1. POPULATION DISTRIBUTION AND ABUNDANCE
Chapter 9
Molles: Ecology 2nd Ed.

2. Physical environment limits geographic distribution of species
Chapter Concepts
Physical environment limits geographic distribution of species
On small scales, individuals within pops. are distributed in random, regular, or clumped patterns; on larger scales, individuals within pop. are clumped
Population density declines with increasing organism size
Rarity influenced by geographic range, habitat tolerance, pop. size; rare species vulnerable to extinction
Molles: Ecology 2nd Ed.

3. Populations
Ecologists define a population as group of individuals of single species inhabiting specific area.
Molles: Ecology 2nd Ed.

4. Habitat
Physical environmental conditions that allow individuals of species to survive AND reproduce
Molles: Ecology 2nd Ed.

5. Ability of environmental conditions to support repro and survival
Habitat quality
Ability of environmental conditions to support repro and survival
Habitat area/volume
Resource concentration
Time
High habitat quality = organisms acquire many resources; high survival + repro = large pop.
Molles: Ecology 2nd Ed.

6. Population numbers vary with habitat quality
Molles: Ecology 2nd Ed.

7. Physical environment limits geographic distribution of species
Distribution Limits
Physical environment limits geographic distribution of species
Organisms can only compensate so much for environmental variation
Molles: Ecology 2nd Ed.

8. Geographical range
Geographic area where species is found (based on macroclimate, salinity, nutrients, oxygen, light, etc.)
Molles: Ecology 2nd Ed.

9. “Large-scale” patterns of distribution:
Refer to variation in species abundance w/in range
due to variation in habitat quality
Molles: Ecology 2nd Ed.

10. Kangaroo Distributions and Climate
Caughley - relationship between climate + distribution of three largest kangaroos in Australia
Molles: Ecology 2nd Ed.

11. Macropus giganteus – eastern grey Eastern 1/3 of continent temperate forest, tropical forest
Molles: Ecology 2nd Ed.

12. Macropus fuliginosus – western grey southern and western regions temperate woodlands and shrubs
Molles: Ecology 2nd Ed.

13. Macropus rufus – red arid / semiarid interior
Molles: Ecology 2nd Ed.

14. Distributions largely based on climate
Fig 9.2
Distributions largely based on climate
Molles: Ecology 2nd Ed.

15. Kangaroo Distributions and Climate
Limited distributions may not be directly determined by climate.
Climate often influences species distributions via:
food production
water supply
habitat
incidence of parasites, pathogens and competitors
Molles: Ecology 2nd Ed.

16. Tiger Beetle of Cold Climates
Tiger beetle (Cicindela longilabris) - higher latitudes + elevations than other NA species
Schultz found metabolic rates of C. longilabris are higher and preferred temps. lower than other species
Physical env. limits species distributions
Molles: Ecology 2nd Ed.

17. Adapted to cool climates
Fig 9.3
Metabolic rates of C. longilabris higher; preferred temps lower than other beetle species
Adapted to cool climates
Molles: Ecology 2nd Ed.

18. Distributions of Plants Along a Moisture-Temperature Gradient
Encelia spp. distributions + variations in temp and precipitation
Fig 9.7
Molles: Ecology 2nd Ed.

19. Fig 9.5
Molles: Ecology 2nd Ed.

20. Distributions of Barnacles - Intertidal Gradient
Organisms in intertidal zone have evolved different degrees of resistance to drying
Barnacles - distinctive patterns of zonation within intertidal zone
Molles: Ecology 2nd Ed.

21. Connell found pattern in barnacles:
Chthamalus stellatus restricted to upper levels; Balanus balanoides limited to middle and lower levels
Molles: Ecology 2nd Ed.

22. Distributions of Barnacles Along an Intertidal Gradient
Balanus - more vulnerable to desiccation, excluded from upper intertidal zone
Chthamalus adults excluded from lower areas by competition with Balanus
Molles: Ecology 2nd Ed.

23. Competition? How do we know that Balanus outcompetes Chthamalus?
Molles: Ecology 2nd Ed.

24. Fig 9.8
Fig 9.9
Molles: Ecology 2nd Ed.

25. Distribution of Individuals on Small Scales
Three basic patterns:
Random: equal chance of being anywhere
Regular: uniformly spaced
Exclusive use of areas
Individuals avoid one another
Clumped: unequal chance of being anywhere
Mutual attraction between individuals
Patchy resource distribution
Molles: Ecology 2nd Ed.

