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Island Biogeography

Dr. MattinglyApril 13, 2015

Biogeography

BiogeographyThe study of the distribution of organisms in space and time

Biogeographers evaluate diversity patterns across geographic gradients

• Area• Isolation• Latitude• Depth• Elevation

Biogeography

Diversity patterns are shaped by four fundamental processes:

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

Biogeography

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

The movement of organisms from a point of origin to a new location

Diversity patterns are shaped by four fundamental processes:

Biogeography

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

The movement of organisms from a point of origin to a new location

Diversity patterns are shaped by four fundamental processes:

The establishment of organisms at a new location (persistent population)

Biogeography

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

The movement of organisms from a point of origin to a new location

Diversity patterns are shaped by four fundamental processes:

The establishment of organisms at a new location (persistent population)

The elimination of a species from a particular area (extirpation)

Biogeography

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

The movement of organisms from a point of origin to a new location

Diversity patterns are shaped by four fundamental processes:

The establishment of organisms at a new location (persistent population)

The elimination of a species from a particular area (extirpation)

The change in allelic frequencies in a population over time (may lead to speciation)

Biogeography

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

The movement of organisms from a point of origin to a new location

Diversity patterns are shaped by four fundamental processes:

The establishment of organisms at a new location (persistent population)

The elimination of a species from a particular area (extirpation)

The change in allelic frequencies in a population over time (may lead to speciation)

Biodiversity

BiodiversityThe variety of living organisms at all levels of biological organization

Genetic diversity:Variation in genetic makeup andmorphological traits among individuals in a population

from genes to ecosystems

Species diversity:Variation in the number and relative abundance of species in a community

Ecosystem diversity:Variation in biotic and abiotic properties amongecosystems in a landscape

Islands provide model systems for studies of biogeography

Charles Darwin Alfred Russel Wallace

Early naturalists cataloged the diversity of island organisms

These patterns led to an understanding of the processes that shape island biodiversity

Islands provide model systems for studies of biogeography

What is an island?

Why do islands provide model systems?

Islands provide model systems for studies of biogeography

What is an island?

Why do islands provide model systems?

Islands represent discrete patches of habitat that vary in isolation and area

• Species-Area Relationship

• Species-Distance Relationship

Islands provide model systems for studies of biogeography

Islands represent discrete patches of habitat that vary in isolation and area

Species richness decreases with increased island isolation

Species richness increases with increased island area

These notions are central to the Theory of Island Biogeography

Dispersal to Oceanic Islands

The movement of organisms from a point of origin to a new location

Propagule: the ‘colonizing unit’ (capable of reproducing in the new location)

Dispersal to Oceanic Islands

The movement of organisms from a point of origin to a new location

Propagule: the ‘colonizing unit’ (capable of reproducing in the new location)

Mechanisms of Dispersal

• Wind dispersal

• Sea Dispersal

• Animal dispersal

Mechanisms of Dispersal

• Wind Dispersal / Aerial Transport

Passivevs.

Active

Mechanisms of Dispersal

• Sea Dispersal (e.g. drifting, rafting, swimming)

Passivevs.

Active

Mechanisms of Dispersal

Abiotic rafts

Volcanic pumice (buoyant)

• Can serve as a raft for algae and invertebrates

Mechanisms of Dispersal

Abiotic rafts

Volcanic pumice (buoyant)

• Can serve as a raft for algae and invertebrates

Vegetation

• Can serve as a raft for vertebrates (and other taxa)

Biotic rafts

Mechanisms of Dispersal

Mechanisms of Dispersal

• Animal Dispersal (e.g. hitchhiking)

endozoochoryvs.

ectozoochory

naturalvs.

human-mediated

Mechanisms of Dispersal

Carlquist, S. 1981. Chance dispersal. Scientific American 69:509-516.

