This presentation discusses factors that effect population distribution

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Biology 205
Ecology and Adaptation
Lecture 8:
Population
distribution and
abundance
Dr. Erik D. Davenport

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Population
 How do ecologist define population?
 What are some characteristics of a
population?

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Population
 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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Outline
 What determines or limits the population
distribution?
 What are the distribution patterns of a
population?
 What is the relationship between Organism
Size and Population Density?
 Commonness and Rarity --- determines
whether a population (species) is a common
species or a rare species.

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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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Distributions and Climate:
Kangaroo study

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Distributions and Climate:
Kangaroo Study
 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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Distributions and Climate:
Kangaroo study
 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

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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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Distributions of Plants along a
Moisture-Temperature Gradient
 Encelia species distributions correspond to
variations in temperature and precipitation.

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Distribution of Individuals on
Small Scales

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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 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
 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.

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Distributions of Desert Shrubs
 Traditional theory suggests desert shrubs are
regularly spaced due to competition of what?
– 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 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

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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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Metapopulations
 A metapopulation is made up of a group of
subpopulations living on patches of habitat
connected by an exchange of individuals.
– Alpine Butterfly - Roland et.al.
– Lesser Kestrels - Serrano and Tella.

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Organism Size and Population
Density

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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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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 and rarity
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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Main concepts
 The physical environment limits the geographic
distribution of species
 On small scales, individuals within populations are
distributed in patterns that maybe random, regular,
or clumped; on larger scales, individuals within a
population are clumped.
 Population density declines with increasing
organisms size.
 Rarity is influenced by geographic range, habitat
tolerance, and population size; rare species are
vulnerable to extinction.