26. Fig 9.10
Molles: Ecology 2nd Ed.

27. Importance of scale in determining distribution patterns:
At one scale pattern may be random, at another scale, might be uniform:
Molles: Ecology 2nd Ed.

28. Distribution of Tropical Bee Colonies
Hubbell and Johnson predicted aggressive bee colonies have regular distributions;
Predicted non-aggressive species have random or clumped distributions
Molles: Ecology 2nd Ed.

29. Hubbell and Johnson results:
4 species with regular distributions were highly aggressive
Fifth non-aggressive and randomly distributed
Molles: Ecology 2nd Ed.

30. Fig 9.11
Molles: Ecology 2nd Ed.

31. What causes overall pattern?
Behavior!
Aggressive bees were uniformly spaced due largely to their interactions.
Non-aggressive species were random - did not interact.
Molles: Ecology 2nd Ed.

32. Fig 9.10
Molles: Ecology 2nd Ed.

33. Distributions of Desert Shrubs
Traditional theory suggests desert shrubs are regularly spaced due to competition
Phillips and MacMahon - distribution of desert shrubs changes from clumped to regular patterns as they grow
Molles: Ecology 2nd Ed.

34. Young shrubs clumped for (3) reasons: Seeds germinate at safe sites
Hypothesis:
Young shrubs clumped for (3) reasons:
Seeds germinate at safe sites
Seeds not dispersed from parent areas
Asexual reproduction
Molles: Ecology 2nd Ed.

35. Distributions of Desert Shrubs
Phillips and MacMahon proposed as plants grow, some individuals in clumps die = reducing clumping
Competition among remaining plants produces higher mortality
Eventually creates regular distributions
Molles: Ecology 2nd Ed.

36. Fig their hypothesis
Molles: Ecology 2nd Ed.

37. Dug up roots, map distribution of 32 bushes
Brisson and Reynolds
Dug up roots, map distribution of 32 bushes
found competitive interactions with neighboring shrubs influences distribution of creosote roots
Molles: Ecology 2nd Ed.

38. Creosote bush roots do not overlap with nearby plant roots
So what?
Creosote bush roots do not overlap with nearby plant roots
Only 4% overlap between bushes
Fig 9.14
Molles: Ecology 2nd Ed.

39. Distributions of Individuals on Large Scales
Bird Pops North America
Root - at continental scale, bird pops have clumped distributions (Christmas Bird Counts)
Clumped patterns in species with widespread distributions
Molles: Ecology 2nd Ed.
Fig 9.14

40. Similar distribution pattern for species with small range: few “hot spots” Fish crow
Fig 9.14
Molles: Ecology 2nd Ed.

41. clumped only during breeding season?
Brown et al. (1995)
Relatively few study sites gave most records for each bird species in Breeding Bird Survey (June):
clumped only during breeding season?
Fig 9.16
Molles: Ecology 2nd Ed.

42. Density = number individuals per unit area/volume
Sedentary organisms: plot approach
Moving/secretive organisms: mark/recapture
Relative abundance = percent cover, CPUE
Molles: Ecology 2nd Ed.

43. Sedentary animals and plants Plot methods Area of known size
Estimating density
Sedentary animals and plants
Plot methods
Area of known size
Randomly located plots
Count individuals in plots
Average / plot
Density = average no. / plot area
Molles: Ecology 2nd Ed.

44. Mobile or secretive animals: mark/recapture 1. Sample animals and mark
Estimating density
Mobile or secretive animals: mark/recapture
1. Sample animals and mark
2. Release (M out of N in pop marked)
3. Wait for mixing
4. Sample (n), count how many marked (m)
5. Compute estimate of pop size:
N = M (n + 1)
(m + 1)
Molles: Ecology 2nd Ed.

45. Example: Estimating Population Size from Mark-Recapture
Number of animals marked in 1st sample = 100
Total number of animals in 2nd sample = 150
Number of marked animals in 2nd sample = 11
Population = M (n + 1) = 100 (151) = 1258
Size (N) (m + 1)
Molles: Ecology 2nd Ed.

