Population Ecology

1. Population Ecology Campbell & Reece Chapter 53

2. Population Ecology • the study of populations in relation to their environment

3. Dynamic Biologic Processes that Influence Population Density • Population: is a group of individuals of a single species living in same generral area • Members of a population rely on same resources & are influenced by same environmental factors • They are likely to interact & breed with each other

4. Populations • often described by their boundaries & #s • boundaries may be natural ones or ecologists may arbitrarily define them

5. Population Density • # of individuals per unit area or volume

6. Dispersion • the pattern of spacing among individuals w/in boundaries of the population

7. Mark-Recapture Method • way to determine population size: • ecologists cannot count all individuals in a population if organisms move too quickly or are hidden from view • Technique: capture a random sample of individuals & “mark” & then release them. • Some species can be identified w/out physically capturing them: dolphin, whale

8. Mark-Recapture Method

9. Population Dynamics • Population density is not a static property: • Births • Deaths • Immigration • Emmigration

10. Patterns of Dispersion

11. Patterns of Dispersion: Clumped • *most common • plants & fungi clumped where soil conditions & other environmental factors favor germination & growth • animal clumping may have to do with being successful in some way: • Mayflies swarm in great #s which increases their chances of mating (only have ~2 days) • Wolf pack more likely to kill a moose or deer than a single wolf

12. Mayfly Swarm

13. Patterns of Dispersion: Uniform • may result from direct interactions between individuals in a population • some plants secrete chemicals that inhibit germination & & growth of nearby individuals that could compete for resources • animals that show territoriality are spaced apart: often as result of antagonistic interactions

15. Patterns of Dispersion: Random • unpredictable spacing • 1 individual’s position is unrelated to other individuals

16. Demographics • study of the vital statistics of a population & how it changes over time • especially important are birth rates & death rates

17. Life Table • age-specific summaries of the survival pattern of a population • construct one by following the fate of a cohort: a group of individuals of the same age, from birth until all of them are dead

18. Life Table of Belding’s Ground Squirrels

19. Survivorship Curves • a graphic method of representing some of the data in a life table • plots proportion or #s in a cohort still alive at each age

20. Idealized Survivorship Curves

21. Type I Curve • flat @ first reflecting low death rate during early & middle life • then steep drop as death rate increases with increasing age • typical curve for large mammals that produce few young but take good care of them

22. Type III Curve • drops sharply at start reflecting high death rate among the young • flattens out as death rate for those individuals that make it out of early life decreases • typical for species that produce many offspring but provide little or no care for them: • fishes, invertebrates

23. Type II Curve • intermediate • have a constant death rate • typical of rodents, some invertebrates, some lizards, & some annuals

24. Survivorship Curves • not all species fall into 1 of the 3 types • some invertebrates have a “stair-step” pattern: more vulnerable during periods when molting, less vulnerable when not molting

25. Reproductive Rates • Demographers tend to just look @ #s of females & how many female offspring they have • simplest way to describe the reproductive pattern of a population is to ask how reproductive output varies with the age of females

26. Reproductive Table • aka a fertility schedule • an age-specific summary of the reproductive rates in a population • constructed by measuring reproductive output of a cohort from birth to death

29. Natural Selection • favors traits that improve an organism’s chances of survival & reproductive success • every species has trade-offs between survival & traits such as frequency of reproduction, # of offspring • traits that affect an organism’s schedule of reproduction & survival make up its life history

30. Life Histories • are very divers but exhibit patterns in the variability • have 3 basic variables: • when reproduction begins • how often the organism reproduces • how many offspring produced during each reproductive episode

31. Big-Bang Reproduction • 1 individual reproduces large # of offspring then die: called semelparity

32. Iteroparity • produce only a few offspring during repeated reproductive episodes

33. Semelparity or Iteroparity? • Which is better? • Critical factor is survival of offspring: • *when survival of offspring is low as in highly variable or unpredictable environments, semelparity is favored • *dependable environments where competition of resources is fierce favors iteroparity

34. Limited Resources means Trade-Offs • sometimes see trade-offs between survival & reproduction when resources are limited • example: red deer females have higher mortality in winters following summers in which they reproduce

35. Species whose Young have High Mortality Rates • often produce large #s of relatively small offspring • example: plants that colonize disturbed environments usually produce many small seeds  only a few reach suitable habitat • smaller seeds allow them to be carried farther

36. Parental Investment Increases Survival of Offspring • example: oak, walnut, & coconut trees have large seeds with large store of nrg & nutrients to help seedlings become established • example: primates provide an extended period of parental care

37. Population Growth • All populations have a tremendous capacity for growth • Unlimited increase does not occur indefinitely for any species, in lab or in nature

38. Exponential Model of Population Growth • unlimited growth does not occur for long in nature but it is assumed to be true in this model • Δ population = (# births + # immigrants ) - (# deaths + # emigrants) • Or, ignoring immigration & emigration: • ΔN/Δt = B - D

39. Exponential Model • now use average #s of births & deaths per individual during a specified period of time • If there are 34 births per year in a population of 1,000 then the annual per capita birth rate is 34/1,000 = 0.034 = b

40. Exponential Growth Equation • ΔN/dt = TMax N: represents a population’s potential growth in an unlimited environment where TMax is the maximum per capita rate of increase & N is the # of individuals in the population

41. Exponential Growth

42. Exponential Model • http://www.slideshare.net/MrDPMWest/population-growth-apbio

43. Logistic Model • in nature, as any population density increases: each individual has access to fewer resources • eventually, there is a limit to the # of individuals that can occupy a habitat • Carrying Capacity: (K) the maximum population size that a particular environment can sustain

44. Carrying Capacity • varies over space & time with the abundance of limiting resources: • Energy • Shelter • Refuge from predators • Nutrients • Water • Suitable nesting sites

45. Carrying Capacity • per capita birth rate decreases if there are not enough nutrients for adults to maintain themselves or if disease or parasitism increases with density • per capita death rate increases for same reasons • either way: results in lower per capita rate of increase

46. Logistic Growth Model • per capita rate of increase approaches 0 as the carrying capacity is reached

47. Logistic Growth Equation

48. Logistic Model • fits few real populations perfectly, but it is useful for estimating possible growth

49. Life History Traits are Products of Natural Selection • traits that affect an organism’s schedule of reproduction & survival make up its: • life history: • 3 main variables: • age @ 1st reproduction • how often the organism reproduces • # offspring produced/ reproductive episode

50. Semelparous Organisms • reproduce once & die • aka “big-bang”