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Chapter 53 Population Ecology PowerPoint Lectures for Biology, Eighth Edition Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp and Janette Lewis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: Counting Sheep • A small population of Soay sheep were introduced to Hirta Island in 1932 • They provide an ideal opportunity to study changes in population size on an isolated island with abundant food and no predators Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 2 of 67 Figure 53.1 What causes a sheep population to fluctuate in size? Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Concept 53.1: Dynamic biological processes influence population density, dispersion, and demographics • A population is a group of individuals of a single species living in the same general area • Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Density and Dispersion • Density is the number of individuals per unit area or volume – Density is the result of an interplay between processes that add individuals to a population and those that remove individuals – Immigration is the influx of new individuals from other areas – Emigration is the movement of individuals out of a population Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 5 of 67 Density and Dispersion • Dispersion is the pattern of spacing among individuals within the boundaries of the population – Clumped: individuals aggregate in patches – Uniform: individuals are evenly distributed (territoriality) – Random: position of each individual is independent of other individuals Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 6 of 67 Figure 52.3 Patterns of dispersion within a population’s geographic range (a) Clumped. (b) Uniform. (c) Random. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Density: A Dynamic Perspective • In most cases, it is impractical or impossible to count all individuals in a population • Sampling techniques can be used to estimate densities and total population sizes • Population size can be estimated by either extrapolation from small samples, an index of population size, or the mark-recapture method Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 8 of 67 Figure 53.2 Determining population size using the mark-recapture method APPLICATION Hector’s dolphins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Figure 53.3 Population dynamics Births Births and immigration add individuals to a population. Immigration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Deaths Deaths and emigration remove individuals from a population. Emigration Slide 10 of 67 Demographics • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 11 of 67 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, a group of individuals of the same age – The life table of Belding’s ground squirrels reveals many things about this population Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 12 of 67 Table 53-1 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 14 of 67 Number of survivors (log scale) Figure 53.5 Survivorship curves for male and female Belding’s ground squirrels 1,000 100 Females 10 Males 1 0 2 4 6 Age (years) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 8 10 Survivorship Curves • Survivorship curves can be classified into three general types: – Type I: low death rates during early and middle life, then an increase among older age groups. (Humans, large mammals) – Type II: the death rate is constant over the organism’s life span. (Squirrels) – Type III: high death rates for the young, then a slower death rate for survivors. (Fish, clams, insects) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 16 of 67 Number of survivors (log scale) Figure 53.6 Idealized survivorship curves: Types I, II, and III 1,000 I 100 II 10 III 1 0 50 Percentage of maximum life span Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 100 Concept 53.2: Life history traits are “products of natural selection” • An organism’s life history comprises the traits that affect its schedule of reproduction and survival: – The age at which reproduction begins – How often the organism reproduces – How many offspring are produced during each reproductive cycle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Life History Diversity • Life histories are very diverse, two contrasting types: 1. Species that exhibit semelparity, or big-bang reproduction, reproduce once and die • Examples: salmon, agave plant 2. Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly • Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 19 of 67 Concept 53.3: The exponential model describes population growth in an idealized, unlimited environment • Exponential population growth model – 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Per Capita Rate of Increase • Population growth rate = Per capita rate of increase – If immigration and emigration are ignored, a population’s growth rate equals birth rate minus death rate during a time interval – N = population size r=b–d N Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 21 of 67 Per Capita Growth Rate • For a population sample size of 1000 (N = 1000) there were 60 births and 10 deaths over a one year period. What is the growth rate? A. .5 per individual per year B. .05 per individual per year C. .005 per individual per year 60-10/1000 = .05 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 22 of 67 Population Growth Rate • Per capita birth and death rates – The average # of birth (b) or