Population Dynamics

1. Population Dynamics • Chapters 4,5, & 6

2. Group and Individual Selection • Group Selection Theory: Groups of individuals control their numbers to avoid extinction • Successful groups – individuals would not act selfishly • Selfish groups – overexploit their environment and die out.

3. Arguments against Group Selection • Mutation • Immigration • Individual selection • Individuals die out more quickly than groups • Individual selection is a more powerful evolutionary force • Group selection needs a reliable and predictable source of food

4. Self-Regulation • Intraspecific competition • Individuals strive to command as much resources as they can. • Act in self-interest. • Ex. Male lions that kill existing cubs when they take over pride. Increase their own offspring

5. Altruism • Apparent cooperation • Grooming • Hunting • Warning signals

6. Group Living • Dense living • Promote intense competition • Significant advantages to compensate for over crowding ex. Protection against predators

7. 7 6 School cohesion 5 Many Few 1 2 3 4 5 6 Predator abundance (streams in rank order)

8. “Many-eyes hypothesis” • Success of predator attacks • Prey alerted to attack • Ex. Goshawks less successful attacking large flocks of pigeons (Columba palumbus) • The bigger the flock (more eyes) the more likely the prey will be alerted to the presence of a predator

9. 100 80 60 Attack success (%) 40 20 0 1 2-10 11-50 50 Number of pigeons in flock

10. Selfish-herd theory . • Predators usually only take one prey per attack. • The bigger the herd, the lower the probability of an individual prey being taken • Larger herds are attacked more, but probability of being taken would still favor individual

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12. Reproductive Strategies • Semelparity • Organisms that produce all of their offspring in a single reproductive event. • May live several years before reproducing or lifespan is one year (ex. Annual plants, \pacific salmon)

13. Iteroparity • Organisms that reproduce in successive years or breeding seasons • Variation in the number of clutches and number of offspring per clutch.

14. Iteroparity (cont.). • Some species have distinct breeding seasons • Ex. Temperate birds and temperate forest trees • Lead to distinct generations

15. Iteroparity (cont.) • Some species reproduce repeatedly and at any time during the year (continuous iteroparity) • Ex. Some tropical species, many parasites, and humans

16. Environmental Uncertainty • Favors iteroparity • Survival of juveniles is poor and unpredictable • Selection favors • Repeated reproduction • Long reproductive life • Environmental Stable • Favors semelparity

17. Age Structure • Semelparous organisms • Often produce groups of same-aged young – cohorts • Cohorts grow at similar rates • Iteroparous organisms • Many young at different ages

18. Increasing populations – large number of young • Decreasing populations – few young • Loss of age classes • Influence on population

19. Mating Systems • Mating systems in animals • Monogamy • Exclusive mating • Common among birds (~90%) of species

20. Polygamy • Individuals mate with multiple partners • Polygyny • One male mates with multiple females • Females mate with one male • Females care for the young • Mammals tend to be polygynous • Polyandry • One female mates with multiple males

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22. Life History Strategies • Success of populations • Reproductive strategies • Survival strategies • Habitat usage • Competition with other organisms

23. r-Selected • Populations increase rapidly toward the carrying capacity • High reproductive allocations • Semelparous • Low competitive abilities • Ex. weeds

24. K-Selected • Populations increase slowly toward the carrying capacity • Low reproductive allocations • Iteroparous • High competitive abilities • Ex. Oak tree

25. General types of survivorship curves • Type I • Most individuals are lost when they are older • Vertebrates or organisms that exhibit parental care and protect their young • Small dip at young age due to predators

26. Type II • Almost linear rate of loss • Many birds and some invertebrates • Type III • Large fraction are lost in the juvenile stages • Invertebrates, many plants, and marine invertebrates that do not exhibit parental care

27. Type I 1000 Many birds, small mammals, lizards, turtles Many mammals 100 x Type II Number of survivors (n ) (log scale) 10 Many invertebrates 1 Type III 0.1 Age

28. Survivorship Life history patterns

29. Reproductive Rate • Fecundity • Age-specific birth rates • Number of female offspring produced by each breeding female • Fecundity schedules • Fecundity information in life table

31. Deterministic Models: Geometric Growth • Predicting population growth • Need to know; • Ro population’s net reproduction rate • Initial population size N • Population size at time t • Population size of females at next generation = Nt+1= RoNt

32. Dependency of Ro • Ro < 1; population becomes extinct • Ro = 1; population remains constant • Population is at equilibrium • No change in density

33. Dependency of R (cont.) • Ro > 1; population increases • Even a fraction above one, population will increase rapidly • Characteristic “J” shaped curve • Geometric growth

34. R =1.20 0 500 R =1.15 0 400 300 Population in size (N) N +1 = R N 200 t 0 t R =1.10 0 100 R =1.05 0 10 0 20 30 Generations

35. Ro > 1; population increases (cont.). • Something (e.g., resources) will eventually limit growth • Population crash

36. Logistic Growth • Occurs in populations where resources are or can be limiting • Logistic growth equations • dN / dt= rN[(K-N)/K]; or • dN / dt = Rate of population change • r = per capita rate of population growth • N = population size • K = carrying capacity • (K - N) / K = unused resources remaining • As N gets larger, the amount of resources remaining gets smaller • When N = K, zero growth will occur Chapt. 06

37. Logistic Growth Graph initial carrying capacity new carrying capacity Fig. 45-8, p. 807

38. Logistic Growth

39. 2000 1500 1000 Number of reindeer 500 0 1910 1920 1930 1940 1950 Year

40. Human population growth • Prior to agriculture and domestication of animals (~10,000 B.C.) • Average annual rate of growth: ~0.0001% • After the establishment of agriculture • 300 million people by 1 A.D. • 800 million by 1750 • Average annual rate of growth: ~0.1%

41. Period of rapid population growth • Began 1750 • From 1750 to 1900 • Average annual rate of growth: ~0.5% • From 1900 to 1950 • Average annual rate of growth: ~0.8% • From 1950 to 2000 • Average annual rate of growth: ~1.7% • Reasons for rapid growth • Advances in medicine • Advances in nutrition

42. Demographic Transition Model Demographic transition model

43. 14 Year 13 12 11 2100 2046 10 2033 9 2020 8 2009 7 Billions of people 1998 6 1987 5 1975 4 1960 3 1930 2 1830 1 0 2-5 million Years ago 7,000 BC 6,000 BC 5,000 BC 4,000 BC 3,000 BC 2,000 BC 1,000 BC 1 AD 1,000 AD 2,000 AD 3,000 AD 4,000 AD

44. Human population statistics • Population is increasing at a rate of 3 people every second • Current population: over 6 billion • UN predicts population will stabilize at 11.5 billion by 2150Human population statistics (cont.). • Average annual rate of growth Developed countries 1960-1965: 1.19% 1990-1995: 0.48% Developing countries 1960-1965: 2.35% 1990-1995: 2.38%

45. Age Structure Diagrams Show age distribution of a population Negative Growth Slow Growth Zero Growth Rapid Growth

46. Fig. 45-17b, p.815

47. Age Structure Diagrams Age structure diagrams

48. Effects of Economic Development Current and projected population sizes by region

49. Fig. 45-20, p.817