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Chapter 10 Population Dynamics (Understanding How Populations Work)

Chapter 10 Population Dynamics (Understanding How Populations Work). Chapter 10 Population Dynamics (Understanding How Populations Work). What Processes Determine Current Population Size?. Population size in earlier time period (N t-1 ) Number of births (B) Number of deaths (D)

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Chapter 10 Population Dynamics (Understanding How Populations Work)

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  1. Chapter 10 Population Dynamics (Understanding How Populations Work)

  2. Chapter 10 Population Dynamics (Understanding How Populations Work)

  3. What Processes Determine Current Population Size? • Population size in earlier time period (Nt-1) • Number of births (B) • Number of deaths (D) • Number of immigrants (I) • Number that emigrate (E) • Nt = Nt-1 + (B−D) + (I−E)

  4. Dynamics of Death Survivorship

  5. Age-Specific Survivorship (Lx) • Def: The proportion of individuals born into a population that survive to a specified age x. Lx = nx / n0 • x = age, • nx = number of individuals surviving to age x. • n0 = number of individuals born into population in a single time period (Cohort)

  6. Cohort Survivorship • Mark all individuals born in a single year (called a cohort). n0 • Each year, count the number of surviving individuals in the cohort. nx • Lx = proportion of original cohort still alive for each age class = x. = nx / n0

  7. Example Calculations for Cohort Survivorship

  8. Survivorship From Age-at-Death • Determine age-at-death for a sample of dead organisms. • Often based on annual growth structures. • Annual tree rings • Annual layers in fish scales and ear bones • Enamel layers in bear teeth • Ridges on horns of Dall sheep

  9. Computing Survivorship From Age-at-Death

  10. Computing Survivorship From Age-at-Death

  11. Three Types of Survivorship Curves Logarithmic Scale

  12. Mortality due to predation affects old more than young)

  13. Type 2 Survivorship Curve: Constant Mortality Rate Winter mortality due to freezing affects all ages equally Mortality due to environment affects all ages equally

  14. Type 3 Survivorship Curve: Perennial Plant Species Mortality due to predation affects seeds and seedlings more than mature plants

  15. Dynamics of Birth

  16. Age-Specific Birth Rate (mx) • Definition:The average number of young born to female organisms of a specific age x. • From direct observation of number of young produced by females. • Fecundity schedule: Age-specific birth rates for entire lifetime.

  17. Interactions Between Survivorship and Birth Rates

  18. Net Reproductive Rate (R0) • Definition:Average offspring from an individual organism during entire lifetime. R0 = Sum for all age classes {Lxmx} WHERE: x = age and Lx and mx are age-specific survivorship and birth rates.

  19. Computing Net Reproductive Rate (R0)

  20. Generation Time ( T ) • Definition:Average time from birth to when it reproduces. • = average age of mothers T = Sum (Age)(Lx)(mx) / R0

  21. Computing Generation Time (T) T = 14.67 / 7.79 = 1.88

  22. Per Capita Rate of Increase (r) • The difference Birth Rate − Death Rate • + r means births exceed deaths, so population size is increasing. • − r means births are less than deaths, population size is decreasing.

  23. Estimating r From a Life Table r = Ln (R0) / T “Ln” indicates the natural logarithm function. Generation Time Net Reproductive Rate

  24. End of Part 1:Population Dynamics

  25. Population DynamicsPart 2

  26. Understanding Population Growth Rate Ln (R0) r = _____ T High net reproductive rate results in high r (rapid population growth) Small generation time results in high r .

  27. Effect of Generation Time 20 yrs 20 yrs 20 yrs 60 yrs

  28. Effect of Generation Time 30 yrs 30 yrs 60 yrs

  29. Effect of Net Reproductive Rate

  30. How to Increase R0 = Sum Lx mx? • Increase survivorship: Longer-lived individuals have more opportunities for reproduction during life time.

  31. How to Increase R0 = Sum Lx mx? • Increase survivorship: Longer-lived individuals have more opportunities for reproduction during life time. • Increase birth rates: Increase number of offspring produced by individuals in each age class. • Question: Can an organism do both ???

  32. How to Decrease T ? • Rapid Growth Rate: Organisms reach sexually mature body size sooner. Question: What is required to do this ? • Reproduce at a smaller body size: Less time required to reach sexual maturity. Any disadvantages to this ?

  33. How to Decrease T ? • Rapid Growth Rate:Organisms reach sexually mature body size sooner. • Question: What is required to do this ? • Reproduce at a smaller body size: Less time to reach sexual maturity. • Any disadvantages to this ?

  34. Larger species take longer to grow to mature size. Larger species often reproduce throughout long life span. Higher average age of reproducing individuals Body Size and Generation Time

  35. Trade – Offs(Assuming Limited Resources) • Allocating resources to reproduction reduces resources available for adult survivorship (immune system, fat reserve). mx Lx

  36. Trade - Offs • Reproducing at an earlier age (smaller body size) means more individuals reproduce before they die.

  37. Trade - Offs • Reproducing at an earlier age (smaller body size) means more individuals reproduce before they die. • However: • Small adults produce small offspring with lower Lx than large offspring. • Smaller parents and offspring at disadvantage in competition for resources with larger individuals (lower Lx and mx)

  38. r - selected traits Short generation time Small adult body size Short life span High birth rates Small offspring Low survivorship of offspring Low Parental Care Type III Survivorship K - selected traits Long generation time Large adult body size Long life span Low birth rates Large offspring High survivorship of offspring High Parental Care Type I Survivorship r - vs K - Selected Life History

  39. Dispersal(Immigration and Emigration) • Causes of Dispersal • Over-population and depletion of resources • Environmental change alters habitat quality • Organisms carried by wind or water currents • Spatial/Temporal variation in resources • Human transport

  40. Importance of Dispersal • Gene flow among separate populations • Re-colonization of empty habitats • Enhances utilization of shifting or ephemeral resources • PROBLEM: Exotic species

  41. Dispersing/sedentary stages of organisms

  42. Northward Expansion of Tree Species After Continental Glaciers Receded 12,000 yrs BP

  43. Exotic Species: Invasion of Africanized Honeybees

  44. Expansion of Collared Doves into Europe Due to occasional long-distance dispersal of young doves in search of new territories. Why did the collared dove not occur in Europe before ???

  45. The End

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