Chapter 36 Population Ecology

Transcription

1 Chapter 36 Population Ecology PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE TAYLOR SIMON DICKEY HOGAN Lecture by Edward J. Zalisko

2 Introduction Has parental care evolved by natural selection? Consider parental investment, the time and energy expended on offspring. The females of some species carry the embryos internally and give birth after they have hatched. Some fish do not provide any form of parental care and instead invest in quantity, producing millions of eggs at a time.

3 Introduction In a small Mediterranean species known as the peacock wrasse, the largest males build seaweed nests, where they court and mate with females, fertilize a nest full of eggs, and then lose interest in mating and guard the nest from predators. Females must search for a large nesting male, spending time and energy, to increase her chances of reproductive success.

4 Figure

5 Figure Chapter 36: Big Ideas Male 1989 Female Population Structure and Dynamics The Human Population

6 POPULATION STRUCTURE AND DYNAMICS

7 36.1 Population ecology is the study of how and why populations change A population is a group of individuals of a single species that occupy the same general area. Individuals in a population rely on the same resources, are influenced by the same environmental factors, and are likely to interact and breed with one another.

8 36.1 Population ecology is the study of how and why populations change Population ecology is concerned with the changes in population size and factors that regulate populations over time. Populations increase through birth and immigration to an area and decrease through death and emigration out of an area.

9 36.2 Density and dispersion patterns are important population variables Population density is the number of individuals of a species per unit area or volume. Examples of population density include the number of oak trees per square kilometer in a forest or the number of earthworms per cubic meter in forest soil. Ecologists use a variety of sampling techniques to estimate population densities.

10 36.2 Density and dispersion patterns are important population variables Within a population s geographic range, local densities may vary greatly. The dispersion pattern of a population refers to the way individuals are spaced within their area.

11 Video: Flapping Geese (clumped)

12 Video: Albatross Courtship (uniform)

13 Video: Prokaryotic Flagella (Salmonella typhimurium) (random)

14 36.2 Density and dispersion patterns are important population variables Dispersion patterns can be clumped, uniform, or random. In a clumped dispersion pattern, resources are often unequally distributed and individuals are grouped in patches.

15 Figure 36.2a

16 36.2 Density and dispersion patterns are important population variables In a uniform dispersion pattern, individuals are most likely interacting and equally spaced in the environment.

17 Figure 36.2b

18 36.2 Density and dispersion patterns are important population variables In a random dispersion pattern of dispersion, the individuals in a population are spaced in an unpredictable way, without a pattern, perhaps resulting from the random dispersal of windblown seeds.

19 Figure 36.2c

20 36.3 Life tables track survivorship in populations Life tables track survivorship, the chance of an individual in a given population surviving to various ages. Survivorship curves plot survivorship as the proportion of individuals from an initial population that are alive at each age. There are three main types of survivorship curves. Type I Type II Type III

21 Table 36.3

22 Figure 36.3b Percentage of survivors (log scale) I II III 50 Percentage of maximum life span 100

23 36.4 Idealized models predict patterns of population growth The rate of population increase under ideal conditions is called exponential growth. It can be calculated using the exponential growth model equation, G = rn, in which G is the growth rate of the population, N is the population size, and r is the per capita rate of increase (the average contribution of each individual to population growth). Eventually, one or more limiting factors will restrict population growth.

24 Figure 36.4a-0 Population size (N) Time (months)

25 Figure 36.4a-1 Population size (N) Time (months)

26 Table 36.4a

27 36.4 Idealized models predict patterns of population growth The logistic growth model is a description of idealized population growth that is slowed by limiting factors as the population size increases. To model logistic growth, the formula for exponential growth, rn, is multiplied by an expression that describes the effect of limiting factors on an increasing population size. K stands for carrying capacity, the maximum population size a particular environment can sustain.

28 Figure 36.4b-0 Breeding male fur seals (thousands) Year Data from K. W. Kenyon et al., A population study of the Alaska fur-seal herd, Federal Government Series: Special Scientific Report Wildlife 12 (1954).

29 Figure 36.4b-1 Breeding male fur seals (thousands) Year Data from K. W. Kenyon et al., A population study of the Alaska fur-seal herd, Federal Government Series: Special Scientific Report Wildlife 12 (1954).

30 Figure 36.4c Number of individuals (N) K 0 G = rn Time G = rn (K N) K

31 36.4 Idealized models predict patterns of population growth Table 36.4B demonstrates how the expression (K N)/K in the logistic growth model produces the S-shaped curve.

