Population Ecology

1. Population Ecology Chapter 52

2. Study of populations in relation to environment • Environmental influences on: • population density and distribution • age structure • variations in population size

3. Definition of Population: • Group of individuals of a single species living in the same general area

4. Density and Dispersion • Density • Is the number of individuals per unit area or volume • Dispersion • Is the pattern of spacing among individuals within the boundaries of the population

5. Density: A Dynamic Perspective • Determining the density of natural populations is possible, but difficult to accomplish • In most cases it is impractical or impossible to count all individuals in a population • How do wildlife biologists approximate populations?

6. Estimating Wildlife Population Size Defined Populations Undefined Populations

7. Births and immigration add individuals to a population. Births Immigration PopuIationsize Emigration Deaths Deaths and emigration remove individuals from a population. • Density is the result of a dynamic interaction of processes that add individuals to a population and those that remove individuals from it • How do these factors • Contribute to Population Size?? • Births • Deaths • Immigration • Emigration Figure 52.2

8. Patterns of Dispersion • Environmental and social factors influence the spacing of individuals in a population

9. Clumped Dispersion • Individuals aggregate in patches • May be influenced by resource availability and behavior

10. Uniform Dispersion • Individuals are evenly distributed • May be influenced by social interactions such as territoriality

11. Random Dispersion • Position of each individual is independent of other individuals (c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

12. Life history traits are products of natural selection • Life history traits are evolutionary outcomes • Reflected in the development, physiology, and behavior of an organism

13. Figure 52.6 Semelparity: Big Bang • Reproduce a single time and die

14. Iteroparity – Repeated Reproduction • Produce offspring repeatedly over time

15. 100 Male Female 80 60 Parents surviving the following winter (%) 40 20 The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents. CONCLUSION 0 Reduced brood size Normal brood size Enlarged brood size “Trade-offs” and Life Histories • Which may lead to trade-offs between survival and reproduction • Organisms have finite resources RESULTS • Kestrels: • Produce a few eggs? • Can invest more into each, ensuring greater survival • Produce many eggs? • Costly but if all survive, fitness is better

16. (a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least somewill grow into plants and eventually produce seeds themselves. Figure 52.8a More is Better? • Some plants produce a large number of small seeds • Ensuring that at least some of them will grow and eventually reproduce

17. (b) Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring. Figure 52.8b Fewer is Better? • Other types of plants produce a moderate number of large seeds • That provide a large store of energy that will help seedlings become established

18. Demography • Study of the vital statistics of a population • And how they change over time • Death rates and birth rates • Zero population growth • Occurs when the birth rate equals the death rate

19. Exponential Population Growth Population increase under idealized conditions No limits on growth • Under these conditions • The rate of reproduction is at its maximum, called the intrinsic rate of increase

20. Example-understanding growth Question: I offer you a job for 1 cent/day and your pay will double every day. You will be hired for 30 days. Will you take my job offer? Answer: If you said YES, you will have made $~21 million dollars for 30 days of work. How is this possible?????

21. Amount of Pay/Day # of Days 1ST DAY OF WORK: 1 cent pay/day 30TH DAY OF WORK: ~10.2 million/day How is this possible?????

22. dN  rmaxN dt Exponential Growth Model *Idealized population in an unlimited environment *Very rapid doubling time; steep J curve *r=N=(b-d)N t r=instrinsic rate of growth

23. 8,000 6,000 Elephant population 4,000 2,000 0 1900 1920 1940 1960 1980 Year Exponential Growth in the Real World • Characteristic of some populations that are rebounding • Cannot be sustained for long in any population

24. Logistic Population Growth • A more realistic population model • Limits growth by incorporating carrying capacity

25. Logistic Population Growth • Carrying capacity (K) • Is the maximum population size the environment can support • In the logistic population growth model • The per capita rate of increase declines as carrying capacity is reached

26. (K  N) dN  rmax N dt K Logistic Growth Equation • Includes K, the carrying capacity

27. 2,000 dN  1.0N Exponential growth dt 1,500 (K  N) K  1,500 dN  rmax N Logistic growth dt K Population size (N) 1,000 dN 1,500  N  1.0N dt 1,500 500 0 0 5 10 15 Number of generations Logistic Population Growth • Produces a sigmoid (S-shaped) curve Figure 52.12

