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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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