Download Population Structures

Robert E.
Chapter 10: Distribution and
Spatial Structure of
Populations
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 Friday: we’ll do both Chapters 14 and 15
 We’re skipping chapters 11-13
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Habitat Fragmentation and Landscape
Ecology
 Fragmentation of formerly continuous habitats is
happening worldwide, caused by:
 clearing of forests
 construction of roads
 channeling of rivers
 Landscape ecology addresses how size and
arrangement of habitat patches influence:
 activities of individuals
 growth and regulation of populations
 interactions between species
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Consequences of Habitat Fragmentation
 Plants and animals can use a habitat patch only if they have
access to it.
 Changes in human land use patterns have reduced such
access.
 Small, isolated populations in habitat fragments are
vulnerable to extinction:
 they suffer from loss of genetic diversity
 they are subject to random perturbations
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Forest Fragmentation in the Amazon
 When habitats are fragmented, most areas are
closer to edges, often with negative
consequences.
 Forest fragmentation in the Amazon basin results
in greater exposure of trees within 100 m of
forest edges, resulting in:
 drying of vegetation
 excessive wind damage
 losses of up to 15 tons of biomass per hectare annually
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Habitat fragmentation (Brazil)
Brown-Headed Cowbirds and Kentucky
Warblers
 Cowbirds are nest parasites of other birds, such as the
Kentucky warbler. Cowbirds:
 prefer open farms and fields
 venture into the edges of forests in search of host nests
 In one study, parasitism on warblers was as high as
60% within 300 m of forest edges:
 warblers had to be 1.5 km from the forest edge to escape
parasitism
 Nest predators also venture into forest from edges.
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Habitat fragmentation can affect population
dynamics
Many species are in decline.
 Warblers and many other forest birds of eastern
North America have declined in recent years.
 Populations of most species have persisted for
millions of years:
 many species are now threatened with extinction, a
cause for great concern
 to understand this problem and potential solutions
we must explore population structures
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Terminology 1
 A population is made up of the individuals of a species within
a particular area:
 each population lives in patches of suitable habitat
 Habitats naturally exist as a mosaic of different patches:
 many populations are thus broken into somewhat isolated
subpopulations
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Terminology 2
 Population structure refers to:
 the density and spacing of individuals within suitable
habitat
 the proportions of individuals in various age classes
 mating system
 genetic structure
 Populations exhibit dynamic behavior,
changing through time because of births,
deaths, and movements of individuals.
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Geographic distributions are determined by
suitable habitats.
 The distribution of a species is its geographic
range.
 Consider the geographic range of sugar maple in
eastern North America:
 broader outlines of the range are determined by
climatic factors
 within its range, the species only occurs in suitable
habitats (absent from many habitats such as marshes
and serpentine barrens)
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Within a population’s
geographic range, only
suitable habitats are occupied
Barriers to long-range dispersal limit
geographic distribution.
 Introduced species often expand successfully
into new regions:
 160 European starlings were introduced near New
York City in 1890 and 1891; within 60 years, the North
American population of starlings covered more than
3 million square miles
 Other examples of successful introductions:
 dogs in Australia, pigs and rats in Pacific islands
 fast-growing pines and eucalyptus trees worldwide
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migration
 The geographic range of a population includes ALL of the
areas its members occupy during their ENTIRE life history
 Sockeye salmon  spawning grounds in Canada plus vast
areas of the North Pacific Ocean where individuals grow to
maturity before making the long migration back
 East Africa: migration of many large ungulates…following the
geographic pattern of seasonal rainfall and fresh vegetation
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Ecological niche modeling
 Map the known occurrences of a species in a geographic
space
 Catalog the combination of ecological conditions (temp and
precipitation) at the locations were the species has been
recorded
 Catalog of ecological conditions = ecological envelope
 Then map the geographic area that has the same
combination of conditions to predict the broader occurrence
of species w/in a region
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Dispersion of Individuals within
Populations
 Dispersion of individuals within a population
describes their spacing with respect to one
another.
 A variety of patterns is possible:
 clumped (individuals in discrete groups)
 evenly spaced (each individual maintains a minimum
distance from other individuals)
 random (individuals distributed independently of
others within a homogeneous area)
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Dispersion patterns describe the spacing
of individuals
Causes of Dispersion
 Even spacing may arise from direct interactions
among individuals:
 maintenance of minimum distance between
individuals or direct competition for limited resources
may cause this pattern
 Clumped distribution may arise from:
 social predisposition to form groups
 clumped distribution of resources
 tendency of progeny to remain near parents
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Evenly spaced desert shrubs in Sonora, Mexico
Vegetative reproduction  clumped distribution
Start here
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Populations exist in heterogeneous
landscapes.
 Uniform habitats are the exception rather than
the rule:
 most populations are divided into subpopulations
living in suitable habitat patches
 Degree to which members of subpopulations
are isolated from one another depends on:
 distances between subpopulations
 nature of intervening environment
 mobility of the species
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Causes of Isolation
 Whether or not areas of unfavorable habitat are
substantial barriers to mobility depends on:
 distance between subpopulations
 nature of intervening habitat
 mobility of the species
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Mobility and Isolation
 The extent of isolation of subpopulations depends on
the mobility of the species:
 snail kites in southern Florida are linked into a single
population
 geckos in Australia are separated by agriculture into
subpopulations between which there is little
movement of individuals
 Several models address patchiness...
