Population ecology


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Population ecology

  1. 1.  Population ecology is the study of populations in relation to the environment. It includes environmental influences on population density and distribution, age structure, and variations in population size.
  2. 2.  Population size Population density Dispersion patterns Demographics Survivorship curves Population growth
  3. 3.  “In population genetics and population ecology, population size (usually denoted N) is the number of individual organisms in a population”. Factors that Govern Population Size1. Crude Birth Rate (CBR)2. Crude Death Rate (CDR)3. Immigration4. Emigration
  4. 4. Natality The birthrate, which is the ratio of total live births to total population in a particular area over a specified period of timeMortality The death rate, which is also the ratio of the total number of deaths to the total population.Immigration The number of organisms moving into area occupied by the population is called immigration.Emigration The number of organisms moving out of the area occupied by the population is called emigration.
  5. 5.  Factors that increase population size1. Natality is recruitment to a population through reproduction.2. Immigration from external populations e.g. Bird migration. Factor reducing population size1. Mortality which is the death rate from any source e.g. predation.2. Emigration, where individuals leave the population for another habitat.
  6. 6.  Population Change = (births + immigration) – (deaths + emigration) Parameters that effect size or density of a population
  7. 7.  “Population density is a measurement of the number of people in an area. It is an average number”. It is usually shown as the number of people per square kilometer.Density = Population/ Area
  8. 8. 1. How to quantify nature – ecologist role2. Estimates are allows for comparisons between different populations in terms of space and time measure.
  9. 9. Mobility Based on movements of these organisms 2 attributes SizeSmall animals/plants are usually more abundant than large animals/plants
  10. 10.  Density is measured by two methods.1. Total count method2. Sampling method
  11. 11.  Direct counting of populations. Possible for few animals. Breeding colonies can be photographed then later counted.Examples Human pop census Trees in a given area
  12. 12.  Depends on the type of organism and its natural abundance and distribution. Two broad categories:1. Plot-based (quadrant) methods2. Capture-based methods
  13. 13.  Widely used in plant studies Total population = Average number per quadrat × Total area/Area of quadrat
  14. 14.  Used for very mobile or elusive species First used in the 1890s by C. G. Peterson to estimate fish abundance.
  15. 15.  The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. random clumped 3 types uniform
  16. 16.  Clumped is a pattern when individuals are aggregated in patches. Most frequent pattern of distribution in a populationReasons: Some area of habitat are more suitable than others Heterogeneous environment with resources concentrated in patches Tendency of offspring to remain with parents Mating or social behavior of the individuals
  17. 17.  Evenly spaced distributions, in which members of the population maintain a minimum distance from one another. In plants due to competition for water, sunlight, or available nutrientsExampleCreosote bushes in the Mojave desert In animals due to strong territorialityExampleThe desert lizard Uta sp
  18. 18.  It is a spacing pattern based on total unpredictability. Least common pattern of distributionReasons Members of a species do not frequently interact with one another Not heavily influenced by the microenvironments within their habitat
  19. 19. Density Independent Density Dependant Floods  Competition for Hurricanes Resources Unseasonable  Predation Weather  Parasitism Fire  Infectious Disease Clear Cutting Pesticide Spraying
  20. 20.  Demography is the study of the vital statistics of a population and how they change over time Twostatistics that are of particular import are a populations age structure and a populations sex ratio.
  21. 21.  Itis the ratio of males to females in a population. Primary sex ratio Secondary sex ratio Tertiary sex ratio The human sex ratio is of particular interest to anthropologists and demographers.
  22. 22.  The age structure describes the number of individuals in each age class as a ratio of one class to another. Age classes can be specific categories, such as people in the same age range.
  23. 23.  Age structure diagram Graphical illustration that shows the distribution of various age groups & sex ratio in a population. Three age categories:1. Prereproductive (ages 0-14)2. Reproductive (ages 15-44)3. Postreproductive (ages 45 and up)
  24. 24. A life table (mortality table ) is a table which shows, for each age, what the probability is that a person of that age will die before his or her next birthday. From this starting point, a number of inferences can be derived.1. The probability of surviving any particular year of age2. Remaining life expectancy for people at different ages Separately for men and for women because of their substantially different mortality rates.
  25. 25.  Graph showing the number or proportion of individuals surviving at each age for a given species or group (e.g. males/females). Constructed for a given cohort (a group of individuals of roughly the same age) based on a life table. Three types1. Type I2. Type II3. Type III
  26. 26.  Type I survivorship curves are characterized by high survival in early and middle life, followed a rapid decline in survivorship in later life.Example: Humans Type II curves are an intermediate between Type I and III, where roughly constant mortality rate is experienced regardless of age.Example: Some birds In Type III curves, the greatest mortality is experienced early on in life, with relatively low rates of death for those surviving this bottleneck.Example: Octopus
  27. 27.  Refers to how the number of individuals in a population increases (or decreases) with time. Controlled by the rate at which new individuals are added to the population -- the birth rate, and the rate at which individuals leave the population -- the death rate.
  28. 28. Exponential population growth dN = rmaxN dt 2 types of pop growth Logistic populationPopulation Growth growthMathematicallyDefined dN = rmaxN (K-N) dt K
  29. 29. N=K/2
  30. 30.  If a population has a constant birth rate through time and is never limited by food or disease, it has what is known as exponential growth. With exponential growth the birth rate alone controls how fast (or slow) the population grows.
  31. 31.  Continuouspopulation growth in an unlimited environment can be modeled exponentially. dN / dt = rmax N As population size (N) increases, rate of population increase (dN/dt) gets larger.
  32. 32.  For an exponentially growing population, size at any time can be calculated as: Nt = Noert Nt = number individuals at time t. N0 = initial number of individuals. e = base of natural logarithms. r (rmax ) = per capita rate of increase. t = number of time intervals.
  33. 33.  As resources are depleted, population growth rate slows and eventually stops: logistic population growth. Sigmoid (S-shaped) curve Carryingcapacity (K) is the number of individuals of a population the environment can support. Finite amount of resources can only support a finite number of individuals.
  34. 34. dN/dt = rmaxN(1-N/K) rmax= Maximum per capita rate of increase under ideal conditions. When N nears K, the right side of the equation nears zero. As population size increases, logistic growth rate becomes a small fraction of growth rate. Highest when N=K/2
  35. 35. Carrying capacity Environmental limits result in logistic growth No limitsNew or changed environment Logistic growth curve
  36. 36.  Environment limits population growth by altering birth and death rates. Density-dependent factors  Disease, Parasites, Resource Competition  Populations do not show continuous geometric increase  When density increases other organisms reduces the fertility and longevity of the individuals in the population  This reduces the rate of increase of the pop until eventually the pop ceases to grow  The growth curve is defined as the sigmoid curve, S – shaped  K = carrying capacity (upper asymptote or maximum value) – the maximum number of individuals that environment can support Density-independent factors  Natural disasters  Climate
  37. 37. K and r selection (MacArthur and Wilson 1967)r-selected species•r refers to the per capita rate ofincrease•Selection favoring rapid growth•Should be favored in new or disturbedenvironments•Less competitionK-selected species•K refers to carrying capacity•More prominent in species that aretypically at their carrying capacity•Favors more efficient use of resources•Live with competition
  38. 38.  The history of human population growth Figure 35.8A