32 Table 36.4b

33 36.5 Multiple factors may limit population growth The logistic growth model predicts that population growth will slow and eventually stop as population density increases. At higher population densities, density-dependent rates result in declining births and/or increases in deaths.

34 Figure 36.5a-0 Mean number of offspring per female Density of females Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population. Ecology 73: (1992).

35 Figure 36.5a-1 Mean number of offspring per female Density of females Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population. Ecology 73: (1992).

36 36.5 Multiple factors may limit population growth Intraspecific competition is competition between individuals of the same species for limited resources and is a density-dependent factor that limits growth in natural populations. Limiting factors may include food, nutrients, or nesting sites.

37 Figure 36.5b-0 Kelp perch Proportional mortality Kelp perch density (number/plot) Data from T. W. Anderson, Predator Responses, Prey Refuges, and Density-Dependent Mortality of a Marine Fish, Ecology 82: (2001).

38 Figure 36.5b-1 Proportional mortality Kelp perch density (number/plot) Data from T. W. Anderson, Predator Responses, Prey Refuges, and Density-Dependent Mortality of a Marine Fish, Ecology 82: (2001).

39 36.5 Multiple factors may limit population growth In many natural populations, abiotic factors such as weather may affect population size well before density-dependent factors become important. Density-independent factors are unrelated to population density. These may include fires, storms, habitat destruction by human activity, or seasonal changes in weather (for example, in aphids).

40 Figure 36.5c-0 Number of aphids Exponential growth Sudden decline Apr May Jun Jul Aug Sep Oct Nov Dec Month

41 Figure 36.5c-1 Number of aphids Exponential growth Sudden decline Apr May Jun Jul Aug Sep Oct Nov Dec Month

42 36.6 SCIENTIFIC THINKING: Some populations have boom-and-bust cycles Some populations fluctuate in density with regularity. Boom-and-bust cycles may be due to food shortages or predator-prey interactions. A striking example is shown in Figure 36.6, which shows estimated populations of the snowshoe hare and the lynx based on the number of pelts sold by trappers in northern Canada to the Hudson Bay Company over a period of nearly 100 years.

43 Figure Snowshoe hare Hare population size (thousands) Lynx Lynx population size (thousands) Year Data from C. Elton and M. Nicholson, The ten-year cycle in numbers of the lynx in Canada, Journal of Animal Ecology 11 : (1942). 0

44 Figure Snowshoe hare Hare population size (thousands) Lynx Lynx population size (thousands) Year Data from C. Elton and M. Nicholson, The ten-year cycle in numbers of the lynx in Canada, Journal of Animal Ecology 11 : (1942). 0

45 36.6 SCIENTIFIC THINKING: Some populations have boom-and-bust cycles But what causes the boom-and-bust cycles of snowshoe hares? One hypothesis proposed that when hares are abundant, they overgraze their winter food supply, resulting in high mortality. Another hypothesis attributed hare population cycles to excessive predation.

46 36.6 SCIENTIFIC THINKING: Some populations have boom-and-bust cycles Using radio collars to track individual hares, researchers determined that 90% of hares had been killed by predators, and none had died of starvation. These results support the predation hypothesis. However, further experiments showed that although fluctuating food availability is not the primary factor controlling hare population cycles, it does play an important role.

47 36.7 EVOLUTION CONNECTION: Evolution shapes life histories The traits that affect an organism s schedule of reproduction and death make up its life history. Key life history traits include age of first reproduction, frequency of reproduction, number of offspring, and amount of parental care.

48 36.7 EVOLUTION CONNECTION: Evolution shapes life histories Populations with r-selected life history traits grow rapidly in unpredictable environments, where resources are abundant, have a large number of offspring that develop and reach sexual maturity rapidly, and offer little or no parental care.

49 36.7 EVOLUTION CONNECTION: Evolution shapes life histories Populations with K-selected traits tend to be long-lived animals (such as bears and elephants) that develop slowly and produce few, but well-cared-for, offspring and maintain relatively stable populations near carrying capacity. Most species fall between these two extremes.

50 36.7 EVOLUTION CONNECTION: Evolution shapes life histories A long-term project in Trinidad studied guppy populations, provided direct evidence that life history traits can be shaped by natural selection, and demonstrated that questions about evolution can be tested by field experiments.