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Genetic differences between populations in small, isolated
habitat patches indicate lack of movement between them
(gecko Oedura reticulata in SW Australia)
Metapopulation Model
 The metapopulation model views a population as a set
of subpopulations occupying patches of a particular
habitat:
 intervening habitat is referred to as the habitat
matrix:
 the matrix is viewed only as a barrier to movement of
individuals between subpopulations
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Source-Sink Model
 The source-sink model recognizes differences
in quality of suitable habitat patches:
 in source patches, where resources are abundant:
 individuals produce more offspring than needed to
replace themselves
 surplus offspring disperse to other patches
 in sink patches, where resources are scarce:
 populations are maintained by immigration of
individuals from elsewhere
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Landscape Model
 The landscape model considers effects of
differences in habitat quality within the habitat
matrix:
 the quality of a habitat patch can be affected by the
nature of the surrounding matrix
 quality is enhanced by presence of resources, such as nesting
materials or pollinators
 quality is reduced by presence of predators or disease
organisms
 some matrix habitats are more easily traversed than
others
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Models of population structure
Ideal Free Distributions 1
 Individuals can make decisions regarding the quality of
habitat patches. Where to live? Where not to live?
 How do they make these decisions?
 Quality of a patch depends on:
 its intrinsic quality
 density of other individuals:
 occupied patches may have fewer remaining resources
 competing individuals may engage in conflicts
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In an ideal free distribution, each individual
exploits a patch of the same apparent quality
Ideal Free Distributions 2
 Each individual chooses among patches to maximize
its access to resources.
 As a patch with high intrinsic quality fills with
individuals, its resources are depleted and its apparent
quality decreases:
 at some point, a patch with lower intrinsic quality has greater
apparent quality and it too begins to fill
 thus all patches reach the same apparent quality, despite
different intrinsic qualities and different densities of
individuals, the ideal free distribution
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Individuals move from sources to sinks.
 The ideal free distribution would suggest
equivalent reproductive success among
individuals occupying habitat patches of differing
intrinsic quality:
 this ideal is rarely realized:
 individuals do not have perfect knowledge of patch
quality
 free choice may be reduced in subordinate
individuals
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Sources and Sinks
 Some patches are population sources and others are
population sinks with net movement from sources to
sinks:
 evident in European blue tit:
 populations in deciduous oak habitats are sources, with
potential for rapid growth
 populations in evergreen oak habitats are sinks, with potential
for rapid decline
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Estimating population size.
 Population size (number of individuals) is the ultimate measure of a
population:
 we typically wish to know what factors cause size to change and
processes that regulate size
 Total population size has two components:
 density, the number of individuals per unit area
 area occupied
 Density  area occupied = size.
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How do we measure density?
 A total count may be feasible:
 suitable for small populations where individuals can be
distinctively marked
 often employed for endangered species, particularly for
larger animals such as mammals and birds
 For sessile organisms, local density may be
determined in plots, then extrapolated to entire area
occupied.
 Other techniques may be needed for mobile
organisms.
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Mark-Recapture Method
 Mark-recapture methods are often used with animal
populations:
 an initial sample is collected and all individuals are distinctively
marked
 marked animals are released into the population and allowed to
mix
 a second sample is collected and marked and unmarked animals
are tallied
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Mark-Recapture Computations
 For an initial marked sample of size M, a second
sample of size n, containing x marked
individuals, the population size N is:
N = nM/x
 If 20 fish are captured, marked, and returned to
a small pond, and a second sample of 50 fish
contains 6 marked fish, the population estimate
is 50(20)/6 = 167.
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Variation in Populations over Space and
Time
 Populations tend to vary in size over time.
 Long-term records often reveal fluctuations that might be
overlooked in shorter term:
 infestation by chinch bugs in Illinois monitored over decades reveals
populations fluctuations:
 in some years, populations averaged 1000/m2 over an area of 300,000 km2
 in other years farmers reported little damage
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Movement of individuals maintains spatial
coherence.
 Movements within populations are referred to as dispersal.
 Movements between subpopulations are referred to as
migrations, or more specifically:
 emigration (leaving a subpopulation)
 immigration (entering a subpopulation)
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Monitoring Dispersal
 Monitoring dispersal is difficult:
 organisms must be marked and recaptured
 large areas must be covered to ensure an adequate sampling of
movements
 Population biologists often use ingenious methods to monitor
dispersal:
 dispersal in fruit flies was investigated by releasing individuals
with a visible mutation
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Lifetime Dispersal Distance
 Average lifetime dispersal distance is a useful index of
movement within populations:
 this indicates how far an individual moves from its birthplace to
where it settles and reproduces:
 a circle with radius equal to the lifetime dispersal distance is the lifetime
dispersal area
 the number of individuals within this circle is the neighborhood size of
the population
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Neighborhood size: # of individuals w/in a circle whose
radius is the average lifetime dispersal distance
European starling
marcoecology
 Analyzing and interpreting the patterns revealed by large
samples of species
 The distribution and population size of a species reflects the
distribution of conditions to which individuals of the species
were well adapted. If these conditions are common 
population should also be common
 Plus: a species should be more abundant at the center of its
distribution where its conditions are most favorable

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Range size and pop density
 Species adapted to a broad range variety of resources and
conditions are likely to be more abundant in the center of
their distribution than are species that are more specialized
 geographic range and population density in the center of the
distribution should be positively correlated
 Can be tested in a sample of species for which both
geographic range and population density have been
measured – eg: populations of birds within North America
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Range size and pop density
 1. area occupied by a species
and its max population
density are positively
correlated
 Among457 species - those
species with larger ranges
have higher max abundances
Body size, distribution, and
abundance
Population density decreases
with increasing body size
Why?
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Pop density in carnivores is closely
related to their food supply