Ecology of Populations

1. Ecology of Populations

2. Population ecology is the study of populations in relation to environment

3. Dynamic biological processes influence population density, dispersion, and demography • A population is a group of individuals of a single species living in the same general area

4. Four main factors affecting population size • Natality = number of births • Immigration = number of individuals arriving from other places • Emigration = number of individuals leaving the population

5. The exponential model describes population growth in an idealized, unlimited environment • It is useful to study population growth in an idealized situation

6. Exponential Growth • Exponential population growth • Under these conditions • The rate of reproduction is at its maximum, called the intrinsic rate of increase (rmax)

7. Exponential population growth • Results in a J-shaped curve

8. The J-shaped curve of exponential growth • Is characteristic of some populations that are rebounding

9. Environmental factors that restrict population growth are called population-limiting factors • Environmental factors limit the growth of populations, preventing exponential growth • Produces an S-shaped or logistic growth curve

10. Exponential growth • Cannot be sustained for long in any population • A more realistic population model • Limits growth by incorporating carrying capacity

11. Carrying capacity (K) • Is the maximum population size the environment can support with no net increase or decrease • The value of K varies, depending on species and habitat

12. Carrying capacity is defined by limiting factors such as: • Availability of resources • Build up of wastes, CO2 • Predation • Disease

13. The logistic model of population growth • Produces a sigmoid (S-shaped) curve

14. Sigmoid or Logistic Growth Curve • Exponential phase • Transitional phase • Plateau phase

15. Causes of each phase • Exponential phase - under ideal conditions, population doubles on a regular basis, due to: • Plentiful resources • Favorable abiotic factors • Little or no predation or disease

16. Causes of each phase • Transitional phase - population growth rate slows because of : • Increased competition for resources • More predators

17. Causes of each phase • Plateau phase - population size has stabilized, no more growth, this is due to: • Predators and disease increase mortality and growth curve levels off • Births + immigration = deaths + emigrations

18. The Logistic Model and Real Populations • The growth of laboratory populations of paramecia • Fits an S-shaped curve

19. Some populations overshoot K • Before settling down to a relatively stable density

20. Some populations • Fluctuate greatly around K

21. The logistic model fits few real populations

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

23. K-selection, or density-dependent selection • r-selection, or density-independent selection • Selects for life history traits that maximize reproduction

24. Survivorship Curves • A survivorship curve is a graphic way of representing the data in a life table that shows the differential mortality rates in relation to age

25. The survivorship curve for Belding’s ground squirrels • Shows that the death rate is relatively constant

26. Survivorship curves can be classified into three general types • Type I, Type II, and Type III

27. Type I curve shows low infant mortality • These species produce few offspring but provide them with a high degree of parental care. This increases the likelihood that they will survive to maturity • Show a low rmax

28. Type II curve mortality is more constant over the life span

29. Type III curve have high death rates for very young. Death rates drop as individuals survive to increased ages • Species produce very large numbers of offspring but provide little or no care for them • Have a high rmax

30. An organisms life history influences the growth rate of a population

31. Opportunistic life history or r-strategists • Small bodied species, reproduce when young, produce many offspring, populations tend to grow exponentially when conditions are favorable • Such populations typically live in unstable or disrupted environments and are controlled by density-independent factors • Exhibit type III survivorship curves • Use a tactic called r-selection • Insects and other invertebrates

32. Equilibrial life history or K-strategists • Larger bodied species, produce few slowly maturing offspring but provide care for their young (nurturing) • Occurs in stable, balanced environments • Exhibit type I survivorship curve • Use tactic called K-selection • Large animals like mammals

33. Some plants produce a large number of small seeds • Ensuring that at least some of them will grow and eventually reproduce

34. Other types of plants produce a moderate number of large seeds • That provide a large store of energy that will help seedlings become established

35. Population Cycles • Many populations • Undergo regular boom-and-bust cycles

36. Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors • Prey populations may be influenced by predation, and by fluctuations in the availability of the plants they feed on

37. Population ecology is the study of populations in relation to environment • Including environmental influences on population density and distribution, age structure, and variations in population size

38. Dynamic biological processes influence population density, dispersion, and demography

39. Demographics of Populations • Demography is the statistical study of a population, which includes its density, distribution, rate of growth

40. Density and Distribution of Populations • Population Density - Number of individuals per unit area or volume • Population Distribution - Pattern of dispersal of individuals within a space of interest • Ecologists analyze what causes the spatial and temporal “patchiness” of organisms • Affected by the availability of resources

41. Density: A Dynamic Perspective • Determining the density of natural populations • In most cases • It is impractical or impossible to count all individuals in a population

42. Density is the result of a dynamic interplay • Between processes that add individuals to a population and those that remove individuals from it

43. Patterns of Dispersion • Environmental and social factors • Influence the spacing of individuals in a population

44. A clumped dispersion • Is one in which individuals aggregate in patches • May be influenced by resource availability and behavior

45. A uniform dispersion • Is one in which individuals are evenly distributed

46. A random dispersion • Is one in which the position of each individual is independent of other individuals

47. Population Growth • Exponential Growth • Biotic Potential • Maximum population growth that can possibly occur under ideal circumstances • Environmental Resistance • All environmental conditions that prevent populations from achieving biotic potential • Biotic potential is having it’s full effect and birthrate is at it’s maximum during exponential growth

48. dN  rN dt • Zero population growth • Occurs when the birth rate equals the death rate • The population growth equation can be expressed as

49. dN  rN dt G = rN G = growth rate of population N = population size r = intrinsic rate of increase If r is constant, then the rate at which population grows depends on the number of individuals already in the population (N), value of r depends on the kind of organism G = dN/dt (∆N/∆t) = change in # of individuals over a given time r = (b-d) = birth rate - death rate G = rN or ∆N/∆t = (b-d)N

50. dN  rmaxN dt • The equation of exponential population growth is