Notes 12: Population Ecology: Attributes
• Attributes
o Population: a group of potentially interbreeding organisms at the same time and
place that share a common gene pool
o Population size: number of individuals
o Population growth rate: change in number over time as a function of (birth +
immigration) – (death + emigration)
o Population density: number per unit area
o Range/distribution: geographic area over which the individuals are dispersed
o Population structure
§ Age class structure
§ Sex ratio
§ Genetic structure
§ Spatial structure
• Pattern of how individuals are distributed through the range
• Distribution
o Determining Factors
§ Environmental Tolerance: The Niche Concept
• Organisms will only live in ecologically suitable habitats
• Fundamental niche: describes abiotic conditions that can be
tolerated
o Environmental conditions in which a species can live
• Realized niche: includes biotic effects like distribution of food,
predators, and competitors
o Where the organism does live
• For each variable, there is an optimal zone where conditions are
ideal
o Zones of tolerance can be survived in
o Zones of intolerance are beyond survivable conditions
• Reproductive fitness highest in fundamental niche where optimal
zones for all variables overlap
o As population experiences any departure from optimal
conditions, performance will decline
o Depart from optimum along any N-dimensional axes
• Variable restricting survival is limiting variable
o Limits population’s distribution
o Can be abiotic or biotic
o May relate to fundamental or realized niche
• The more variables added, the less space of optimal zone
o For example, soil moisture and temperature must both be
in the fundamental niche for a plant to grow
o Niche defined by multiple axes is n-dimensional
hypervolume
,§ Barriers to Dispersal
• Organisms may be absent from an otherwise habitable area
because they can’t disperse to it
o Usually ocean for terrestrial species
o Creates different biorealms dominated by iconic sets of
species that evolved on one continent and could not leave
until continents collided
• All habitats are islands at some scale, surrounded by differentially
permeable barrier
§ Changes in Distributions Through Time: Seasonal Migration
• Many animals migrate following the availability of food resources
o Move or adapt
• Wildebeests
o Tanzania just below equator, from 1-11°S
o Rainy season in southern half of the country for
September-December
o Rainy season in northern half of country from January-July
§ Changes in Distributions Through Time: Climate Change
• As environments change, geographical area containing the
fundamental niche requirements may move, expand or contract
• Necessities a behavioral (migratory) or evolutionary (adapting to
new local environment) response
• Marine fishes are moving north into the North Sea as water
temperatures increase
• Alpine mammals, birds, and plants are increasing their elevation
o And decreasing the size of their habitat and population
o
o Can’t keep going up a mountain
• Some scientists estimate that 35% of species may go extinct in the
next 50 years because of climate change and range shifts
§ Niche Modeling and Invasive Species
, • By quantifying the environmental attributes of a species’ niche,
you can predict where it might live elsewhere on the globe if
limited by dispersal barrier
• If it crosses a barrier, we can predict how far its range might
increase
• Problems
o Doesn’t account for biological aspects of the environment
o Range may increase when separated from a
predator/competitor
o Maybe it exploits a particular place because of a foodplant
with its own environmental limitations limiting the species
to these environments
o In a new area with a new foodplant, it might expand
across the entire range of this new foodplant into abiotic
environments not yet exploited
o Doesn’t account for the possibility of adaptation
• Ex: Eucalyptus
o On sedimentary rock vs. granitic rock
o Draw up a lot of water from the water table
o Aggression to water depends on environmental conditions
• Principal Component Analyses
o Derive new variables that explain the most variance in a
data set and can correlate real variables to these derived
ones
o Highest abundance in center of range
o Where all fundamental niches overlap
o
o Living things do not remain static
o Dispersion
, § Types
• Regular
o Variance is less than the mean
o Variance can be zero if equally dispersed
o Usually caused by intraspecific competition, like
allelopathy or territoriality
• Clumped
o Variance is greater than the mean
o Usually caused by sociality or common response to
clumped resources
• Random
o Variance equals the mean
o Rather unusual because clumped resources and
competition are so common
o Previously clumped or regular distributions can degrade to
random over time, such as when seedlings (clumped) grow
up and compete (regular) and then die of other causes
§ Canopy trees, later in succession
o Patchy resource, social effects, limited dispersal
o High variance
§ Complexities
• Can vary with type of dispersal
• Can vary with development
o As the example of tree seedlings for random dispersion
o Life history stage
• Can vary with environment and resource availability and
distribution
o We can expect organisms to move with their resources
across space if they are able, creating patterns of seasonal
migration
• Varies with spatial scale of analysis
o Set by the experimenter
o Populations can respond at different scales to their
environment and create patterns at different scales
o At a large scale, we might find populations distributed in a
clumped over a range
§ Localized to places where resources are found
o At a smaller scale within patches, individuals may be
regularly spaced as they compete for these resources
o Analyses at different scales can reveal different things that
are important about the biology of the species
o Population Density
