Chapter 54

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Chapter 54

  1. 1. Population Ecology
  2. 2. Studying Populations <ul><li>A population consists of all the individuals of a species in a given area. </li></ul><ul><li>Population structure describes the age distribution of individuals, and how those individuals are spread over the environment. </li></ul>
  3. 3. <ul><li>The number of individuals per unit area or volume is the population density . </li></ul><ul><li>Density has strong influence over how individuals react with one another and with populations of other species. </li></ul>Studying Populations
  4. 4. <ul><li>Population structure changes over time due to demographic events : births, deaths, immigration, and emigration. </li></ul><ul><li>These events create population dynamics . Study of these events is called demography . </li></ul>Studying Populations
  5. 5. <ul><li>Population ecologists measure number and density of individuals, rates of demographic events, and locations of individuals. </li></ul><ul><li>Individuals are often tagged or marked in some way to facilitate research. </li></ul><ul><li>Tracking devices are also used. They may provide additional physiological and environmental data. </li></ul>Studying Populations
  6. 6. Figure 54.1 By Their Marks You May Know Them
  7. 7. <ul><li>A life table can be constructed by tracking a group of individuals born at the same time: a cohort . </li></ul><ul><li>Numbers that are still alive at later dates ( survivorship ) are determined. </li></ul><ul><li>Some life tables include fecundity : number of offspring produced in a time interval. </li></ul>Studying Populations
  8. 8. <ul><li>Life tables can be used to predict future trends. </li></ul><ul><li>The data can be plotted to show survivorship in relation to age. </li></ul><ul><li>Survivorship curves fall into three different general patterns. </li></ul>Studying Populations
  9. 9. Figure 54.3 Survivorship Curves
  10. 10. <ul><li>Age distribution reveals information about recent births and deaths. </li></ul><ul><li>Example: human population of the U.S. </li></ul>Studying Populations
  11. 11. Life Histories <ul><li>An organism’s life history describes how it allocates time and energy among the various activities throughout its life. </li></ul><ul><li>For example, some animals have a single offspring per reproductive episode, some have many. Some species, such as salmon and agave reproduce only once and then die. </li></ul>
  12. 12. Population Densities <ul><li>All populations have the potential for explosive growth. </li></ul><ul><li>Even when per capita growth rate remains constant, as population size increases, number of new individuals added per time unit increases: exponential growth . </li></ul>
  13. 13. <ul><li>The term Δ N/ Δ t is the rate of change of the population over time. </li></ul><ul><li>r is the net reproductive rate . </li></ul>Population Densities
  14. 14. <ul><li>The highest possible value for r is r max or the intrinsic rate of increase . </li></ul>Population Densities
  15. 15. <ul><li>Real populations cannot maintain exponential growth for long. </li></ul><ul><li>Environmental limits cause birth rates to decrease and death rates to increase. </li></ul><ul><li>The environmental carrying capacity ( K ) is the number of any particular species that can be supported in an environment. </li></ul>Population Densities
  16. 16. <ul><li>Carrying capacity is determined by availability of resources such as food or shelter, plus factors such as diseases and parasites, and social interactions. </li></ul><ul><li>Growth of a population usually slows when it nears carrying capacity. </li></ul><ul><li>A graph of population size over time forms an </li></ul><ul><li>S-shaped curve , and is known as logistic growth . </li></ul>Population Densities
  17. 17. Figure 54.8 Logistic Population Growth
  18. 18. <ul><li>Logistic growth can be modeled by adding a term for carrying capacity to the equation for population growth: </li></ul><ul><li>Growth stops when N = K . </li></ul>Population Densities
  19. 19. <ul><li>Birth rates and death rates are influenced by density-dependent factors: </li></ul><ul><li>As population density increases, food supplies may be depleted, reducing amount of food available to individuals. </li></ul><ul><li>Predators may be attracted to high densities of prey, increasing death rate. </li></ul><ul><li>Diseases can spread more easily. </li></ul>Population Densities
  20. 20. <ul><li>Other factors that influence populations are density-independent , such as weather-related phenomena. </li></ul>Population Densities
  21. 21. <ul><li>In general, more stable population numbers are seen in species with long-lived individuals and low reproductive rates. </li></ul><ul><li>For example, insect populations tend to fluctuate more than those of birds and mammals. </li></ul><ul><li>Environmental factors can change carrying capacity for species. </li></ul>Population Densities
  22. 22. <ul><li>Some species tend to be more common than </li></ul><ul><li>others. Four factors have strong influence on the </li></ul><ul><li>variation of population density among species: </li></ul><ul><li>Resource abundance </li></ul><ul><li>Size of individuals </li></ul><ul><li>Length of time a species has lived in an area </li></ul><ul><li>Social organization </li></ul>Population Densities
  23. 23. <ul><li>Species that use abundant resources generally reach higher population densities than those using scarce resources. </li></ul><ul><li>Animals that eat plants are generally more abundant than animals that eat other animals. </li></ul>Population Densities
  24. 24. <ul><li>Species with small body size generally reach higher population densities. </li></ul><ul><li>Small individuals require less energy to survive than large ones. </li></ul><ul><li>This is illustrated by mammal species worldwide. </li></ul>Population Densities
  25. 25. Figure 54.11 Population Density Decreases as Body Size Increases
  26. 26. <ul><li>Complex social organization can lead to high population densities. </li></ul><ul><li>Highly social species, such as ants, termites, and humans, can achieve very high densities. </li></ul>Population Densities
  27. 27. Space & Population Dynamics <ul><li>Most populations are divided into live in habitat patches . </li></ul>
  28. 28. Managing Populations <ul><li>Numbers of births and growth of individuals tend to be highest when population is below carrying capacity. </li></ul><ul><li>If humans wish to maximize the number of individuals harvested from a population, we should try to maintain it below carrying capacity. </li></ul>
  29. 29. <ul><li>Whaling has also resulted in declining populations. </li></ul><ul><li>Most whale populations have failed to recover. </li></ul><ul><li>Whales are large animals with slow reproductive rates. Many adults are needed to produce a small number of offspring. </li></ul>Managing Populations
  30. 30. <ul><li>Humans wish to decrease the size of populations of many pest species. </li></ul><ul><li>Reducing population numbers below carrying capacity stimulates higher birth rates and growth of the population. </li></ul><ul><li>A more effective approach is to remove the resources for the population, (e.g., making garbage unavailable for rats). </li></ul>Managing Populations
  31. 31. <ul><li>The size of the human population now contributes to most environmental problems. </li></ul><ul><li>Human social organization and specialization has allowed us to increase the carrying capacity for humans. </li></ul>Managing Populations
  32. 32. <ul><li>Earth’s current carrying capacity for humans is set in part by the biosphere’s ability to absorb our by-products, especially CO 2 from fossil fuels; also by water availability and our willingness to cause extinction of other species to accommodate our increasing use of Earth’s resources. </li></ul>Managing Populations

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