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Biology 205Ecology 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.