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  1. 1. Population EcologyPopulation Ecology Cyra Mae R. Soreda
  2. 2. Population EcologyPopulation Ecology 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.
  3. 3. Characteristics of PopulationCharacteristics of Population Population size Population density Dispersion patterns Demographics Survivorship curves Population growth
  4. 4. Population sizePopulation size “In population genetics and population ecology, population size (usually denoted N) is the number of individual organisms in a population”. Factors that Govern Population Size 1.Crude Birth Rate (CBR) 2.Crude Death Rate (CDR) 3.Immigration 4.Emigration
  5. 5. Factors That Increase Population Size 1.Natality is recruitment to a population through reproduction. 2.Immigration from external populations e.g. Bird migration. Factor Reducing Population Size 1.Mortality which is the death rate from any source e.g. predation. 2.Emigration, where individuals leave the population for another habitat.
  6. 6. NatalityNatality The production of new individuals by birth, hatching, germination or fission 2 aspects of reproduction must be distinguished:  Fecundity  fertility
  7. 7. NatalityNatality Fecundity-physiological notion that refers to an organism’s potential reproductive capacity Fertility-ecological concept based on the no. of viable offspring produced during a period time Realized fertility and potential fecundity-we must be able to distinguish between them
  8. 8. NatalityNatality E.g, realized fertility rate for a human pop may be only 1 birth per 15 years per female in the child- bearing ages While the potential fecundity rate for humans is 1 birth per 10 to 11 months per female in the childbearing ages
  9. 9. MortalityMortality Biologists-interested not only in why organisms die but also why they die at a given age Longevity-the age of death of individuals in a population 2 types: ◦ Potential longevity ◦ Realized longevity
  10. 10. MortalityMortality Potential longevity ◦ The maximum life span of an individual of a particular sp is a limit set by the physiology of the organism, such that it simply dies of old age ◦ The average longevity of individuals living under optimum conditions ◦ However, organisms rarely live under optimum conditions-most die from disease, or eaten by predators or succumb to a number of natural hazards
  11. 11. MortalityMortality Realized longevity ◦ The actual life span of an organism ◦ Can be measured in the field, while potential longevity only in labs or zoos
  12. 12. examplesexamples European robin has an average life expectation of 1 year in the wild, whereas it can live at least 11 year in captivity
  13. 13. Have more births than deaths? ◦ Population increases Have more deaths than births? ◦ Population decreases Have equal amounts of births and deaths? ◦ Population remains constant What happens to the populationWhat happens to the population when we….when we….
  14. 14. ImmigrationImmigration “im”= in Migrate= to move from one place to another Immigration is the individual movement into an area Animals in search of mates and food in new areas
  15. 15. EmigrationEmigration “E” means ‘out’ Migrate means to move from one place to another Emigrate means individuals moving out of one place and into another Young wolves and bears leaving as they mature Shortage of food
  16. 16. NATALITYNATALITY The birthrate, which is the ratio of total live births to total population in a particular area over a specified period of time MORTALITYMORTALITY The death rate, which is also the ratio of the total number of deaths to the total population. IMMIGRATIONIMMIGRATION The number of organisms moving into area occupied by the population is called immigration. EMIGRATIONEMIGRATION The number of organisms moving out of the area occupied by the population is called emigration.
  17. 17. How to estimate populationHow to estimate population density?density? Techniques differ between organisms such that the technique to estimate deer cannot be applied to bacteria or protozoa or vice versa There are 2 fundamental attributes that affect and ecologists choice of technique for population estimation
  18. 18. 2 attributes Size -small animals/plants are usually more abundant than large animals/plants Mobility -based on movements of these organisms
  19. 19. Why the need to estimateWhy the need to estimate population density?population density? Estimates of population are made for two reasons: ◦ How to quantify nature – ecologist role ◦ Estimates are allows for comparisons between different populations in terms of space and time measure
  20. 20. 2 BROAD APPROACHES TO ESTIMATE POP DENSITY Absolute density No of individual per area/ per volume Important for conservation and management Relative density Comparative no of organisms Two areas of equal sizes, which area has more organism e.g, between area x and y Area x has more organism than area y
  21. 21. ABSOLUTE DENSITYABSOLUTE DENSITY Making total counts and by using sampling methods Total counts - direct counting of populations - human pop census, - trees in a given area, - breeding colonies can be photographed then later counted - in general total counts are possible for few animals
  22. 22. Measurements of Absolute densityMeasurements of Absolute density  Sampling methods ◦ to count only a small proportion of the population - sample  Using the sample to estimate the total population  2 general sampling techniques: 1) Use of quadrats 2) Capture-recapture method
  23. 23. Use of Quadrats Count all individuals on several quadrats of known size, then extrapolate the average count to the whole area Quadrat- a sampling area of any shape (may be a rectangle, triangle or circle) 3 requirements: • the pop in the quadrat must be determined exactly • area of the quadrant must be known • quadrant/s must be representative of the area • achieved by random sampling
  24. 24. Quadrant sampling in plantQuadrant sampling in plant populationpopulation Conduct a transect in the upland hardwood forest 3 transect line, 110 meters long, count all trees taller than 25cm within 1meter of each line By utilizing the quadrant method sampling for old trees and seedlings, we can determine if populations were likely to change over time
  25. 25. Capture Recapture Method Capture, marking, release, and recapture-important for mobile animals Why?-it allows not only an estimate of density but also estimates of birth rate and death rate for the population being studied Capture animal, mark (tag) them and then release them Peterson method: Involves 2 sampling periods Capture, mark and release at time 1 Capture and check for marked animals at time 2 Time intervals between the 2 samples must be short because this method assumes a closed population with no recruitment of new individuals into the Population between time 1 and 2 and no losses of marked individuals
  26. 26. Formula for capture-recaptureFormula for capture-recapture methodmethod Marked animals in 2nd sample = Marked animals in 1st sample Total caught in 2nd sample Total population size
  27. 27. e.g of capture recapture methode.g of capture recapture method Dahl marked trout in small Norwegian lakes to estimate the size of the population that was subject to fishing. He marked and released 109 trout, and in 2nd sample a few days later caught 177 trout, of which 57 were marked. From the data, what is the estimate population size?
