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Chapter 9 Concepts
Physical environment limits geographic distributions
Distribution patterns differ depending on the scale of the investigation
Populations are not always spatially contiguous
Extinction risk is influenced by population size, habitat tolerance and geographic
range
What is a Population?
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Classic definition:
“Any group of organisms living together at the same time, in the same place, and
capable of interbreeding with one another.”
Applying the Definition
• Two considerations:
– space
– time
• How broadly should the spatial limits of our population be drawn to justify “in the
same place”?
• How far forward and backward should we go to say “at the same time?”
Spatial Scales
• Galapagos tortoise on the Island of Pinta
– rarely crosses water
– forms local colonies on Pinta
• Chinook salmon in the Pacific Ocean
– large schools of adult fish in the ocean
– individual “groups” breed in specific streams
• Honey mushroom
– fruiting bodies occur sporadically
– mycelia connect a single
individual over hectares of land
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Temporal Scales
Nature of the population usually
defines the temporal scale:
– Redwood tree generation time is centuries
– East African elephant generation time is decades
– White tail deer generation time is two years
– Dandelion generation time is months
– Bacterial generation time may be hours
– Honey mushroom generation time is...?
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Population Density
Density is simply the number of individuals per unit area.
This assumes you can define:
– area of interest
– individuals
Also need to differentiate between:
– crude density (# per total area), and
– ecological density (# per usable area).
Distribution
Distribution is a species characteristic
– Geographic distribution encompasses the entire geographic area that the species
may inhabit
– Ecological distribution includes only those areas within a geographic range that
species actually inhabits.
– Ecological distribution may be “nested”
Dispersion
(Small Scales)
Dispersion is a measure of the spatial distribution of the individuals in a population.
Three patterns:
– Uniform (or regular)
– Random
– Clumped (or aggregated)
Dispersion and Grain
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Grain relates:
– the size of patches in an organism’s
ecological distribution, and
– the ability of the organism to move
among those patches
Coarse-grained
– low movement capability relative to
patch size
Fine-grained
– high movement capability relative to
patch size
Geographic Distributions
At large spatial scales, organisms tend to have clumped distributions
Distributions most often related to variation in:
– Environmental variables
– Resource abundance
Horned Lark Distribution
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Distribution, Abundance and Rarity
Within their geographic range, most organisms show:
– High abundance in a few areas
– Low abundance in many areas
Distributions are most commonly related to environmental gradients
Zones of Tolerance
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V. E. Shelford, tiger beetles and
“microenvironments”
– moisture
– temperature
– light
Critical maximum, critical
minimum define “zone of
tolerance”
Highest abundance where
most conditions are optimal
Range of Tolerances
Differing organisms have
differing tolerances
Two useful prefixes:
– steno = narrow
– eury = wide
Wide tolerance = wide distribution?
Narrow tolerance = narrow distribution?