.
Irregular: erratic changes possibly due to chaos or drastic change.

21. Types of Population Change Curves in Nature
Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.
Figure 8-7

22. Population size (thousands)
Hare
Lynx
Population size (thousands)
Figure 8.7
Population cycles for the snowshoe hare and Canadian lynx. At one time scientists believed these curves provided circumstantial 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—predation by lynx and other predators—and bottom-up population control. In the latter, changes in the availability of the food supply for hares help determine hare population size, which in turn helps determine the lynx population size. (Data from D. A. MacLulich)
Year
Fig. 8-7, p. 166

23. Case Study: Exploding White-Tailed Deer Populations in the United States
Since the 1930s the white-tailed deer population has exploded in the United States.
Nearly extinct prior to their protection in 1920’s.
Today million white-tailed deer in U.S. pose human interaction problems.
Deer-vehicle collisions (1.5 million per year).
Transmit disease (Lyme disease in deer ticks).

24. REPRODUCTIVE PATTERNS
Some species reproduce without having sex (asexual).
Offspring are exact genetic copies (clones).
Others reproduce by having sex (sexual).
Genetic material is mixture of two individuals.
Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly.
Major advantages: genetic diversity, offspring protection.

25. Sexual Reproduction: Courtship
Courtship rituals consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners.
Figure 8-8

26. Reproductive Patterns: Opportunists and Competitors
Large number of smaller offspring with little parental care (r-selected species).
Fewer, larger offspring with higher invested parental care (K-selected species).
Figure 8-9

27. Carrying capacity K K species; experience K selection
Number of individuals
r species;
experience
r selection
Figure 8.9
Positions of r-selected and K-selected species on the sigmoid (S-shaped) population growth curve.
Time
Fig. 8-9, p. 168

28. Reproductive Patterns
r-selected species tend to be opportunists while K-selected species tend to be competitors.
Figure 8-10

29. Little or no parental care and protection of offspring
r-Selected Species
Cockroach
Dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental conditions
High population growth rate (r)
Population size fluctuates wildly above and below carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
Figure 8.10
Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes.
Fig. 8-10a, p. 168

30. Fewer, larger offspring High parental care and protection of offspring
K-Selected Species
Elephant
Saguaro
Fewer, larger offspring
High parental care and protection
of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental conditions
Lower population growth rate (r)
Population size fairly stable and usually close to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
Figure 8.10
Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes.
Fig. 8-10b, p. 168

32. Survivorship Curves: Short to Long Lives
The way to represent the age structure of a population is with a survivorship curve.
Late loss population live to an old age.
Constant loss population die at all ages.
Most members of early loss population, die at young ages.

33. Survivorship Curves: Short to Long Lives
The populations of different species vary in how long individual members typically live.
Figure 8-11

34. Percentage surviving (log scale)
Late loss
Constant loss
Percentage surviving (log scale)
Figure 8.11
When does death come? Survivorship curves for populations of different species, show the percentages of the members of a population surviving at different ages. Most members of a late loss population (such as elephants, rhinoceroses, and humans) live to an old age. Members of a constant loss population (such as many songbirds) die at all ages. In an early loss population (such as annual plants and many bony fish species), most members die at a young age. These generalized survivorship curves only approximate the realities of nature.
Early loss
Age
Fig. 8-11, p. 169