Population Ecology

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 • http://www.ibiblio.org/lunarbin/worldpop • Current world r value = 1.2% birth rate – 19.95 births/1,000 people death rate - 7.9 deaths/1,000 people Doubling time = 70/% growth rate At current r value – by 2050, population will grow to 9.6 billion people http://www.npr.org/2011/10/31/141816460/visualizing-how-a-population-grows-to-7-billion

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

13. 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

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)( )