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Ch 14: Population Growth + Regulation dN/dt = rN dN/dt = rN(K-N)/K

NEXT WEEK: Computer sessions all on MONDAY: R AM 7-9 R PM 4-6 F AM 7-9 Lab: last 1/2 of manuscript due Lab VII Life Table for Human Pop Bring calculator! Will complete Homework 8 in lab.

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Ch 14: Population Growth + Regulation dN/dt = rN dN/dt = rN(K-N)/K

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  1. NEXT WEEK:Computer sessions all on MONDAY:R AM 7-9 R PM 4-6 F AM 7-9 Lab: last 1/2 of manuscript due Lab VII Life Table for Human PopBring calculator! Will complete Homework 8 in lab

  2. Ch 14: Population Growth + Regulation dN/dt = rN dN/dt = rN(K-N)/K

  3. Sample Exam ? A moth species breeds in late summer and leaves only eggs to survive the winter. The adult die after laying eggs. One local population of the moth increasd from 5000 to 6000 in one year. Does this species have overlapping generations? Explain. What is  for this population? Show calculations. Predict the population size after 3 yrs. Show calculations. What is one assumption you make in predicting the future population size?

  4. Objectives • Age structure • Life table + Population growth • Growth in unlimited environments • Geometric growth Nt+1 =  Nt • Exponential growth Nt+1 = Ntert • Model assumptions

  5. Exponential growth of the human population

  6. Population growth can be mimicked by simple mathematical models of demography. • Population growth (# ind/unit time) = recruitment - losses • Recruitment = births and immigration • Losses = death and emigration • Growth (g) = (B + I) - (D + E) • Growth (g) = (B - D) (in practice)

  7. How fast a population grows depends on its age structure. • When birth and death rates vary by age, must know age structure • = proportion of individuals in each age class

  8. Age structure varies greatly among populations with large implications for population growth.

  9. Population Growth: (age structure known) How fast is a population growing? per generation = Ro instantaneous rate = r per unit time =  What is doubling time?

  10. Life Table: A Demographic Summary Summary of vital statistics (births + deaths) by age class; Used to determine population growthSee printout for Life Table for example…

  11. Values of , r, and Roindicate whether population is decreasing, stable, or increasing Ro < 1 Ro =1 Ro >1

  12. Life Expectancy: How many more years can an individual of a given age expect to live?How does death rate change through time?Both are also derived from life table…Use Printout for Life Table for example…

  13. Survivorship curves: note x scale… death rate constant +plants

  14. Sample Exam ? In the population of mice we studied, 50% of each age classof females survive to the following breeding season, at which time they give birth to an average of three female offspring. This pattern continues to the end of their third breeding season, when the survivors all die of old age.

  15. Fill in this cohort life table. • Is the population increasing or decreasing? Show formula used. • How many female offspring does a female mouse have in her lifetime? • At what precise age does a mouse have her first child? Show formula used. • Draw a graph showing the surivorship curve for this mouse population. Label axes carefully. Explain how you reached your answer.

  16. Cohort life table: follows fate of individuals born at same time and followed throughout their lives. mx

  17. Survival data for a cohort(all born at same time) depends strongly on environment + population density.

  18. What are advantages and disadvantages of a cohort life table? Advantages: • Describes dynamics of an identified cohort • An accurate representation of that cohort’ behavior Disadvantages: • Every individual in cohort must be identified and followed through entire life span - can only do for sessile species with short life spans • Information from a given cohort can’t be extrapolated to the population as a whole or to other cohorts living at different times or under different conditions

  19. Static life table: based on individuals of known age censused at a single time.

  20. Static life table: avoids problem of variation in environment; can be constructed in one day (or season) n = 608

  21. E.g.: exponential population growth  = 1.04

  22. Two models of population growth with unlimited resources : • Geometric growth: • Individuals added at one time of year (seasonal reproduction) • Uses difference equations • Exponential growth: • individuals added to population continuously (overlapping generations) • Uses differential equations • Both assume no age-specific birth /death rates

  23. Difference model for geometric growthwith finite amount of time • ∆N/ ∆t = rate of ∆ = (bN - dN) = gN, • where bN = finite rate of birth or per capita birth rate/unit of time • g = b-d, gN = finite rate of growth