§ Correlations with range
• Attributes
o Population: a group of potentially interbreeding organisms at the same time and
place that share a common gene pool
o Population size: number of individuals
o Population growth rate: change in number over time as a function of (birth +
immigration) – (death + emigration)
o Population density: number per unit area
o Range/distribution: geographic area over which the individuals are dispersed
o Population structure
§ Age class structure
§ Sex ratio
§ Genetic structure
§ Spatial structure
• Pattern of how individuals are distributed through the range
• Distribution
o Determining Factors
§ Environmental Tolerance: The Niche Concept
• Organisms will only live in ecologically suitable habitats
• Fundamental niche: describes abiotic conditions that can be
tolerated
o Environmental conditions in which a species can live
• Realized niche: includes biotic effects like distribution of food,
predators, and competitors
o Where the organism does live
• For each variable, there is an optimal zone where conditions are
ideal
o Zones of tolerance can be survived in
o Zones of intolerance are beyond survivable conditions
• Reproductive fitness highest in fundamental niche where optimal
zones for all variables overlap
o As population experiences any departure from optimal
conditions, performance will decline
o Depart from optimum along any N-dimensional axes
• Variable restricting survival is limiting variable
o Limits population’s distribution
o Can be abiotic or biotic
o May relate to fundamental or realized niche
• The more variables added, the less space of optimal zone
o For example, soil moisture and temperature must both be
in the fundamental niche for a plant to grow
o Niche defined by multiple axes is n-dimensional
hypervolume
,§ Barriers to Dispersal
• Organisms may be absent from an otherwise habitable area
because they can’t disperse to it
o Usually ocean for terrestrial species
o Creates different biorealms dominated by iconic sets of
species that evolved on one continent and could not leave
until continents collided
• All habitats are islands at some scale, surrounded by differentially
permeable barrier
§ Changes in Distributions Through Time: Seasonal Migration
• Many animals migrate following the availability of food resources
o Move or adapt
• Wildebeests
o Tanzania just below equator, from 1-11°S
o Rainy season in southern half of the country for
September-December
o Rainy season in northern half of country from January-July
§ Changes in Distributions Through Time: Climate Change
• As environments change, geographical area containing the
fundamental niche requirements may move, expand or contract
• Necessities a behavioral (migratory) or evolutionary (adapting to
new local environment) response
• Marine fishes are moving north into the North Sea as water
temperatures increase
• Alpine mammals, birds, and plants are increasing their elevation
o And decreasing the size of their habitat and population
o
o Can’t keep going up a mountain
• Some scientists estimate that 35% of species may go extinct in the
next 50 years because of climate change and range shifts
§ Niche Modeling and Invasive Species
, • By quantifying the environmental attributes of a species’ niche,
you can predict where it might live elsewhere on the globe if
limited by dispersal barrier
• If it crosses a barrier, we can predict how far its range might
increase
• Problems
o Doesn’t account for biological aspects of the environment
o Range may increase when separated from a
predator/competitor
o Maybe it exploits a particular place because of a foodplant
with its own environmental limitations limiting the species
to these environments
o In a new area with a new foodplant, it might expand
across the entire range of this new foodplant into abiotic
environments not yet exploited
o Doesn’t account for the possibility of adaptation
• Ex: Eucalyptus
o On sedimentary rock vs. granitic rock
o Draw up a lot of water from the water table
o Aggression to water depends on environmental conditions
• Principal Component Analyses
o Derive new variables that explain the most variance in a
data set and can correlate real variables to these derived
ones
o Highest abundance in center of range
o Where all fundamental niches overlap
o
o Living things do not remain static
o Dispersion
, § Types
• Regular
o Variance is less than the mean
o Variance can be zero if equally dispersed
o Usually caused by intraspecific competition, like
allelopathy or territoriality
• Clumped
o Variance is greater than the mean
o Usually caused by sociality or common response to
clumped resources
• Random
o Variance equals the mean
o Rather unusual because clumped resources and
competition are so common
o Previously clumped or regular distributions can degrade to
random over time, such as when seedlings (clumped) grow
up and compete (regular) and then die of other causes
§ Canopy trees, later in succession
o Patchy resource, social effects, limited dispersal
o High variance
§ Complexities
• Can vary with type of dispersal
• Can vary with development
o As the example of tree seedlings for random dispersion
o Life history stage
• Can vary with environment and resource availability and
distribution
o We can expect organisms to move with their resources
across space if they are able, creating patterns of seasonal
migration
• Varies with spatial scale of analysis
o Set by the experimenter
o Populations can respond at different scales to their
environment and create patterns at different scales
o At a large scale, we might find populations distributed in a
clumped over a range
§ Localized to places where resources are found
o At a smaller scale within patches, individuals may be
regularly spaced as they compete for these resources
o Analyses at different scales can reveal different things that
are important about the biology of the species
o Population Density
§ Correlations with range
