1. Population Ecology
Ch 52

2. Population
A group of individuals of a single species living in the same geographic area
How to describe populations:
Density - # of individuals per unit area
Dispersion – the spacing between individuals in an area
Demographics – study of vital statistics of populations and how they change over time

3. Patterns of dispersion
Uniform, clumped, or random?

4. Clumped
– due to resources, mating, defense
Uniform
– often due to competition for resources by same species
Random
– in absences of strong attractions or repulsions between individuals

5. Demographics Birth rates & death rates Patterns of life expectancy
Life tables – summaries of the survival pattern of a population

6. Idealized survivorship curves
Type I – most individuals die late in life (humans, elephants)
Type II – constant death rate over lifespan (coral, birds)
Type III - large decline in young (plants, fish)

7. Exponential model Population growth in an ideal environment
Abundant resources
No external restrictions
Density independent growth

8. Change in population =

9. Births + Immigrants – Deaths – Emigrants
Ignore immigration, emigration
Per capita birth rate – per capita death rate = per capita rate of increase
rmax – maximum per capita rate for the species, under ideal conditions

10. J shaped exponential growth curve

11. Current world population: 7.3 billion
Current world r value = 1.2%
birth rate – births/1,000 people
death rate deaths/1,000 people
Doubling time = 70/% growth rate
At current r value – by 2050, population will grow to 9.6 billion people

12. Which age structure diagram represents – Italy, Kenya, and US?

13. Why hasn’t this happened?
Darwin calculated that if you started with 2 elephants and exponential growth, after 700 years the world population of elephants would be 19,000,000
Why hasn’t this happened?
available resources
competition for these resources
Resources are limited in the real world

14. Logistic Model K = carrying capacity
The maximum size that a particular environment can sustain

15. S-curve

16. Lab population of flour beetles

17. Life history What determines an organism’s reproduction & survival
3 main variables:
When reproduction begins
How often the organism reproduces
How many offspring produced per reproductive episode
There is a trade-off between present & future reproduction

18. Evolution & life history
Natural selection maximizes total lifetime reproductive output
Single, massive reproductive episode
Don’t need resources for future survival & reproduction
Repeated reproductive episodes
Produce fewer but larger offspring each time, provide more resources for offspring

19. “Big Bang” reproduction
Semelparity
Pacific Salmon – produces thousands of eggs in single reproductive opportunity
Annual plants, all grain crops
Spiders
Death may occur after single reproductive event
Advantageous if adult survival rate is low

20. Repeated reproduction or “bet-hedge”
Iteroparity
Some Lizards - few large, nutrient containing eggs each year
Perennial plants
Most mammals, all birds, most reptiles, most fish
Advantageous in highly variable conditions that affect juvenile survivorship

21. Factors for evolution Survival rate of offspring
Likelihood that adult will survive to reproduce again
Low survival of offspring – highly variable environment – big bang
More dependable environment - repeated

22. Trade offs Trade offs due to limited resources
between the number and size of the offspring
Between reproduction & survival

23. K- selection Density dependent selection
selection for traits that are sensitive to population density, and favored at high densities
Mature trees at old-growth forests
In stable environments, organisms tend to make fewer “expensive” offspring

24. r- selection Density independent selection
selection for traits that maximize reproductive success in low density (uncrowded) environments
Weeds
In unstable environments, organisms tend to make more “cheaper” offspring

25. Extreme r & K selection Unstable environment, density independent
Stable environment, density dependent
Small organisms
Larger organisms
Energy used to make each individual low
Energy used to make each individual high
Early maturity
Late maturity
Short life expectancy
Long life expectancy
Individual reproduces once
Individuals reproduce repeatedly
Type III survivor curve
Type I & II survivor curve

26. Density independent factors
Not affected by density of population Natural disasters i.e. drought, temperature extremes, hurricanes

27. Density dependent factors
Dependent on population density - competition for resources - predation - toxic waste – i.e. ethanol produced by fermentation in yeast -intrinsic factors - i.e. hormonal changes that delay sexual maturation & depress immune system in white-footed mice -territoriality - disease

28. Population dynamics
Fluctuations in populations

29. Practice problems If carrying capacity = 500 individuals
Population size (N) = 300
Maximum rate of increase (rmax) = 1.0
Solve for:
Per capita rate of increase = (rmax)( )
Population growth rate = (rmax)(N)( )

30. Practice problems If carrying capacity = 500 individuals
Population size (N) = 400
Maximum rate of increase (rmax) = 1.0
Solve for:
Per capita rate of increase = (rmax)( )
Population growth rate = (rmax)(N)( )