1. Population Ecology
Chapter 53

2. YOU MUST KNOW
How density, dispersion, and demographics can describe a population
The differences between exponential and logistic models of population growth
How density-dependent and density-independent factors can control population growth
The effect of age distributions and fecundity on human populations as presented in age-structure pyramids

3. Reminder:
A population is a group of individuals of a single species living in the same general area

4. Populations
A. Population Ecology – how biotic and abiotic factors influence density, distribution, size, and age structure of populations
Density – number of individuals per unit of area or volume
1. Increases by births & immigration, decreases by deaths and emigration

5. Population Sampling:
Sampling techniques can be used to estimate densities and total population sizes
Population size can be estimated by either extrapolation from small samples, an index of population size, or the mark-recapture method

6. Dispersion – pattern of spacing among individuals
1. Clumped, uniform, or random

7. Demography – study of vital statistics of a population, especially birth and death rates, represented by survivorship curves

8. Survivorship curves
Type I – low death rates during early and mid life, increase sharply among older groups (large organisms with long life spans
Type II – constant death rate over life span, often prey
Type III – high death rates early, flat in older groups

9. Births and immigration add individuals to a population.
Fig. 53-3
Births
Deaths
Births and immigration
add individuals to
a population.
Deaths and emigration
remove individuals
from a population.
Figure 53.3 Population dynamics
Immigration
Emigration

10. Population Growth
Useful to study in ideal situation of no migration
dN – size of population at a particular instant in time
dt - time interval of the calculation
B = births
D = deaths
Change in Population size over time

11. Sample Problem
In 2006 the United States had a population of about 300 million people. If there were 14 births and 8 deaths per 1000 people, what was the country’s net population growth that year? (ignore immigration and emigration)
Net gain of 6/1000, (300,000,000/1000 x 6) dN dt 
bN-dN

12. Sample Problem
In 2006 the United States had a population of about 300 million people. If there were 14 births and 8 deaths per 1000 people, what was the country’s net population growth that year? (ignore immigration and emigration)
Net gain of 6/1000, (300,000,000/1000 x 6)
Net gain of 6 per every (14-8)
(300,000,000/1000) x 6
1,800,000 dN dt 
bN-dN

13. Population Growth
A. Exponential - occurs under ideal conditions
rmax – maximum per capita rate of increase
N – population size

14. B. Logistic growth – increases exponentially at first but then levels off as it approaches carrying capacity
K = carrying capacity

15. 1. Carrying capacity – max population size that the environment
1. Carrying capacity – max population size that the environment can support at a given time without degrading the habitat
2. If there is no migration, population growth rate equals birth rate – death rate

16. Sample problem:
If a population is 1600, and the rmax is 1.0, what is the growth of a population with a carrying capacity of 1500? dN dt 
(K  N) K
rmax N

17. Sample problem:
If a population is 1600, and the rmax is 1.0, what is the growth of a population with a carrying capacity of 1500? dN dt 
(K  N) K
rmax N
(1.0) (1600) [( )/1500] =

18. 1. How early? (at what age does reproduction begin?)
Life History – traits that affect an organism’s schedule of reproduction and survival rate
A. 3 factors
1. How early? (at what age does reproduction begin?)
2. How often? (repeated reproduction or big-bang?)
3. How many? (litter size)
Evolutionary adaptations, NOT conscious decisions

19. 1. Operates in populations living near carrying capacity
K-selection – selection for life history traits that are sensitive to population density and carrying capacity
1. Operates in populations living near carrying capacity
C. r-selection – for populations not near, carrying capacity, favors
maximizing reproductive success
More competition, more demanding young, more often in logistic growth
Little competition, undemanding young, often in exponential growth

20. Regulation of population growth (limiting factors)
Density-dependent factors – factors that limit population size that are dependent on population density
Cause death rate to rise and birth rate to fall as population density increases
Includes competition for resources & territory, disease, and predation

21. Density-independent factors – population density is irrelevant, usually things like natural disasters, drought

22. Human population growth
A. Exponential until the 1960s (2.2% at its peak, about 1.1% in 2011)
B. Demographic transition – change in birth/death rates

23. Age-structure pyramids – show relative numbers of individuals in each age group in a population
1. Used to predict and explain demographic patterns

24. Global carrying capacity 1. Currently unknown
2. Ecological footprints – total resources a person consumes
In hectares – currently thought that 1.7 hectares/person is sustainable