46. Sample M = 38 squirrels, marked, released
Another Example
Sample M = 38 squirrels, marked, released
After 2 weeks, resample, n = 120
m = 12 of 120 marked
Estimate of pop. size:
N = M (n + 1) / (m + 1)
= 38 ( ) / (12 + 1) = 353.7
~ 354
Molles: Ecology 2nd Ed.

47. 20 randomly located plots, 10 x 10 m squares (area = 100 m2)
Example: maple trees
20 randomly located plots, 10 x 10 m squares (area = 100 m2)
Average sugar maple stems per plot = 4.5
Unit area for trees = hectare (10,000 m2)
Density = 4.5 maples per plot / 0.01 hectare plots = 450 maples / ha
Molles: Ecology 2nd Ed.

48. Example: zooplankters
35 lake water samples, 50 ml each
Average copepods per sample = 78
Unit volume for zooplankton = liters
Sample volume = 0.05 l
Density = 78 copepods per sample / 0.05 l samples
= 1560 copepods / l
Molles: Ecology 2nd Ed.

49. Organism Size and Population Density
Population density decreases with larger organism size
Why?
Bigger organisms need more space and resources
Bigger organisms have lower repro rates
Molles: Ecology 2nd Ed.

50. Damuth (1981)
Pop density of 307 spp. of herbivorous mammals decreased with increased body size
Fig 9.19
Molles: Ecology 2nd Ed.

51. Peters and Wassenberg (1983)
Aquatic invertebrates had higher pop densities than terrestrial invertebrates of similar size;
mammals have higher pop densities than birds of similar size
Fig 9.20
Molles: Ecology 2nd Ed.

52. Plant Size and Population Density
Plant population density decreases with increasing plant size
Underlying details different from animals
Molles: Ecology 2nd Ed.

53. White (1985)
Tree seedlings can live at high densities, but as trees grow, density declines until mature trees are at low densities
Molles: Ecology 2nd Ed.

54. Rabinowitz - 7 forms of rarity
Rarity and Extinction
Rabinowitz - 7 forms of rarity
commonness classification based on (3) factors:
Geographic Range of Species
Habitat Tolerance
Local Population Size
Molles: Ecology 2nd Ed.

55. All seven other other combinations create some kind of rarity
Non-rare populations have large geographic ranges, broad habitat tolerances, some large local populations
All seven other other combinations create some kind of rarity
= risk of extinction
Molles: Ecology 2nd Ed.

56. Large Range: Broad Habitat Tolerance: Small Local Pops
Rarity
Rarity I
Large Range: Broad Habitat Tolerance: Small Local Pops
Peregrine Falcons
Molles: Ecology 2nd Ed.

57. Large Range: Narrow Habitat Tolerance: Small Local Pops
Rarity II
Large Range: Narrow Habitat Tolerance: Small Local Pops
Passenger Pigeons
Molles: Ecology 2nd Ed.

58. Small Range: Narrow Habitat Tolerance: Small Pops Mountain Gorilla
Rarity
Rarity III
Small Range: Narrow Habitat Tolerance: Small Pops
Mountain Gorilla
Molles: Ecology 2nd Ed.

59. Least
vulnerable to
extinction
Increasing
Rarity
Increasing
vulnerability to
extinction

60. Moderate
vulnerability to
extinction

61. High
vulnerability to
extinction

62. Highest
vulnerability to
extinction
Other Example ?

63. White sucker - large range Broad habitat requirements Large body size
Example: NA suckers
White sucker - large range
Broad habitat requirements
Large body size
Molles: Ecology 2nd Ed.

64. Yacqui sucker - small range Narrow habitat requirements
Small body size
Molles: Ecology 2nd Ed.

65. Physical environment limits geographic distribution of species
Summary
Physical environment limits geographic distribution of species
On small scales, individuals w/in pops. are distributed in random, regular, or clumped patterns; on larger scales, individuals w/in pop. are clumped
Population density declines with increasing body size
Rarity influenced by geographic range, habitat tolerance, pop size; rare species vulnerable to extinction
Molles: Ecology 2nd Ed.

66. Molles: Ecology 2nd Ed.