51 Figure Pool 1 Predator: Killifish; preys mainly on small guppies Guppies: Larger at sexual maturity than those in pike-cichlid pools Pool 2 Predator: Pikecichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in killifish pools Pool 3 Data from D. N. Reznick and H. Bryga, Life-History Evolution in Guppies (Poecilia reticulata): 1. Phenotypic and genetic changes in an introduction experiment, Evolution 41: (1987). Experiment: Transplant guppies Pools with killifish, but no guppies prior to transplant Hypothesis: Predator feeding preferences caused difference in life history traits of guppy populations. Results Mass of guppies at maturity (mg) Age of guppies at maturity (days) Males Males Females Females Control: Guppies from pools with pike-cichlids as predators Experimental: Guppies transplanted to pools with killifish as predators

52 Figure Pool 1 Predator: Killifish; preys mainly on small guppies Experiment: Transplant guppies Guppies: Larger at sexual maturity than those in pike-cichlid pools Pool 2 Predator: Pikecichlid; preys mainly on large guppies Guppies: Smaller at sexual maturity than those in killifish pools Pool 3 Pools with killifish, but no guppies prior to transplant Hypothesis: Predator feeding preferences caused difference in life history traits of guppy populations. Data from D. N. Reznick and H. Bryga, Life-History Evolution in Guppies (Poecilia reticulata): 1. Phenotypic and genetic changes in an introduction experiment, Evolution 41: (1987).

53 Figure Mass of guppies at maturity (mg) Males Females Age of guppies at maturity (days) Males Females Control: Guppies from pools with pike-cichlids as predators Experimental: Guppies transplanted to pools with killifish as predators

54 36.8 CONNECTION: Principles of population ecology have practical applications Sustainable resource management involves harvesting crops without damaging the resource. In terms of population growth, this means maintaining a high population growth rate to replenish the population. According to the logistic growth model, the fastest growth rate occurs when the population size is at roughly half the carrying capacity of the habitat.

55 36.8 CONNECTION: Principles of population ecology have practical applications Fish are hunted on a large scale and are particularly vulnerable to overharvesting. The northern Atlantic cod fishery was overfished, collapsed in 1992, and still has not recovered. Resource managers may try to provide additional habitat or improve the quality of existing habitat to raise the carrying capacity and thus increase population growth.

56 Figure 36.8 Yield (thousands of metric tons) Data from Stock Assessment of Northern (2J3KL) Cod, Science Advisory Report 2011/041, Fisheries and Oceans Canada (2011).

57 THE HUMAN POPULATION

58 36.9 The human population continues to increase, but the growth rate is slowing The human population grew rapidly during the 20th century and currently stands at about 7 billion. An imbalance between births and deaths is the cause of population growth (or decline). The human population is expected to continue increasing for at least the next several decades.

59 Figure 36.9a Annual increase (in millions) Population increase Total population size Total population (in billions) Year Adapted from The World at Six Billion, United Nations Publications (1999).

60 36.9 The human population continues to increase, but the growth rate is slowing The demographic transition is a shift from zero population growth, in which birth rates and death rates are high but roughly equal, to zero population growth, characterized by low but roughly equal birth and death rates. Figure 36.9B shows the demographic transition of Mexico, which is projected to approach zero population growth with low birth and death rates in the next few decades.

61 Figure 36.9b 50 Birth or death rate per 1,000 population Birth rate Death rate Rate of increase Year Adapted from Transitions in World Population, Population Bulletin 59: 1 (2004).

62 36.9 The human population continues to increase, but the growth rate is slowing In the developing world death rates have dropped, but high birth rates persist, and these populations are growing rapidly.

63 Table 36.9

64 36.9 The human population continues to increase, but the growth rate is slowing The age structure of a population is the number of individuals in different age-groups and affects the future growth of the population.

65 36.9 The human population continues to increase, but the growth rate is slowing The fertility rate is the average number of children produced by a woman over her lifetime. Population momentum is the continued growth that occurs despite reduction of the fertility rate to replacement level and is a result of girls in the 0 14 age-group of a previously expanding population reaching their childbearing years.

66 Figure 36.9c-0 Age Male Female Population in millions Total population size = 83,366,836 Adapted from International Data Base, U.S. Census Bureau (2013). Male 2035 Female Male Female Estimated population in millions Total population size = 114,975,406 Projected population in millions Total population size = 139,457,070

67 Figure 36.9c-1 Age Male 1989 Female Population in millions Total population size = 83,366,836 Adapted from International Data Base, U.S. Census Bureau (2013).

