Transcript Distribution

Population Distribution and Abundance
(Chapter 9) Introduction
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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 local abundance.
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Distribution Limits
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Physical environment limits geographic
distribution of a species.
http://plants.usda.gov/index.html
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Two needle pinyon
Giant Sequoia
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Distribution
of three
Peromyscus
species
Temperature and
Water Balance
(Blackwell and Pivorun, 1979)
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Kangaroo Distributions and Climate
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Distributions of Plants along a MoistureTemperature Gradient
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Encelia species distributions correspond to
variations in temperature and precipitation.
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Microenvironmental forces
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Distributions of Barnacles along an
Intertidal Gradient
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Distribution of Individuals on Small Scales
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Random: Equal chance of being anywhere.
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Regular: Uniformly spaced.
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Clumped: Unequal chance of being
anywhere.
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Distribution of Individuals on Small Scales
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Distribution of Tropical Bee Colonies
Hubbell and Johnson: aggressive behavior
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Distributions of Desert Shrubs
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Traditional theory suggests desert shrubs
are regularly spaced due to competition.
 Phillips and MacMahon found distribution
of desert shrubs changes from clumped to
random 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
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Phillips and MacMahon proposed as plants
grow, some individuals in clumps die,
reducing clumping.
 Competition among remaining plants
produces higher mortality.
 Root zone competition
 Eventually creates regular distributions.
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Clumped Distributions of Individuals
on Large Scales (resource hot spots)
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Animal Size and Population Density
Relationship holds up within trophic levels
(e.g., herbivores)
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Plant Size and Population Density
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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
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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
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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
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Rarity III
 Restricted Range, Narrow Habitat
Tolerance, Small Populations
 California Condor
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