  28. 28. e.g of capture recapture methode.g of capture recapture method By using the formula 57 = 109 177 Total pop size Total pop size = (109 X 177) 57 = 338 trout
  29. 29. RELATIVE DENSITYRELATIVE DENSITY Traps – no caught per day per trap – animals caught will depend on their density, activity and range of movement, skill in placing traps – rough idea of abundance – night flying insects, pitfall traps for beetles, suction traps for aerial insects Fecal pellets – rabbits, deer, field mice – provides an index of pop size Vocalization frequency – bird calls per 10 mins, can be used for frogs, cicadas, crickets Pelt records – trapper records dates back 300 years – of lynx
  30. 30. Relative densityRelative density Catch per unit effort – index of fish abundance – no of fish per cast net or no of fish per 1 hour trawling Number of artifacts – thing left behind – pupal cases of emerging insects Questionnaires – to sportsmen (eg fish)and trappers Cover - % ground surface covered – in botany, invertebrate studies of the rocky intertidal zone Feeding capacity – bait taken – for rats and mice – index of density Roadside counts – birds observed while driving standard distances
  31. 31. Population dispersionPopulation dispersion patternspatterns 3 types random uniformclumped
  32. 32. Population dispersion patterns Random-when the position of each individuals in a pop is independent of the others Uniform-it results as a form of some negative interactions Common among animal pop where individuals defend an area for their own exclusive use (territoriality) or in plant pop where severe competition exist for belowground resources, i.e water or nutrients
  33. 33. Population dispersion patternsPopulation dispersion patterns Clumped-where individuals occur in groups Reason-suitable habitat or resources may be distributed as patches on a larger landscape
  35. 35. Population growthPopulation growth Refers to how the number of individuals in a population increases or decreases with time (N, t) Reflects the difference between rates of birth and death  in pop, if new births occur  in pop, if death occurs
  36. 36. 2 types of pop growth Exponential population growth dN = rmaxN dt Logistic population growth dN = rmaxN (K-N) dt K Population Growth Mathematically Defined
  37. 37. N=K/2
  38. 38. Exponential GrowthExponential Growth Continuous population growth in an unlimited environment can be modeled exponentially. dN / dt = rmax N Appropriate for populations with overlapping generations. ◦ As population size (N) increases, rate of population increase (dN/dt) gets larger.
  39. 39. Exponential GrowthExponential Growth 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.
  40. 40. PracticePractice If the human population size in 1993 was 5.4 billion, what was the projected population size in the year 2000? r=0.0139 No = population size in 1993 = 5.4 billion t = 7 years (year 2000 - 1993) r = 0.0139
  41. 41. Nt = No ert Nt = (540,000,000) e(0.0139)(7) Nt /540,000,000 = e 0.0973 Dust off your high school math skills. To get rid of the exponent, simply take the (ln) of both sides of the equation. Remember, when we take the natural log of a quotient we end up taking the ln of one value and subtracting it from the ln of the other value (see below).
  42. 42. ln (Nt /540,000,000) = ln (e 0.0973) [here we're taking the natural log of the quotient] = ln(Nt) - ln(540,000,000) = 0.0973 [rewrite it as natural log of one value minus natural log of the other value] Nt = 595,000,000 or 5.95 billion
  43. 43. Logistic Population GrowthLogistic Population Growth As resources are depleted, population growth rate slows and eventually stops: logistic population growth. ◦ Sigmoid (S-shaped) population growth curve. ◦ Carrying capacity (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.
  44. 44. Logistic Population GrowthLogistic Population Growth 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.  N/K = Environmental resistance.
  45. 45. ProblemProblem Suppose a population of butterflies is growing according to the logistic equation. If the carrying capacity is 500 butterflies and r = 0.1 individuals/ (individual*month), what is the maximum possible growth rate for the population?
  46. 46. To solve this, you must first determine N, population size. From the plot of dN/dt vs. N, we know that the maximum possible growth rate for a population growing according to the logistic model occurs when N = K/2, here N = 250 butterflies. Plugging this into the logistic equation: DN/dt = rN [1- (N/K)] = 0.1(250)[1-(250/500)] = 12.5 individuals / month
  47. 47. Fig. 11.9Fig. 11.9
  48. 48. Limits to Population GrowthLimits to Population Growth 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
  49. 49. r- and k-speciesr- and k-species Characteristics of r- species  high biotic potential  Rapid development  Early reproduction  Single period reproduction per individual  Short lifecycle  Small body size  Regulated by the density- independent factor
  50. 50. Characteristics of k- species  low biotic potential  slow development  delayed reproduction  multiple period reproduction per individual  long lifecycle  large body size  Regulated by the density-dependent factor
  51. 51. Life history strategiesLife history strategies K and r selection (MacArthur and Wilson 1967) r-selected species •r refers to the per capita rate of increase •Selection favoring rapid growth •Should be favored in new or disturbed environments •Less competition K-selected species •K refers to carrying capacity •More prominent in species that are typically at their carrying capacity •Favors more efficient use of resources •Live with competition