deaths (m for mortality) per individual per unit time – If there are 34 births per year in a population of 1000, • b = 0.034 – If the death rate in a population is 16 deaths per year, • m= 0.016 – Zero population growth occurs when the birth rate equals the death rate Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 23 of 67 • Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time: – per capita birth rate - per capita death rate = rN: per capita rate of change in a population size ΔN = Δt rN where N = population size, t = time, and r = per capita rate of increase = birth – death Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 24 of 67 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 – Equation of exponential population growth: dN = dt rmaxN Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 25 of 67 Figure 53.10 Exponential Population Growth • Exponential population growth results in a Jshaped curve Population size (N) 2,000 dN 1.0N dt = 1,500 dN 0.5N dt = 1,000 500 0 0 5 10 Number of generations Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 15 Slide 26 of 67 Exponential Population Growth Model • Populations with a high rN (per capita rate of increase) will have a steeper J-shaped curve than ones with a lower value • The J-shaped curve of exponential growth characterizes some rebounding populations Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 27 of 67 Figure 53.11 Exponential growth in the African elephant population of Kruger National Park Elephant population 8,000 6,000 4,000 2,000 0 1900 1920 1940 Year Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1960 1980 Concept 53.4: The logistic model describes how a population grows more slowly as it nears its carrying capacity (K) • The logistical growth model – 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 • Influenced by many factors such as: available water and food, predators, soil nutrients, suitable nesting sites Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 approaches K (K − N) dN = rmax N dt K Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 30 of 67 Population growth predicted by the logistic model Population size (N) • The logistic model of population growth produces a sigmoid (S-shaped) curve Exponential growth dN 1.0N = 2,000 dt 1,500 K = 1,500 Logistic growth 1,000 dN 1.0N dt = 1,500 – N 1,500 500 0 0 5 10 Number of generations Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 15 Slide 31 of 67 The Logistic Model and Real Populations Number of Paramecium/mL • The growth of laboratory populations of paramecia fits an S-shaped curve if the experimenter maintains a constant environment. 1,000 800 600 400 200 0 0 5 10 Time (days) 15 A Paramecium population in the lab Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 32 of 67 The Logistic Model and Real Populations Number of Daphnia/50 mL • Some populations overshoot K before settling down to a relatively stable density 180 150 120 90 60 30 0 0 20 40 60 80 100 120 140 160 Time (days) (b) A Daphnia population in the lab Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 33 of 67 The Logistic Model and Real Populations • Some populations fluctuate greatly and make it difficult to define K – Some populations have regular cycles of boom and bust – Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small – The logistic model fits few real populations but is useful for estimating possible growth Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 34 of 67 The Logistic Model and Real Populations 80 60 Number of females • Some populations fluctuate greatly around K: not well described by the logistical model 40 20 0 1975 1980 1985 1990 1995 2000 Time (years) (c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 35 of 67 The Logistic Model and Real Populations 160 120 Lynx 9 80 6 40 3 0 1850 Figure 52.21 Snowshoe hare Lynx population size (thousands) Hare population size (thousands) • Many populations undergo regular boom-andbust cycles 0 1875 1900 Year Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1925 Slide 36 of 67 Figure 53.14 White rhinoceros mother and calf Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 37 of 67 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 38 of 67 Concept 53.5: Many factors that regulate population growth are density dependent • There are two general questions about regulation of population growth: – What environmental factors stop a population from growing indefinitely? – Why do some populations show radical fluctuations in size over time, while others remain stable? Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 40 of 67 Fig. 53-15 Birth or death rate per capita Density-dependent birth rate Density-dependent birth rate Densitydependent death rate Equilibrium density Equilibrium density Population density (a) Both birth rate and death rate vary. Birth or death rate per capita Densityindependent death rate Densityindependent birth rate Density-dependent death rate Equilibrium density Population density (c) Death rate varies; birth rate is constant. Population density (b) Birth rate varies; death rate is constant. 