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Chapter 53

Chapter 53. Population Ecology. Concept 53.1: Dynamic biological processes influence population density, dispersion, and demographics.

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Chapter 53

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  1. Chapter 53 Population Ecology

  2. Concept 53.1: Dynamic biological processes influence population density, dispersion, and demographics. • Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size • A population is a group of individuals of a single species living in the same general area

  3. 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

  4. Fig. 53-2 APPLICATION Hector’s dolphins

  5. Fig. 53-3 Births Deaths Births and immigration add individuals to a population. Deaths and emigration remove individuals from a population. Immigration Emigration

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

  7. 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

  8. Table 53-1

  9. Fig. 53-6 1,000 I 100 II Number of survivors (log scale) 10 III 1 0 50 100 Percentage of maximum life span

  10. 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 • Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

  11. Evolution and 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 • Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction

  12. “Trade-offs” and Life Histories • Natural selection cannot maximize all reproductive variables simultaneously. • Organisms have finite resources, which may lead to trade-offs between survival and reproduction. • Plants and animals whose young are subject to high mortality rates often produce large numbers of relatively small offspring. • In other organisms, extra investment on the part of the parent greatly increases the offspring’s chances of survival.

  13. Fig. 53-9 (a) Dandelion (b) Coconut palm

  14. Fig. 53-8 RESULTS 100 Male Female 80 60 Parents surviving the following winter (%) 40 20 0 Reduced brood size Normal brood size Enlarged brood size

  15. Concept 53.3: 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 • If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

  16. N  rN t Per Capita Rate of Increase • 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: where N = population size, t = time, and r = per capita rate of increase = birth – death

  17. 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 • Equation of exponential population growth:

  18. Fig. 53-10 2,000 dN 1.0N = dt 1,500 dN 0.5N = dt Population size (N) 1,000 500 0 0 5 10 15 Number of generations

  19. Concept 53.4: The logistic model describes how a population grows more slowly as it nears its 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

  20. (K  N) dN  rmax N dt K 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

  21. Table 53-3

  22. Fig. 53-12 Exponential growth 2,000 dN 1.0N = dt 1,500 K = 1,500 Population size (N) Logistic growth 1,000 1,500 – N dN 1.0N = 1,500 dt 500 0 0 5 10 15 Number of generations

  23. Fig. 53-13 180 1,000 150 800 120 Number of Paramecium/mL Number of Daphnia/50 mL 600 90 400 60 200 30 0 0 0 5 10 15 0 20 40 60 80 100 120 140 160 Time (days) Time (days) (a) A Paramecium population in the lab (b) A Daphnia population in the lab

  24. 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

  25. 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?

  26. 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

  27. Fig. 53-15 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 (a) Both birth rate and death rate vary. (b) Birth rate varies; death rate is constant. Density-dependent death rate Density- independent birth rate Birth or death rate per capita Equilibrium density Population density (c) Death rate varies; birth rate is constant.

  28. 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, toxic wastes, and intrinsic factors

  29. Fig. 53-16 100 80 60 Percentage of juveniles producing lambs 40 20 0 200 300 400 500 600 Population size

  30. Fig. 53-17 (a) Cheetah marking its territory (b) Gannets

  31. Other Density-Dependent Limiting Factors • 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.

  32. Population Dynamics • The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size • Changes in predation pressure can drive population fluctuations • Mathematical and computer models are used to illustrate and investigate population interactions within and environmental impacts on a community.

  33. Fig. 53-19 2,500 50 Wolves Moose 2,000 40 30 1,500 Number of wolves Number of moose 20 1,000 10 500 0 0 1955 1965 1975 1985 1995 2005 Year

  34. Calculating Lag Time • Use the graph below to calculate lag time in months between the change in the densities of the prey and the predator populations. • To find lag time, calculate the difference in the predator/prey size between peaks or valleys.

  35. Population Cycles: Scientific Inquiry • Some populations undergo regular boom-and-bust cycles • Lynx populations follow the 10 year boom-and-bust cycle of hare populations • Three hypotheses have been proposed to explain the hare’s 10-year interval

  36. Fig. 53-20 Snowshoe hare 160 120 9 Lynx Number of lynx (thousands) Number of hares (thousands) 80 6 40 3 0 0 1850 1875 1900 1925 Year

  37. Variation and Population Dynamics • The level of variation in a population affects population dynamics. • A population’s ability to respond to changes in the environment is affected by genetic diversity. • Species and populations with little genetic diversity are at risk for extinction. • Illustrative Examples: California condors, prairie chickens

  38. Variation and Population Dynamics • Genetic diversity allows individuals in a population to respond differently to the same changes in environmental conditions: • Not all animals in a population stampede • Not all individuals in a population are equally affected in a disease outbreak

  39. Concept 53.6: The human population is no longer growing exponentially but is still increasing rapidly. • No population can grow indefinitely, and humans are no exception • The human population increased relatively slowly until about 1650 and then began to grow exponentially • Though the global population is still growing, the rate of growth began to slow during the 1960s

  40. Fig. 53-22 7 6 5 4 Human population (billions) 3 2 The Plague 1 0 8000 B.C.E. 4000 B.C.E. 3000 B.C.E. 2000 B.C.E. 1000 B.C.E. 0 1000 C.E. 2000 C.E.

  41. Fig. 53-23 2.2 2.0 1.8 1.6 1.4 2005 1.2 Annual percent increase Projected data 1.0 0.8 0.6 0.4 0.2 0 1950 1975 2000 2025 2050 Year

  42. 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

  43. Fig. 53-24 50 40 30 Birth or death rate per 1,000 people 20 10 Sweden Mexico Birth rate Birth rate Death rate Death rate 0 1850 1900 1750 1800 1950 2000 2050 Year

  44. 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

  45. Fig. 53-25 Rapid growth Slow growth No growth Afghanistan United States Italy Male Female Age Male Female Age Male Female 85+ 85+ 80–84 80–84 75–79 75–79 70–74 70–74 65–69 65–69 60–64 60–64 55–59 55–59 50–54 50–54 45–49 45–49 40–44 40–44 35–39 35–39 30–34 30–34 25–29 25–29 20–24 20–24 15–19 15–19 10–14 10–14 5–9 5–9 0–4 0–4 10  8 6 4 2 0 2 4 6 8 10  8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 Percent of population Percent of population Percent of population

  46. Fig. 53-26 60 80 50 60 40 Life expectancy (years) Infant mortality (deaths per 1,000 births) 30 40 20 20 10 0 0 Less indus- trialized countries Less indus- trialized countries Indus- trialized countries Indus- trialized countries

  47. Global Carrying Capacity • How many humans can the biosphere support? • The carrying capacity of Earth for humans is uncertain • The average estimate is 10–15 billion • Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes

  48. Fig. 53-27 Log (g carbon/year) 13.4 9.8 5.8 Not analyzed

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