Plant dispersal to the Galapagos Islands

Galapagos flora came from S. America via an estimated 378 colonization events

60% animal dispersed (birds)

31% wind dispersed

9% sea dispersed (drifting)

Mechanisms of Dispersal

Krakatau

Krakatau, 1883 eruption

The relative importance of differentdispersal mechanisms may changeover time

Long-distance dispersal events are rare and largely random

Dispersal to Oceanic Islands

Sweepstakes route

Jump Dispersal: the colonization of areas over long distances

Green iguana

(e.g. Hawaii, Galapagos)

Dispersal to Oceanic Islands

• Colonizers share adaptations that facilitate successful dispersal to islands

(but these adaptations may not ensure survival upon reaching an island)

• Colonization of an island often requires different traits than the ones

that permitted dispersal to the island

Long-distance dispersal events are rare and largely random

Sweepstakes route

Jump Dispersal: the colonization of areas over long distances(e.g. Hawaii, Galapagos)

Natural Selection

Natural Selection

Cody & Overton (1996)

pappus

achene

Dispersal to Oceanic Islands

Long-distance dispersal events are rare and largely random

Sweepstakes route

Jump Dispersal: the colonization of areas over long distances

Island hopping: organisms may disperse over long distances by moving from one island to another

Source Pop.

(e.g. Hawaii, Galapagos)

Species traits that are suited for long-distance dispersal

Species traits that are suited for long-distance dispersal

• Small propagule size (for passive transport)

• Large body size (for active transport)

• Tolerance of desiccation

• Tolerance of temperature extremes

• Tolerance of intense sunlight exposure

• Sufficient energy reserves (or a low metabolic rate)

• Colonizers share adaptations that facilitate successful dispersal to islands

(but these adaptations may not ensure survival upon reaching an island)

Following dispersal, an organism must overcome obstacles to establishment

• Must persist in an environment with a tolerable range of abiotic conditions(e.g. temperature, salinity, moisture)

• Must be able to access resources necessary for its maintenance, growth,and reproduction

• Must be able to find a mate and reproduce

• Must be able to avoid pre-reproductive mortality

Island populations are more prone to extinction than those on the mainland

• Island populations are typically smaller

• Island populations typically have less genetic diversity

• Upon arrival, organisms generally are not adapted to the island habitat

Biogeography

1. Dispersal

2. Colonization

3. Extinction

4. Evolution

The movement of organisms from a point of origin to a new location

Diversity patterns are shaped by four fundamental processes:

The establishment of organisms at a new location (persistent population)

The elimination of a species from a particular area (extirpation)

The change in allelic frequencies in a population over time (may lead to speciation)

• Species-Area Relationship

• Species-Distance Relationship

Islands provide model systems for studies of biogeography

Islands represent discrete patches of habitat that vary in isolation and area

Species richness decreases with increased island isolation

Species richness increases with increased island area

These notions are central to the Theory of Island Biogeography

Species-Distance Relationships

Bird species richness decreases with increased isolation

Species-Distance Relationships

Mammal species richness decreases with increased isolation

Mount St. Helens, 1980 eruption

Species-Distance Relationships

Plant species richness decreases with increased isolation

Species-Distance Relationships

Birds in montane forest islands

Bird species richness decreases with increased isolation

Why might species richness decrease with increased island isolation?

Why might species richness decrease with increased island isolation?

• Dispersal limitation

The ratio of observed : expected species richnessdeclines with increased island isolation

Species-Area Relationships

Species-Area Relationships

Species-Area Relationships

Species-Area Relationships

Why might species richness increase with increased island area?

Why might species richness increase with increased island area?

• More available resources on large islands

• Large islands support larger populations

• Large populations are less prone to extinction

• Increased habitat diversity on large islands

• Large islands provide large ‘targets’ for dispersing propagules

The Theory of Island Biogeography

Islands generally contain fewer species than mainland habitats of comparable size

• Non-equilibrium theory of island biogeography:

Islands contain fewer species because there has not been sufficient time

for species to accumulate via immigration

The Theory of Island Biogeography

Islands generally contain fewer species than mainland habitats of comparable size

• Non-equilibrium theory of island biogeography:

Islands contain fewer species because there has not been sufficient time

for species to accumulate via immigration

• Equilibrium theory of island biogeography:

MacArthur & Wilson (1967)

Explains patterns of species richness on islands

Species richness is determined by a balance between

two processes: immigration and extinction

The number of species on an island is determined by:

1) The rate of immigration of new species

2) The rate of local extinction

Assumption: An island can hold a fixednumber of individuals, based on its size

Number of species on islandEquilibrium numberRa

te o

f im

mig

ratio

n or

ext

inct

ion

The number of species on an island is determined by:

1) The rate of immigration of new species

2) The rate of local extinction

Assumption: An island can hold a fixednumber of individuals, based on its size

Number of species on islandEquilibrium numberRa

te o

f im

mig

ratio

n or

ext

inct

ion Immigration:

As the number of specieson an island increases, immigration rates declinebecause available nichesbecome more limited

The number of species on an island is determined by:

1) The rate of immigration of new species

2) The rate of local extinction

Assumption: An island can hold a fixednumber of individuals, based on its size

Number of species on islandEquilibrium numberRa

te o

f im

mig

ratio

n or

ext

inct

ion Immigration:

As the number of specieson an island increases, immigration rates declinebecause available nichesbecome more limited

Extinction: As the number of specieson an island increases, extinction rates increasebecause species competemore intensely for limitedresources

The number of species on an island is determined by:

1) The rate of immigration of new species

2) The rate of local extinction

Number of species on islandEquilibrium numberRa

te o

f im

mig

ratio

n or

ext

inct

ion Equilibrium:

When the number ofspecies on an islandstabilizes.

This is achieved whenimmigration rates equalextinction rates.

Equilibrium number willvary with island size andisland isolation.

I > E E > I

Island size and Island isolation influence the number of species on an island

Immigration and Extinction rates vary with both island size and island isolation

Species-Area Relationship

Species-Distance Relationship

Immigration varies with the distance of the island from the mainland

• More distant islands are colonized less frequently.

• The rate of colonization declines as species richness increases,because there are fewer potential colonists and fewer unexploited niches.

• More distant islands have lower species richness

Rat

e

Extinction varies with the size of the island

• Large islands can support larger population sizes, and large populations are less prone to extinction. So, the smaller the island, the greater the probability of extinction.

• Extinction rates rise as species richness increases, as a result ofincreased interspecific competition for limited resources.

• Small islands have lower species richness

Rat

e

What type of island is predicted to have the greatest number of species at equilibrium?

Together, island size and island isolation shape patterns of species richness

What type of island is predicted to have the greatest rate of species turnover?

Species Turnover

The Theory of Island Biogeography

Equilibrium theory and non-equilibrium theory make very different predictions

about the diversity of species on islands

The Theory of Island Biogeography

Equilibrium theory and non-equilibrium theory make very different predictions

about the diversity of species on islands

• Equilibrium theory predicts that island species richness will remain constant

over time, whereas non-equilibrium theory predicts that species richness

will increase over time

• Equilibrium theory predicts that although species richness will remain

constant, species composition will change over time (species turnover)

Testing the Theory of Island Biogeography

Wilson & Simberloff (1969)

• Arthropod richness matched pre-treatment levels(species richness remains constant per island)

• Species composition differed from pre-treatmentcommunities

Plant diversity in the South Pacific

Circles: near islands

Triangles: distant islands

Limitations to the equilibrium theory

Limitations to the equilibrium theory

1. Relationships between immigration rates(or extinction rates) and species richnessmay not be linear

1. Relationships between immigration rates(or extinction rates) and species richnessmay not be linear

2. Individuals may arrive from neighboring islands(not just from the mainland)

Limitations to the equilibrium theory

1. Relationships between immigration rates(or extinction rates) and species richnessmay not be linear

2. Individuals may arrive from neighboring islands(not just from the mainland)

3. All species are assumed to be equal, regardlessof differences in dispersal ability, competitiveability, etc.

Limitations to the equilibrium theory

How might Island Biogeography theory inform reserve design and species conservation?

Application of the equilibrium theory

1. We can use the theory to predict the effects of habitat fragmentation

2. Knowledge of species-area effects can inform the design of nature reserves that will maximize long-term species diversity

Habitat fragmentationMajor drivers of biodiversity loss

How might Island Biogeography theory inform reserve design and species conservation?

Application of the equilibrium theory

Principles of Reserve DesignWhy

• may support more species and largerpopulations; more interior per edge

• intact better than fragmented; moreinterior per edge; no dispersal issues

• easier to disperse among patches(increases likelihood of recolonization)

• easier to disperse among multiple patches

• corridors facilitate dispersal

• the amount of edge (relative to area)is minimized when a reserve is round

Island Biogeography

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