Chapter 36 Population Ecology Individual emperor penguins face the rigors of the Antarctic climate and have special adaptations, including a downy underlayer of feathers for insulation and thick layer of fat for energy storage and insulation. The entire population of emperor penguins reflects group characteristics, including the survivorship of chicks and growth rate of the population. © 2012 Pearson Education, Inc. 1

POPULATION STRUCTURE AND DYNAMICS POPULATION STRUCTURE AND DYNAMICS Population ecologists study natural population structure and dynamics. © 2012 Pearson Education, Inc. 2

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. © 2012 Pearson Education, Inc. 3

36.1 Population ecology is the study of how and why populations change A population can be described by the number and distribution of individuals. Population dynamics, the interactions between biotic and abiotic factors, cause variations in population sizes. © 2012 Pearson Education, Inc. 4

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. © 2012 Pearson Education, Inc. 5

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 number of earthworms per cubic meter in forest soil. Ecologists use a variety of sampling techniques to estimate population densities. © 2012 Pearson Education, Inc. 6

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. © 2012 Pearson Education, Inc. 7

36.2 Density and dispersion patterns are important population variables Dispersion patterns can be clumped, uniform, or random. In a clumped pattern resources are often unequally distributed and individuals are grouped in patches. © 2012 Pearson Education, Inc. 8

Figure 36.2A Clumped dispersion of ochre sea stars at low tide 9

36.2 Density and dispersion patterns are important population variables In a uniform pattern, individuals are most likely interacting and equally spaced in the environment. © 2012 Pearson Education, Inc. 10

Figure 36.2B Uniform dispersion of sunbathers at Coney Island 11

36.2 Density and dispersion patterns are important population variables In a random pattern of dispersion, the individuals in a population are spaced in an unpredictable way. © 2012 Pearson Education, Inc. 12

Figure 36.2C Random dispersion of dandelions 13

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 © 2012 Pearson Education, Inc. 14

Table 36.3 Table 36.3 Life Table for the U.S. Population in 2008 15

Figure 36.3 Three types of survivorship curves 100 I 10 Percentage of survivors (log scale) II 1 III Figure 36.3 Three types of survivorship curves 0.1 50 100 Percentage of maximum life span 16

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. © 2012 Pearson Education, Inc. 17

Figure 36.4A Exponential growth of rabbits 500 450 400 350 Population size (N) 300 250 200 Figure 36.4A Exponential growth of rabbits 150 100 50 1 2 3 4 5 6 7 8 9 10 11 12 Time (months) 18

Table 36.4A Table 36.4A Exponential Growth of Rabbits, r  0.3 19

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. © 2012 Pearson Education, Inc. 20

Figure 36.4B Logistic growth of a population of fur seals 10 8 Breeding male fur seals (thousands) Figure 36.4B Growth of a population of fur seals 6 4 2 1915 1925 1935 1945 Year 21

Figure 36.4C Logistic growth and exponential growth compared rN   G rN (K  N) K K Number of individuals (N) Figure 36.4C Logistic growth and exponential growth compared Time 22

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 increasing population densities, density-dependent rates result in declining births and increases in deaths. © 2012 Pearson Education, Inc. 23

Figure 36.5A Declining reproductive success of song sparrows (inset) with increasing population density 6 5 4 3 2 1 Mean number of offspring per female Figure 36.5A Declining reproductive success of song sparrows (inset) with increasing population density 0 10 20 30 40 50 60 70 80 Density of females Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population. Ecology 73: 805–882 (1992). 24

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. © 2012 Pearson Education, Inc. 25

36.5 Multiple factors may limit population growth Limiting factors may include food, nutrients, retreats for safety, or nesting or denning sites. © 2012 Pearson Education, Inc. 26

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). © 2012 Pearson Education, Inc. 27

Exponential growth Sudden decline Figure 36.5C Weather change as a density-independent factor limiting aphid population growth Exponential growth Sudden decline Number of aphids Figure 36.5C Weather change as a density-independent factor limiting aphid population growth Apr May Jun Jul Aug Sep Oct Nov Dec Month 28

36.6 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. © 2012 Pearson Education, Inc. 29

Figure 36.6 Population cycles of the snowshoe hare and the lynx 160 Snowshoe hare 120 9 Hare population size (thousands) Lynx Lynx population size (thousands) 80 6 Figure 36.6 Population cycles of the snowshoe hare and the lynx 40 3 1850 1875 1900 1925 Year 30

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. © 2012 Pearson Education, Inc. 31

36.7 EVOLUTION CONNECTION: Evolution shapes life histories Populations with so-called r-selected life history traits produce more offspring and grow rapidly in unpredictable environments. Populations with K-selected traits raise fewer offspring and maintain relatively stable populations. Most species fall between these two extremes. © 2012 Pearson Education, Inc. 32

36.7 EVOLUTION CONNECTION: Evolution shapes life histories A long-term project on life histories 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. © 2012 Pearson Education, Inc. 33

Figure 36.7 The effect of predation on the life history traits of guppies Experiment: Transplant guppies Pool 1 Predator: Killifish; preys on small guppies 200 185.6 161.5 160 Mass of guppies at maturity (mg) 120 76.1 Results 80 67.5 40 Guppies: Larger at sexual maturity Males Females Pool 3 Pools with killifish but no guppies prior to transplant 100 85.7 92.3 80 Age of guppies at maturity (days) 58.2 60 48.5 40 Pool 2 Predator: Pike- cichlid; preys on large guppies 20 Males Females Control: Guppies from pools with pike-cichlids as predators Guppies: Smaller at sexual maturity Figure 36.7 The effect of predation on the life history traits of guppies Experimental: Guppies transplanted to pools with killifish as predators Hypothesis: Predator feeding preferences caused difference in life history traits of guppy populations. 34

