Population Distribution and Abundance

1
Population Distribution and Abundance

• Chapter 9

2
Outline

• Distribution Limits
• Distribution Patterns
• Organism Size and Population Density
• Commonness and Rarity

3
Introduction

• Ecologists usually define a population as a group
  of individuals of a single species inhabiting a
  specific area.
• Characterized by the number of individuals and
  their density.
• Additional characteristics of a population
  include age distributions, growth rates,
  distribution, and abundance.

4
Introduction

• The distribution of a population includes the
  size, shape, and location of the area it
  occupies.
• The density of the population is the number of
  individuals per unit area.

5
Distribution Limits

• Physical environment limits geographic
  distribution of a species.
• Organisms can only compensate so much for
  environmental variation.

6
Kangaroo Distributions and Climate

• The family Macropodidae includes the kangaroos,
  wallabies, rat kangaroos and tree kangaroos.
• Some species of macropods can be found in nearly
  every part of Australia.
• No single species ranges across the entire
  continent.
• All confined to limited number of climatic zones.

7
Kangaroo Distributions and Climate

• Caughley found a close relationship between
  climate and distribution of the three largest
  kangaroos in Australia.

8
Kangaroo Distributions and Climate

• Macropus giganteus - Eastern Grey
• Eastern 1/3 of continent.
• Macropus fuliginosus - Western Grey
• Southern and western regions.
• Macropus rufus - Red
• Arid / semiarid interior.

9
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.

10
Tiger Beetle of Cold Climates

• The tiger Beetle (Cicindela longilabris) lives at
  higher latitudes and elevations than most other
  species in North America.

11
Tiger Beetle of Cold Climates

• Schultz et. al. found metabolic rates of C.
  longilabris are higher and preferred temperatures
  lower than most other species.
• Supports generalization that the physical
  environment limits species distributions.

12
Distributions of Plants along a
Moisture-Temperature Gradient

• Encelia species distributions correspond to
  variations in temperature and precipitation.

13
Distributions of Plants along a
Moisture-Temperature Gradient

• Leaf pubescence helps to keep a plant cooler
  because the leaves are highly reflective.
• Reduces light absorption by the leaves.

14
Distributions of Plants along a
Moisture-Temperature Gradient

• E. farinosa and E. frutescens overlap in range.
• E. farinosa has pubescent leaves and grows on
  slope habitats with shallow soil (limited water).
• E. frutescens doesnt have pubescent leaves.
• Keeps cool through rapid transpiration.
• Lives in desert washes with deep soil and better
  access to water.

15
Distributions of Barnacles along an Intertidal
Exposure Gradient

• Organisms living in an intertidal zone have
  evolved different degrees of resistance to
  drying.
• Barnacles show distinctive patterns of zonation
  within intertidal zone.

16
Distributions of Barnacles along an Intertidal
Gradient

• Connell found Chthamalus stellatus restricted to
  upper levels while Balanus balanoides is limited
  to middle and lower levels.

17
Distributions of Barnacles along an Intertidal
Gradient

• Balanus appears to be more vulnerable to
  desiccation, excluding it from the upper
  intertidal zone.
• Chthamalus adults appear to be excluded from
  lower areas by competition with Balanus.

18
Distribution of Individuals on Small Scales

• Random Equal chance of being anywhere.
• Uniform distribution of resources.
• Regular Uniformly spaced.
• Exclusive use of areas.
• Individuals avoid one another.
• Clumped Unequal chance of being anywhere.
• Mutual attraction between individuals.
• Patchy resource distribution.

19
Distribution of Individuals on Small Scales
20
Distribution of Tropical Bee Colonies

• Hubbell and Johnson predicted aggressive bee
  colonies would show regular distributions while
  non-aggressive species would show random or
  clumped distributions.

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Distribution of Tropical Bee Colonies

• As predicted, four species with regular
  distributions were highly aggressive.
• Fifth was non-aggressive and randomly
  distributed.
• Prospective nest sites marked with pheromones.

22
Distributions of Desert Shrubs

• Traditional theory suggests desert shrubs are
  regularly spaced due to competition.
• Phillips and MacMahon found distribution of
  desert shrubs changes from clumped to regular
  patterns as they grow.

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Distributions of Desert Shrubs

• Young shrubs clumped for (3) reasons
• Seeds germinate at safe sites
• Seeds not dispersed from parent areas
• Asexual reproduction

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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.

25
Distributions of Desert Shrubs

• Brisson and Reynolds found competitive
  interactions with neighboring shrubs appear to
  influence distribution of creosote roots, Larrea
  tridentata.

26
Distributions of Individuals on Large Scales

• Bird Populations Across North America
• Root found at continental scale, bird populations
  showed clumped distributions in Christmas Bird
  Counts.
• Clumped patterns occur in species with widespread
  distributions.

27
Distributions of Individuals on Large Scales

• Brown found a relatively small proportion of
  study sites yielded most of records for each bird
  species in Breeding Bird Survey.
• Opposite season from Roots study.

28
Plant Abundance along Moisture Gradients

• Whittaker examined distributions of woody plants
  along moisture gradients in several North
  American mountain ranges.

29
Plant Abundance Along Moisture Gradients

• Documented moisture gradient from moist canyon
  bottoms up to the dry southwest-facing slopes.
• Tree species showed a highly clumped distribution
  along moisture gradients, with densities
  decreasing substantially toward the edges of
  their distribution.

30
Plant Abundance Along Moisture Gradients

• Similar distributions were found in the Great
  Smoky Mountains as well as the Santa Catalina
  Mountains of Arizona.

31
Organism Size and Population Density

• In general, population density declines with
  increasing organism size.
• Damuth found the population density of
  herbivorous mammals decreased with increased body
  size.

32
Organism Size and Population Density

• Peters and Wassenberg found aquatic invertebrates
  tend to have higher population densities than
  terrestrial invertebrates of similar size.
• Mammals tend to have higher population densities
  than birds of similar size.

33
Plant Size and Population Density

• Plant population density decreases with
  increasing plant size.
• Underlying details are very different.
• Tree seedlings can live at very high densities,
  but as the trees grow, density declines
  progressively until mature trees are at low
  densities.

34
Rarity Vulnerability to Extinction

• Some populations seem to be more vulnerable to
  extinction than others.
• Distribution abundance are important factors.

35
Commonness and Rarity

• Rabinowitz devised commonness classification
  based on (3) factors
• Geographic Range of Species
• Habitat Tolerance
• Local Population Size

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Commonness and Rarity

• Populations that are least threatened by
  extinction, have extensive geographic ranges,
  broad habitat tolerances, and some large local
  populations.
• All seven other combinations create some kind of
  rarity.

37
Rarity

• Rarity I
• Extensive Range, Broad Habitat Tolerance, Small
  Local Populations
• Peregrine Falcon

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Rarity

• Rarity II
• Extensive Rage, Large Populations, Narrow Habitat
  Tolerance
• Passenger Pigeon

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Rarity

• Rarity III
• Restricted Range, Narrow Habitat Tolerance, Small
  Populations
• California Condor