Overview: Earth’s Fluctuating Populations

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

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 • 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

4. Density • 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

5. Immigration Births Population size Emigration Deaths

6. Patterns of Dispersion • Environmental and social factors influence spacing of individuals in a population • In a clumped dispersion, individuals aggregate in patches • A clumped dispersion may be influenced by resource availability and behavior

7. 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.

8. 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.

9. Random. Dandelions grow from windblown seeds that land at random and later germinate.

10. 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 • 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

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

13. 1,000 100 Number of survivors (log scale) Females 10 Males 1 2 0 10 6 8 4 Age (years)

14. 1,000 I 100 II Number of survivors (log scale) 10 III 1 100 50 0 Percentage of maximum life span

15. 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

16. 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.

17. 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.

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

19. dN  rN dt • If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate • 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:

20. dN  rmaxN dt 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 • Exponential population growth results in a J-shaped curve

21. 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

22. 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 • 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 • 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

23. Maximum Per capita rate of increase (r) Positive N = K 0 Negative Population size (N)

24. (K  N) dN  rmax N dt K • The logistic growth equation includes K, the carrying capacity • The logistic model of population growth produces a sigmoid (S-shaped) curve

26. 2,000 dN = 1.0N Exponential growth dt 1,500 K = 1,500 Logistic growth Population size (N) 1,000 dN 1,500 – N = 1.0N dt 1,500 500 0 0 15 10 5 Number of generations

27. The Logistic Model and Real Populations • The growth of laboratory populations of paramecia fits an S-shaped curve • Some populations overshoot K before settling down to a relatively stable density • Some populations fluctuate greatly around K

28. 1,000 800 600 Number of Paramecium/mL 400 200 0 5 10 15 0 Time (days) A Paramecium population in the lab

29. 180 150 120 Number of Daphnia/50 mL 90 60 30 0 20 40 80 60 100 120 140 0 160 Time (days) A Daphnia population in the lab

30. 80 60 Number of females 40 20 0 1980 1985 1995 1990 2000 1975 Time (years) A song sparrow population in its natural habitat

31. The Logistic Model and Life Histories • Life history traits favored by natural selection may vary with population density and environmental conditions • K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density • r-selection, or density-independent selection, selects for life history traits that maximize reproduction • The concepts of K-selection and r-selection are somewhat controversial and have been criticized by ecologists as oversimplifications

32. Populations are regulated by a complex interaction of biotic and abiotic influences • There are two general questions about regulation of population growth: • What environmental factors stop a population from growing? • Why do some populations show radical fluctuations in size over time, while others remain stable?

33. 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

34. Density-dependent birth rate Density-dependent birth rate Density- independent death rate Density- dependent death rate Birth or death rate per capita Equilibrium density Equilibrium density Population density Population density Density-dependent death rate Density- independent birth rate Equilibrium density Population density

35. Density-Dependent Population Regulation • Density-dependent birth and death rates are an example of negative feedback that regulates population growth • They are affected by many factors, such as competition for resources, territoriality, health, predation, toxic wastes, and intrinsic factors • In crowded populations, increasing population density intensifies intraspecific competition for resources

36. 4.0 10,000 3.8 3.6 Average number of seeds per reproducing individual (log scale) 1,000 Average clutch size 3.4 3.2 3.0 100 2.8 0 10 60 20 30 70 40 100 10 80 1 50 Plants per m2 (log scale) Females per unit area Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases. Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

37. Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries

38. Oceanic birds exhibit territoriality in nesting behavior

39. Health, Predation and Toxic Wastes • Population density can influence the health and survival of organisms • In dense populations, pathogens can spread more rapidly • As a prey population builds up, predators may feed preferentially on that species • Accumulation of toxic wastes can contribute to density-dependent regulation of population size

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

41. Stability and Fluctuation • Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time • Extreme fluctuations in population size are typically more common in invertebrates than in large mammals

42. 2,500 Steady decline probably caused largely by wolf predation 2,000 1,500 Moose population size 1,000 Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population 500 0 2000 1960 1980 1990 1970 Year

43. 730,000 100,000 Commercial catch (kg) of male crabs (log scale) 10,000 1960 1980 1990 1950 1970 Year

44. Metapopulations and Immigration • Metapopulations are groups of populations linked by immigration and emigration • High levels of immigration combined with higher survival can result in greater stability in populations • Many populations undergo boom-and-bust cycles • Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors

45. 60 50 40 Mandarte Island Number of breeding females 30 20 Small islands 10 0 1988 1990 1991 1989 Year

46. Snowshoe hare 160 120 9 Lynx Lynx population size (thousands) Hare population size (thousands) 80 6 40 3 0 0 1925 1850 1875 1900 Year

47. 6 5 4 Human population (billions) 3 2 The Plague 1 0 2000 B.C. 1000 B.C. 4000 B.C. 3000 B.C. 0 1000 A.D. 8000 B.C. 2000 A.D.

48. 2.2 2 1.8 1.6 2003 1.4 Annual percent increase 1.2 1 0.8 0.6 0.4 0.2 0 2000 2050 1975 1950 2025 Year

49. Regional Patterns of Population Change • No population can grow indefinitely, and humans are no exception • To maintain population stability, a regional human population can exist in one of two configurations: • Zero population growth = High birth rate – High death rate • Zero population growth =Low birth rate – Low death rate • The demographic transition is the move from the first state toward the second state • The demographic transition is associated with various factors in developed and developing countries

50. 50 40 30 Birth or death rate per 1,000 people 20 10 Sweden Mexico Birth rate Birth rate Death rate Death rate 0 2050 1950 1850 1750 1900 2000 1800 Year