36.8 Principles of population ecology have practical applications Sustainable resource management involves harvesting crops and eliminating damage to the resource. The cod fishery off Newfoundland was overfished, collapsed in 1992, and still has not recovered. Resource managers use population ecology to determine sustainable yields. © 2012 Pearson Education, Inc. 35

Figure 36.8 Collapse of northern cod fishery off Newfoundland 900 800 700 600 Yield (thousands of metric tons) 500 400 300 200 Figure 36.8 Collapse of northern cod fishery off Newfoundland 100 1960 1970 1980 1990 2000 36

THE HUMAN POPULATION © 2012 Pearson Education, Inc. 37

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. © 2012 Pearson Education, Inc. 38

Annual increase (in millions) Total population (in billions) Figure 36.9A Five centuries of human population growth, projected to 2050 100 10 Population increase 80 8 60 6 Annual increase (in millions) Total population (in billions) 40 4 Total population size 20 2 Figure 36.9A Five centuries of human population growth, projected to 2050 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 Year 39

36.9 The human population continues to increase, but the growth rate is slowing The demographic transition is the shift from high birth and death rates to low birth and death rates, and has lowered the rate of growth in developed countries. © 2012 Pearson Education, Inc. 40

Figure 36.9B Demographic transition in Mexico 50 40 30 Rate of increase per 1,000 population Birth or death rate 20 Birth rate 10 Death rate Figure 36.9B Demographic transition in Mexico 1900 1925 1950 1975 2000 2025 2050 Year 41

36.9 The human population continues to increase, but the growth rate is slowing In the developing nations death rates have dropped, birth rates are still high, and these populations are growing rapidly. © 2012 Pearson Education, Inc. 42

Table 36.9 Table 36.9 Population Changes in 2012 (Estimated) 43

36.9 The human population continues to increase, but the growth rate is slowing The age structure of a population is the proportion of individuals in different age groups and affects the future growth of the population. © 2012 Pearson Education, Inc. 44

36.9 The human population continues to increase, but the growth rate is slowing Population momentum is the continued growth that occurs despite reduced fertility and as a result of girls in the 0–14 age group of a previously expanding population reaching their childbearing years. © 2012 Pearson Education, Inc. 45

Figure 36.9C Population momentum in Mexico Figure 36.9C Population momentum in Mexico due to increased proportion of women of childbearing age 80+ 1989 2012 2035 75–79 70–74 65–69 Male Female Male Female Male Female 60–64 55–59 50–54 Age 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 6 5 4 3 2 1 0 1 2 3 4 5 6 5 4 3 2 1 0 1 2 3 4 5 5 4 3 2 1 0 1 2 3 4 5 Figure 36.9C Population momentum in Mexico Population in millions Estimated population in millions Projected population in millions Total population size = 83,366,836 Total population size = 114,975,406 Total population size = 139,457,070 Adapted from International Data Base, U.S. Census Bureau (2013). 46

36.10 Age structures reveal social and economic trends Age-structure diagrams reveal a population’s growth trends and social conditions. © 2012 Pearson Education, Inc. 47

Figure 36.10 Age structures for the United States in 1985, 2010 (estimated), and 2035 (projected). Baby boom after war (tan) will result in older population structure with stress on social security and medicaid. 1989 2012 2035 Birth years Male Female Birth years Male Female Birth years Male Female 85+ before 1905 before 1928 before 1951 80–84 1905–1909 1928–32 1951–55 75–79 1910–14 1933–37 1956–60 70–74 1915–19 1938–42 1961–65 65–69 1920–24 1943–47 1966–70 60–64 1925–29 1948–52 1971–75 55–59 1930–34 1953–57 1976–80 Age 50–54 1935–39 1958–62 1981–85 45–49 1940–44 1963–67 1986–90 40–44 1945–49 1968–72 1991–95 35–39 1950–54 1973–77 1996–2000 30–34 1955–59 1978–82 2001–05 25–29 1960–64 1983–87 2006–10 20–24 1965–69 1988–92 2011–15 15–19 1970–74 1993–97 2016–20 10–14 1975–79 1998–2002 2021–25 5–9 1980–84 2003–2007 2026–30 0–4 1985–89 2008–2012 2031–35 Figure 36.10 Age structures for the United States in 1985, 2010 (estimated), and 2035 (projected) 12 10 8 6 4 2 0 2 4 6 8 10 12 12 10 8 6 4 2 0 2 4 6 8 10 12 12 10 8 6 4 2 0 2 4 6 8 10 12 Population in millions Estimated population in millions Projected population in millions Total population size = 246,819,230 Total population size = 313,847,465 Total population size = 389,531,156 Data from International Data Base, U.S. Census Bureau website, (2013). 48

36.11 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 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. © 2012 Pearson Education, Inc. 49

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, water, housing, and waste disposal. © 2012 Pearson Education, Inc. 50

36.11 CONNECTION: An ecological footprint is a measure of resource consumption The United States has a very large ecological footprint, much greater than its own land, and is running on a large ecological deficit. Some researchers estimate that if everyone on Earth had the same standard of living as people living in the United States, we would need the resources of 4.5 planet Earths. © 2012 Pearson Education, Inc. 51

Ecological Footprint (global hectares per person) Figure 36.11B 8 7 6 5 Ecological Footprint (global hectares per person) 4 World average 3 2 Earth’s biocapacity 1 Haiti Mexico China India Australia Argentina Colombia South Africa Uzbekistan Afghanistan United States United Kingdom Adapted from Living Planet Report 2012: Biodiversity, Biocapacity, and Better Choices, World Wildlife Fund (2012).