Population Distribution and Abundance

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Population Distribution and Abundance

• Chapter 9

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Outline

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

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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.

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Distribution Limits

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

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Kangaroo Distributions and Climate

• Caughley found a close relationship between
  climate and distribution of the three largest
  kangaroos in Australia.
• Macropus giganteus - Eastern Grey
• Eastern 1/3 of continent.
• Macropus fuliginosus - Western Grey
• Southern and western regions.
• Macropus rufus - Red
• Arid / semiarid interior.

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Kangaroo Distributions and Climate
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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.

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Tiger Beetle of Cold Climates

• Tiger Beetle (Cicindela longilabris) lives at
  higher latitudes and elevations than most other
  species in NA.
• 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.

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Tiger Beetle of Cold Climates
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Distributions of Plants along a
Moisture-Temperature Gradient

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

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Distributions of Barnacles along an Intertidal
Exposure Gradient

• Organisms living in an intertidal zone have
  evolved to different degrees of resistance to
  drying.
• Barnacles show distinctive patterns of zonation
  within intertidal zone.
• Connell found Chthamalus stellatus restricted to
  upper levels while Balanus balanoides is limited
  to middle and lower levels.

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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.

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Distributions of Barnacles along an Intertidal
Gradient
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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.

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

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

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

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Plant Abundance along Moisture Gradients

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

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Plant Abundance Along Moisture Gradients
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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.
• 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.

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Organism Size and Population Density
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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.

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Commonness and Rarity

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

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Rarity

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

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Review

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

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