Chapter 10: Distribution and Spatial Structure of

Populations

Robert E.

Next chapter

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

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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