Biology 102 Week 9

1. Biology 102 Week 9 Population Ecology

2. Overview: Earth’s Fluctuating Populations • To understand human population growth, we must consider general principles of population ecology

3. Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size • The fur seal population of St. Paul Island, off the coast of Alaska, has experienced dramatic fluctuations in size

5. Concept 52.1: 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

6. 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

7. Density: A Dynamic Perspective • Determining the density of natural populations is difficult • In most cases, it is impractical or impossible to count all individuals in a population • Density is the result of an interplay between processes that add individuals to a population and those that remove individuals

8. LE 52-2 Immigration Births Population size Emigration Deaths

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

10. In a clumped dispersion, individuals aggregate in patches • A clumped dispersion may be influenced by resource availability and behavior Video: Flapping Geese (clumped)

11. LE 52-3a Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

12. A uniform dispersion is one in which individuals are evenly distributed • It may be influenced by social interactions such as territoriality Video: Albatross Courtship (uniform)

13. LE 52-3b Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

14. In a random dispersion, the position of each individual is independent of other individuals Video: Prokaryotic Flagella (Salmonella typhimurium) (random)

15. LE 52-3c Random. Dandelions grow from windblown seeds that land at random and later germinate.

16. Demography • Demography is the study of the vital statistics of a population and how they change over time • Death rates and birth rates are of particular interest to demographers

17. Life Tables • A life table is an age-specific summary of the survival pattern of a population • It is best made by following the fate of a cohort • The life table of Belding’s ground squirrels reveals many things about this population

19. Survivorship Curves • A survivorship curve is a graphic way of representing the data in a life table • The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate

20. LE 52-4 1,000 100 Number of survivors (log scale) Females 10 Males 1 2 0 10 6 8 4 Age (years)

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

22. LE 52-5 1,000 I 100 II Number of survivors (log scale) 10 III 1 100 50 0 Percentage of maximum life span

23. Reproductive Rates • A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population • It describes reproductive patterns of a population

25. Concept 52.2: Life history traits are products of natural selection • Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

26. Life History Diversity • Life histories are very diverse • Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die • Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly

28. “Trade-offs” and Life Histories • Organisms have finite resources, which may lead to trade-offs between survival and reproduction

29. LE 52-7 100 Male Female 80 60 Parents surviving the following winter (%) 40 20 0 Normal brood size Reduced brood size Enlarged brood size

30. Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce

31. LE 52-8a Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.

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

33. LE 52-8b 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.

34. In animals, parental care of smaller broods may facilitate survival of offspring

35. Concept 52.3: The exponential model describes population growth in an idealized, unlimited environment • It is useful to study population growth in an idealized situation • Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth

36. Per Capita Rate of Increase • If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

37. dN  rN dt • Zero population growth occurs when the birth rate equals the death rate • Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:

38. Exponential Growth • Exponential population growth is population increase under idealized conditions • Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase

39. dN  rmaxN dt • Equation of exponential population growth:

40. Exponential population growth results in a J-shaped curve

41. LE 52-9 2,000 dN = 1.0N dt 1,500 dN = 0.5N dt Population size (N) 1,000 500 0 15 10 5 0 Number of generations

42. The J-shaped curve of exponential growth characterizes some rebounding populations

43. LE 52-10 8,000 6,000 Elephant population 4,000 2,000 0 1920 1900 1980 1960 1940 Year

44. Concept 52.4: The logistic growth model includes the concept of carrying capacity • Exponential growth cannot be sustained for long in any population • A more realistic population model limits growth by incorporating carrying capacity

45. Carrying capacity (K) is the maximum population size the environment can support

46. The Logistic Growth Model • In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached • We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N increases

47. LE 52-11 Maximum Per capita rate of increase (r) Positive N = K 0 Negative Population size (N)

48. (K  N) dN  rmax N dt K • The logistic growth equation includes K, the carrying capacity

50. The logistic model of population growth produces a sigmoid (S-shaped) curve