1. Chapter 8 Population Ecology
Instructor R. Zamora
AP Environmental Science
Edinburg North High School

2. Core Case Study
Southern Sea Otters: Are They Back From the Brink of Extinction?
Historical abundance and distribution: 1 million along NA Pacific Coast
Habitat: Kelp Forests
Habits: Use tools to eat shellfish

3. By early 1900s, hunted to near extinction
Fur
Viewed as competitors for shellfish
 kelp forests disappeared
Keystone species
Depredates herbivorous invertebrates (e.g., sea urchins)
Maintains ecologically and economically important kelps
Recovery of otter populations
 recovery of kelp forests and overall diversity
 upsets commercial and recreational shellfishers
Focus of this chapter, Population dynamics – study of how populations change in their distribution, numbers, age structure, and density.

4. Focus 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?

5. Population Dynamics and Carrying Capacity
Population Distribution
Three patterns of distribution or dispersion:
Clumping – most common
Uniform – when there is intense competition for resources
Random – least common

6. Four reasons for clumping distributions
Resources vary from place to place
Living in groups provides protection against predators
Living in groups gives some predators a better chance at getting food
Mating or caring for young

7. Changes in Population Size: Entrances and Exits
Population size is influenced by:
Births
Deaths
Immigration
Emigration

8. Age structure: Young Populations Can Grow Fast
Rate of population change depends on age structure – proportion of individuals at various ages.
Usually described by three main categories:
Pre-reproductive ages (juvenile or immature)
Reproductive ages (adult)
Post-reproductive (senescent)
Senescents
Adults
Immatures
Growing Pop.
Stable
Pop.
Decreasing Pop.

9. Limits on Population Growth: Biotic Potential vs
Limits on Population Growth: Biotic Potential vs. Environmental Resistance
No pop. can grow indefinitely  limits to growth in nature (lesson from one of nature’s four sustainability principles)
Pops. vary in their biotic potential – capacity for growth.
Intrinsic rate of increase (r) – rate at which a pop would grow if it had unlimited resources.
Populations with high r:
Reproduce early in life
Have short generation times
Can reproduce many times
Have many offspring each time they reproduce
Example: House fly  5.6 x 106 descendants in 13-mo
Example: Bacteria w. generation time of 20-min  0.3-m deep layer over the earth in 36-h

10. There is a size limit to growth imposed by limiting factors.
Limiting factors: water, light, living space, nutrients, competition, predation, and disease.
Environmental resistance – all factors that limit growth of a pop
Negative, or corrective feedback
Biotic potential and environmental resistance lead to carry capacity (K) – the maximum population size that a particular habitat can sustain indefinitely w/o degrading the habitat.

11. Exponential and Logistic Population Growth: J-curves and S-curves
With ample resources a pop can grow rapidly, but as resources become limited, its growth rate slows and levels off.
With few, or no limitations populations grow exponentially (exponential growth) at a fixed rate (e.g., 2%).
N-t plot produces a J-shaped curve
Logistic growth involves rapid growth followed by a steady decline w/ time until pop size levels off.
Decrease occurs as pop experiences environmental resistance
N-t plot produces a S-shaped (or sigmoid) curve

12. Figure 8-3. No population can continue to increase in size indefinitely.

13. Figure 8-4. Logistic growth of sheep after being introduced to the island of Tasmania

14. Brown tree snake Multiplied exponentially Up to 5000 km-2 Venomous
Caused more than 2000 power outages
Caused the extinction of 8 out of 11 of Guam’s forests birds.
Figure 8-5. Brown tree snake was accidentally introduced to Guam during WWII.

15. What influence would a decline in population size of a keystone species have on community composition?
Decrease in populations of species dependent on the keystone species.
Increase in species that move in to occupy part or all of vacant niches.

16. Exceeding Carrying Capacity: Move, Change Habits, or Decline in Size
The transition from exponential growth to logistic growth may not be smooth.
Occurs because of a reproductive lag time.
Dieback, or crash ensues (Fig. 8-6)
Figure 8-6. Exponential growth, overshoot, and population crash after introduction to St. Paul Island in Bearing Sea in 1910.