68 Figure 36.9c-2 Age Male 2012 Female Estimated population in millions Total population size = 114,975,406 Adapted from International Data Base, U.S. Census Bureau (2013).

69 Figure 36.9c-3 Age Male 2035 Female Projected population in millions Total population size = 139,457,070 Adapted from International Data Base, U.S. Census Bureau (2013).

70 36.10 CONNECTION: Age structures reveal social and economic trends Age-structure diagrams reveal a population s growth trends and social conditions. For instance, an expanding population has an increasing need for schools, employment, and infrastructure, and a large elderly population requires that extensive resources be allotted to health care.

71 Figure Age Birth years Male Female before Population in millions Total population size = 246,819,230 Data from International Data Base, U.S. Census Bureau website, (2013) Birth years Male Female before Estimated population in millions Total population size = 313,847, Birth years Male Female before Projected population in millions Total population size = 389,531,156

72 Figure Age Birth years before Male Female Population in millions Total population size = 246,819,230 Data from International Data Base, U.S. Census Bureau website, (2013).

73 Figure Age Birth years before Male Female Estimated population in millions Total population size = 313,847,465 Data from International Data Base, U.S. Census Bureau website, (2013).

74 Figure Age Birth years before Male Female Projected population in millions Total population size = 389,531,156 Data from International Data Base, U.S. Census Bureau website, (2013).

75 36.11 CONNECTION: An ecological footprint is a measure of resource consumption The U.S. Census Bureau projects a global population of 8 billion people within the next 20 years and 9.5 billion by the mid-21st century. Do we have sufficient resources to sustain 8 or 9 billion people? To accommodate all the people expected to live on our planet by 2025, the world will have to double food production.

76 36.11 CONNECTION: An ecological footprint is a measure of resource consumption An ecological footprint is an estimate of the amount of land required to provide the raw materials an individual or a nation consumes, including food, fuel, and housing.

77 36.11 CONNECTION: An ecological footprint is a measure of resource consumption Comparing our demand for resources with Earth s capacity to renew these resources, or biocapacity, gives us a broad view of the sustainability of human activities. When the total area of ecologically productive land on Earth is divided by the global population, we each have a share of about 1.8 global hectares (1 hectare, or ha, = 2.47 acres; a global hectare, or gha, is a hectare with world-average ability to produce resources and absorb wastes).

78 36.11 CONNECTION: An ecological footprint is a measure of resource consumption Figure compares the ecological footprints of several countries to the world average footprint (orange line) and Earth s biocapacity.

79 Figure 36.11a Ecological Footprint (number of Earths) Key Built-up land Carbon Footprint Year World biocapacity Data from B Ewing et al., The Ecological Footprint Atlas, Oakland: Global Footprint Network (2010).

80 Figure 36.11b 8 Ecological Footprint (global hectares per person) World average Earth s biocapacity 0 Adapted from Living Planet Report 2012: Biodiversity, Biocapacity, and Better Choices, World Wildlife Fund (2012).

81 Figure 36.11c

82 You should now be able to 1. Define a population and population ecology. 2. Define population density and describe different types of dispersion patterns. 3. Explain how life tables are used to track mortality and survivorship in populations. 4. Compare Type I, Type II, and Type III survivorship curves. 5. Describe and compare the exponential and logistic population growth models, illustrating both with examples.

83 You should now be able to 6. Explain the concept of carrying capacity. 7. Describe the factors that regulate growth in natural populations. 8. Define boom-and-bust cycles, explain why they occur, and provide examples. 9. Explain how life history traits vary with environmental conditions and with population density. 10. Compare r-selection and K-selection and indicate examples of each.

84 You should now be able to 11. Describe the major challenges inherent in managing populations. 12. Explain how the structure of the world s human population has changed and continues to change. 13. Explain how the age structure of a population can be used to predict changes in population size and social conditions. 14. Explain the concept of an ecological footprint. Describe the uneven reliance upon natural resources in the world.

85 Figure 36.UN01 Percentage of survivors III II I Percentage of maximum life span

86 Figure 36.UN02 Age Male 1989 Female Male 2012 Female Population in millions Total population size = 83,366,836 Estimated population in millions Total population size = 114,975,406

87 Figure 36.UN03 G = rn (K N) K

88 Figure 36.UN04 I II III IV Birth or death rate Time

89 Figure 36.UN05