1. Population Ecology
Chapter 45

2. Overview: Earth’s Fluctuating Populations
To understand human population growth, we must consider general principles of population ecology

3. Population ecology = study of populations relative to environment, including environmental influences on density and distribution, age structure, and population size
Population = group of individuals of a single species living in the same general area

4. Density and Dispersion
Density is the number of individuals per unit area or volume
Dispersion is the pattern of spacing among individuals within the boundaries of the population
Environmental and social factors influence spacing of individuals in a population

5. Clumped distribution may be influenced by resource availability (living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young)
Uniform distribution may be influenced by social interactions such as territoriality
Random distribution—the position of each individual is independent of other individuals

6. Demography
Study of theory and statistics behind population growth and decline
N = size of the population

7. Demographic Statistics
Birth rate = number of offspring produced per time period
Death rate = number of deaths per time period
Sex ratio = proportion of males and females in a population
Generation time = time needed for individuals to reach reproductive maturity

8. Demographic Statistics
Age structure = statistic that compares the relative number of individuals in the population from each age group
Immigration rate = rate at which individuals relocate into a given population
Emigration rate = rate which individuals relocate out of a give population

9. Population Growth
Growth rate of a population is the amount by which a population’s size changes in a given time.

10. Population Growth and Size
Biotic potential = maximum growth rate of a population given unlimited resources, unlimited space, and lack of competition or predators
Rate varies from species to species
Carrying capacity (k) = maximum number of individuals that a population can sustain in a given environment without destroying the habitat

11. Limiting Factors Control Population Sizes
Density-dependent factors come into play when population approaches and/or passes the carrying capacity
Food supplies, waste products, population-crowding diseases

12. Limiting Factors Control Population Sizes
Density-independent factors have nothing to do with the population size
Floods, droughts, earthquakes, other natural disasters and weather conditions

13. Population Growth
Exponential Growth = population grows as if there are no limitations as to how large it can get (biotic potential)
A population increases slowly at first (the "lag phase") and then grows increasingly rapidly as time passes (the "log phase"). When numbers are low, a doubling does not produce much addition to the population, but as numbers increase, each successive doubling adds larger and larger increments.

14. Population Growth
Logistic Growth = population growth slows to zero and population size tends to stabilize because of environmental resistance (limiting factors)

15. Point of Maximum growth (K/2)

16. Exponential Growth Rate Equation
Determining Population Growth per unit time (G):
where:
G = Population Growth per unit time
r = net population growth rate per individual per unit time
N = number of individuals

17. Logistic Growth Rate Equation
The S-shaped (sigmoid) curve that shows the effect of environmental resistance upon population growth can be represented by the following equation, often referred to as the logistic equation:
where K is the carrying capacity (maximum value of N for a given set of environmental conditions)

18. Carrying Capacity Number of generations Population size (N) K = 1,500
2,000
1,000
500 15 10 5
Logistic growth
Exponential
growth
= 1.0N dN dt
1,500 – N

19. Survivorship Patterns
Survivorship is the percentage of an original population that survives to a given age
Involves study of cohort population – individuals of same age
Species differ with respect to mortality patterns
Extreme forms: K versus r

20. Survivorship curves Type I Mortality rises in postreproductive years
Type II
Mortality constant throughout life
Type III
Mortality low after establishment

21. Life History Strategies
K-selected populations are of a roughly constant size whose members have low reproductive rates.
Offspring require extensive postnatal care until sufficiently matured (humans)
R-selected populations experience rapid growth
Offspring are numerous, mature rapidly, and require little postnatal care (bacteria)