17. Carrying capacity if an area or volume is not fixed.
Habitat may be degraded by the population that exceeded K.
Also, K varies temporally increasing or decreasing seasonally or year to year.
Weather
Climate
Other factors
K for a population man increase by developing adaptive traits through natural selection.
Population may migrate when K has been exceeded.
Humans are not exempt from population overshoot and dieback.
Ireland, 1845, 1 million died, 3 million migrated
Polynesians on Eater Island, pop crashed after using up most of island trees
Earth’s carrying capacity for humans has been extended by technological, social, and cultural changes.

18. Population Density and Population Change: Effects of Crowding
Population density – the number of individuals in a population found in a particular area or volume.
Pop density can affect how rapidly it can grow or decline.
Some control factors are not affected by population density.
Density-dependent factors can control population size increase as the density increases.
Competition, predation, parasitism, and diseases (e.g., bubonic plague in the 14th century)
Tend to regulate a pop at a fairly constant size, often near K
Density independent factors control independently of pop density.
Mostly abiotic
Examples: freezes, floods, hurricanes, fire, pollution, and habitat destruction

19. Types of Population Change Curves in Nature
Four general patterns:
Stable – size fluctuates slightly above and below K
Characteristic of species in stable environments
Irruptive – explosive growth to a high peak and then crash.
Characteristic of short-lived, rapidly reproducing species
Linked to seasonal changes in weather and nutrient availability
Cyclic – regular cycles of increase and decrease
Rise and fall of lemmings every 3-4 years
Lynx and snowshoe hare, 10-yr (Fig 8-7)
Top-down pop regulation
Bottom-up regulation
Irregular – no pattern in change of population size
Figure 8-7

20. Cases Study: Exploding White-tailed Deer Population in the US
Since the 1930s the white-tailed deer population in the US has exploded.
By 1900, reduced to
1920s and 30s laws passed to protect deer, and wolves and mountain lions nearly eliminated
Today there are million
Problem with the rebound
Encroachment
Suburbanization
Vector for Lyme disease
Solutions
Change hunting regulations
Trap and relocate
Birth control

21. Reproductive Patterns
Ways to reproduce: Sexual Partners Not Always Needed
Asexual Reproduction
Produces clones
Common in taxa such as bacteria, plants and some animals such as corals.
Sexual Reproduction
Mixes genetic material of two parents producing offspring w/ genetic traits of each parent.

22. Disadvantages of sexual reproduction
First, males don’t give birth; female has to produce twice as many offspring to break even.
Second, increased change of genetic errors separation and recombination of chromosomes.
Third, courtship and mating is expensive (time and energy budgets), can cause disease, and injury may be inflicted in males that combat for mates.
Advantages
Provides genetic diversity in offspring
Males of some species can help raise young
Figure Courtship display

23. Reproductive Patterns: Opportunists and Competitors
Species differ in reproductive strategies to help ensure survival.
(instead opportunists)
(good competitors)

24. Most species have reproductive patterns between extreme r- and K- selected species.
Reproductive patterns may give a species a temporary advantage, but the ultimate population regulator is available habitat.
Figure Positions of r-selected and K-selected species on the sigmoid population growth curve.

25. Survivorship Curves
A representation of age structure that shows the percentage of members surviving at different ages (Fig. 8-11)
There are three generalized curves: late loss, early loss, and constant loss.
A life table shows projected life expectancy and probability of death for individuals at each age in a survivorship curve.
Figure Survivorship curves for populations of different species.

26. The problems to be faced are vast and complex, but come down to this: 6.7 billion people are breeding exponentially. The process of fulfilling their wants and needs is stripping earth of its biotic capacity to support life; a climactic burst of consumption by a single species is overwhelming the skies, earth, waters, and fauna.
-Paul Hawken
The next chapter applies the principles of population dynamics discussed in this chapter to the growth of human population and its environmental impact.
The principle of population dynamics are also used to help us harvest fish and wildlife resources more sustainably.
Figure 9-1. Crowded street in China.