Population
Distribution
and
Abundance
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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
• Scientist 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:
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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.
• scientist 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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Fig. 9.11
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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
– Scientist T. 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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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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