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, disease, predation, accumulation of toxic wastes, and intrinsic factors – Example: Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 42 of 67 Gannets defend their territory by pecking others Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 43 of 67 Population Dynamics • The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size – Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time – Weather can affect population size over time Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 44 of 67 Immigration, Emigration, and Metapopulations • 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 Aland ˚ Islands EUROPE 5 km Occupied patch Unoccupied patch Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 45 of 67 Concept 53.6: The human population is no longer growing exponentially but is still increasing rapidly • Human population growth – No population can grow indefinitely, and humans are no exception Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings The Global Human Population 7 6 5 4 3 2 The Plague 8000 B.C.E. 4000 3000 2000 1000 B.C.E. B.C.E. B.C.E. B.C.E. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 0 1000 C.E. 1 2000 C.E. 0 Human population (billions) • The human population increased relatively slowly until about 1650 and then began to grow exponentially Slide 47 of 67 Human Population Growth Rate • Though the global population is still growing, the rate of growth began to slow during the 1960s Annual percent increase 2.2 2.0 1.8 1.6 1.4 2005 1.2 Projected data 1.0 0.8 0.6 0.4 0.2 0 1950 1975 2000 Year Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 2025 2050 Slide 48 of 67 Regional Patterns of Population Change • 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 49 of 67 Birth or death rate per 1,000 people Figure 53.24 Demographic transition in Sweden and Mexico, 1750–2025 (data as of 2005) 50 40 30 20 10 Sweden Birth rate Death rate 0 1750 1800 Mexico Birth rate Death rate 1850 1900 Year Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 1950 2000 2050 • The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women • Most of the current global population growth is concentrated in developing countries Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 51 of 67 Age Structure • One important demographic factor in present and future growth trends is a country’s age structure – Age structure is the relative number of individuals at each age – Commonly represented in pyramids – Male/female represented by color – Percent of population at each age is represented by a horizontal bar. Young at bottom, elderly at top. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 52 of 67 Figure 53.25 Age-structure pyramids for the human population of three countries (data as of 2005) Rapid growth Afghanistan Male 10 8 Female 6 4 2 0 2 4 6 Percent of population Slow growth United States Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 10 8 Male Female 6 4 2 0 2 4 6 Percent of population Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings No growth Italy Age 85+ 80–84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 8 8 Male Female 6 4 2 0 2 4 6 8 Percent of population Infant Mortality and Life Expectancy • Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 54 of 67 50 Life expectancy (years) Infant mortality (deaths per 1,000 births) 80 60 40 30 20 60 40 20 10 0 0 Indus- Less industrialized trialized countries countries Indus- Less industrialized trialized countries countries Estimates of Global Carrying Capacity • The carrying capacity of Earth for humans is uncertain • The average estimate is 10–15 billion Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 56 of 67 Limits on Human Population Size • The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation – It is one measure of how close we are to the carrying capacity of Earth – Countries vary greatly in footprint size and available ecological capacity Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 57 of 67 Figure 53.27 The amount of photosynthetic products that humans use around the world Log (g carbon/year) 13.4 9.8 5.8 Not analyzed Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings • A population of 500 experiences 55 births and 5 deaths during a one year period. What is the reproductive (growth) rate for the population during the one-year period? A. .01/ year B. .05/year C. .1/year D. 50/year Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 59 of 67 • For a population sample size of 1000 (N = 1000) there were 60 births and 10 deaths over a one year period. If the population maintains the current growth pattern, a plot of its growth would resemble A. exponential growth B. K-selected growth C. ZPG (zero population growth) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 60 of 67 You should now be able to: 1. Define and distinguish between the following sets of terms: density and dispersion; clumped dispersion, uniform dispersion, and random dispersion; life table and reproductive table; Type I, Type II, and Type III survivorship curves; semelparity and iteroparity; r-selected populations and K-selected populations 2. Explain how ecologists may estimate the density of a species Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 61 of 67 3. Explain how limited resources and trade-offs may affect life histories 4. Compare the exponential and logistic models of population growth 5. Explain how density-dependent and densityindependent factors may affect population growth 6. Explain how biotic and abiotic factors may work together to control a population’s growth Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Slide 62 of 67 Fig. 53-UN1 Patterns of dispersion Clumped Uniform Random Population size (N) Fig. 53-UN2 dN = rmax N dt Number of generations Population size (N) Fig. 53-UN3 K = carrying capacity K–N dN = rmax N dt K Number of generations Fig. 53-UN4 Fig. 53-UN5