28. 1,000 800 600 Number of Paramecium/ml 400 200 0 0 5 15 10 Time (days) (a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment. Figure 52.13a The Logistic Model and Real Populations • The growth of laboratory populations of paramecia • Fits an S-shaped curve

29. 180 150 120 90 Number of Daphnia/50 ml 60 30 0 160 0 40 60 100 120 140 20 80 Time (days) (b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size. Figure 52.13b Logistic Growth and The Real World • Some populations overshoot K • Before settling down to a relatively stable density

30. 80 60 40 Number offemales 20 0 1995 2000 1980 1985 1990 1975 Time (years) (c) A song sparrowpopulation in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model. Figure 52.13c Logistic Growth and the Real World • Some populations • Fluctuate greatly around K

31. The Logistic Model and Life Histories • Life history traits favored by natural selection • May vary with population density and environmental conditions

32. Life History and Logistic Growth • K-selection, or density-dependent selection • Selects for life history traits that are sensitive to population density • Reproduce slowly, small litters • r-selection, or density-independent selection • Selects for life history traits that maximize reproduction • Reproduce rapidly, large litters

33. Natural selection (diverse reproductive strategies) a) Relatively few, large offspring (K selected species) b) Many, small offspring (r selected species) (K selected species) (r selected species)

34. Human Populations 6 5 4 Human population (billions) 3 2 The Plague 1 0 8000 B.C. 4000 B.C. 3000 B.C. 2000 B.C. 1000 B.C. 1000 A.D. 2000 A.D. 0 Figure 52.22 • No population can grow indefinitely and humans are no exception

35. Global Carrying Capacity • Just how many humans can the biosphere support? • Carrying capacity of earth is unknown…. http://www.youtube.com/watch?v=9_9SutNmfFk http://www.youtube.com/watch?v=UUOEcNomakw&feature=rec-LGOUT-exp_fresh+div-1r-8-HM http://www.youtube.com/watch?v=4B2xOvKFFz4&feature=related

36. Populations Regulated Biotic and Abiotic Factors Two general questions we can ask about regulation of population growth • What environmental factors stop a population from growing? • 2. Why do some populations show radical fluctuations in size over time, while others remain stable?

37. Population Change and Population Density • In density-independent populations • Birth rate and death rate do not change with population density • In density-dependent populations • Birth rates fall and death rates rise with population density

38. Density-Dependent Population Regulation • Density-dependent birth and death rates • Are an example of negative feedback that regulates population growth • Are affected by many different mechanisms

39. 4.0 10,000 3.8 3.6 Average number of seeds per reproducing individual (log scale) 1,000 3.4 Average clutch size 3.2 3.0 100 2.8 0 0 40 50 60 80 20 30 10 70 0 10 100 Seeds planted per m2 Density of females (a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. (b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply. Competition for Resources • In crowded populations, increasing population density • Intensifies intraspecific competition for resources Figure 52.15a,b

40. Territoriality • In many vertebrates and some invertebrates • Territoriality may limit density

41. Figure 52.16 Territoriality Example: Cheetas • Cheetahs are highly territorial • Using chemical communication to warn other cheetahs of their boundaries

42. Figure 52.17 Territoriality: Ocean birds • Exhibit territoriality in nesting behavior

43. Health • Population density • Can influence the health and survival of organisms • In dense populations • Pathogens can spread more rapidly

44. Predation • As a prey population builds up • Predators may feed preferentially on that species

45. Intrinsic Factors • For some populations • Intrinsic (physiological) factors appear to regulate population size

46. Population Dynamics • The study of population dynamics • Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

47. 730,000 100,000 Commercial catch (kg) of male crabs (log scale) 10,000 1990 1950 1980 1960 1970 Year Figure 52.19 Fluctuations in Population Size • Extreme fluctuations in population size • Are typically more common in invertebrates than in large mammals

48. Metapopulations and Immigration • Metapopulations • Groups of populations linked by immigration and emigration

49. 60 50 40 Mandarte island Number of breeding females 30 20 Small islands 10 0 1991 1988 1989 1990 Year Immigration- Movement Into a Population • High levels of immigration combined with higher survivalcan result in greater stability in populations Figure 52.20

50. Snowshoe hare 160 120 Lynx 9 Lynx population size (thousands) Hare population size (thousands) 80 6 40 3 0 0 1850 1875 1900 1925 Year Population Cycles • Many populations undergo regular boom-and-bust cycles • Influenced by complex interactions between biotic and abiotic factors