  24. Projection model of geometric growth (to predict future population size) • Nt+1 = Nt + gNt • =(1 + g)Nt Let  (lambda) = (1 + g), then • Nt+1 =  Nt •  = Nt+1 /Nt • Proportional ∆, as opposed to finite ∆, as above • Proportional rate of ∆ / time •  = finite rate of increase, proportional/unit time

  25. Geometric growth over many time intervals: • N1 =  N0 • N2 =  N1 = ·  · N0 • N3 =  N2 = ·  ·  · N0 • Nt = t N0 • Populations grow by multiplication rather than addition (like compounding interest) • So if know  and N0, can find Nt

  26. Example of geometric growth (Nt = t N0) • Let =1.12 (12% per unit time) N0 = 100 • N1 = 1.12 x 100 112 • N2 = (1.12 x 1.12) 100 125 • N3 = (1.12 x 1.12 x 1.12) 100 140 • N4 = (1.12 x 1.12 x 1.12 x 1.12) 100 157

  27. Geometric growth:  > 1 and g > 0 N N0  = 1 and g = 0  < 1 and g < 0 time

  28. Differential equation model of exponential growth:dN/dt = rN rate of contribution number change of each of in = individual X individuals population to population in the size growth population

  29. dN / dt = r N • Instantaneous rate of birth and death • r = difference between birth (b) and death (d) • r = (b - d) so r is analogous to g, but instantaneous rates • rates averaged over individuals (i.e. per capita rates) • r = intrinsic rate of increase

  30. Exponential growth:Nt = N0 ert r > 0 • Continuously accelerating curve of increase • Slope varies directly with population size (N) r = 0 r < 0

  31. Exponential and geometric growth are related: • Nt = N0 ert • Nt / N0 = ert • If t = 1, then ert =  • N1 / N0 =  = er •  ln  = r

  32. The two models describe the same data equally well. Exponential TIME

  33. Environmental conditions influence r, the intrinsic rate of increase.

  34. Population growth rate depends on the value of r; r is environmental- and species-specific.

  35. Value of r is unique to each set of environmental conditions that influenced birth and death rates……but have some general expectations of pattern:High rmax for organisms in disturbed habitatsLow rmax for organisms in more stable habitats

  36. Rates of population growth are directly related to body size. • Population growth: • increases directly with the natural log of net reproductive rate (lnRo) • increases inversely with mean generation time • Mean generation time: • Increases directly with body size

  37. Rates of population growth and rmax are directly related to body size. Body Size Ro T r small 2 0.1 6.93 medium 2 1.0 0.69 large 2 10 0.0693 6.9 .69 .069 if Ro=2 Generation time decreases w/ increase in r; T increases w/ decrease in r r 0.1 1 10 T

  38. Assumptions of the model • 1. Population changes as proportion of current population size (∆ per capita) • ∆ x # individuals -->∆ in population; • 2. Constant rate of ∆; constant birth and death rates • 3. No resource limits • 4. All individuals are the same (no age or size structure)

  39. Sample Exam ? A moth species breeds in late summer and leaves only eggs to survive the winter. The adult die after laying eggs. One local population of the moth increasd from 5000 to 6000 in one year. Does this species have overlapping generations? Explain. What is  for this population? Show calculations. Predict the population size after 3 yrs. Show calculations. What is one assumption you make in predicting the future population size?

  40. Sample Exam ? In the population of mice we studied, 50% of each age classof females survive to the following breeding season, at which time they give birth to an average of three female offspring. This pattern continues to the end of their third breeding season, when the survivors all die of old age.

  41. Fill in this cohort life table. • Is the population increasing or decreasing? Show formula used. • How many female offspring does a female mouse have in her lifetime? • At what precise age does a mouse have her first child? Show formula used. • Draw a graph showing the surivorship curve for this mouse population. Label axes carefully. Explain how you reached your answer.

  42. Objectives • Age structure • Life table + Population growth • Growth in unlimited environments • Geometric growth Nt+1 =  Nt • Exponential growth Nt+1 = Ntert • Model assumptions

  43. Vocabulary

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