galapagos islands - sfu.ca - simon fraser university...galapagos islands 2,700 endemic species! why?...
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Galapagos Islands
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Galapagos Islands 2,700 endemic species! WHY?
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Denali, Alaska Low species diversity. Why?
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Patterns of Species Diversity • Latitudinal
– Global pattern—drivers? • Islands (but also mtn. tops, lakes, etc.)
– Patterns with island size • Species-Area curves
– Patterns with island size & distance • Theory of Island Biogeography
• Predicting species diversity as a function of colonization and extinction rates
• Metapopulations* – Modern extension of island biogeography *not related to diversity
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Global patterns of species diversity (land plants)
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Latitudinal diversity gradient: Birds from the new world
Gaston & Blackburn (2000) Pattern and Processes in Macroecology.
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BUT...other features also influence diversity
Evolutionary history, habitat heterogenity, species interactions, etc.
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! Time: ! time since glaciation/major disturbance, which “resets”
diversity ! climate stability = more diversification
! Area: ! global land area dominated by tropics, statistical
artifact?
! Energy: ! Solar input higher, more productivity could allow
multiple species to coexist (more action at the base of the food web)
Why are there latitudinal diversity gradients?
tundra2%
tropical40%
subtropical16%
temperate19%
boreal23%
Rosenzweig (1992) Journal of Mammalogy
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Some exceptions:
! Aphids, sawflies, ichneumonids and bees: show peaks at intermediate or high latitudes.
Some taxa are adapted to life at higher latitudes (e.g. penguins and auks)
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Hotspots
Endemic species are those found only in a single area
ISLANDS AS SPECIAL HOTSPOTS OF DIVERSITY
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Island species richness
• Bigger islands have more species than small islands
• “Species-Area curves”
• Documented for diverse taxa
• Other types of habitat also follow this pattern.....island-like (mtn. tops, forest remnants, lakes, etc.)
Island area
# s
peci
es
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Species richness increases with island AREA
MacArthur and Wilson (1967)
Rich Glor
Log
scal
e
Log scale
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Galapagos land plants
S = c · Az
c is constant of spp./ area z is the slope
z = 0.32 (~0.3 most islands)
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What’s z for species-area curves on continental areas?
z = 0.17 (~0.15-0.24 for land areas)
Nor
th A
mer
ican
bird
s
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Species-Area relationship for N. Wisconsin Lakes
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Island species richness
• If z ~0.3 on islands, and ~0.15-0.24 on land
• What does that tell us about islands??
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An extension of this idea: Island biogeography
! Dynamic equilibrium theory that explains species richness of islands
! Island richness determined by colonization and extinction rates (number of species per unit time)
! Richness increases with size ---Why? ! more habiats to support more species, less extinction....
! Richness decreases with isolation ---Why? ! less likely to be colonized
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Species richness decreases with isolation
More isolated islands less likely to receive colonists (immigration low)
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MA
INLA
ND
large
small
near
far
Extinction more likely for small
populations (small islands)
Colonization more likely for closer islands
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Island biogeography
*Note: Immigration & Colonization used interchangeably
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Experimental test of island biogeography
! Defaunation experiment by Simberloff and Wilson
! Methods: ! Survey small mangrove islands for arthropods. ! Cover islands with plastic and spray with
insecticide (gets rid of all arthropods) ! Observe colonization/ succession over one year. ! How many and what species return?
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Simberloff and Wilson’s experiment Florida Keys
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Experimental Results
! Species richness on islands returned to levels similar to before defaunation
! Closer, larger islands had more species
! The precise species identity was not consistent, only the total number of species
" Order of colonization and species interactions important for “who” composes the community
! Support for dynamic equilibrium
Simberloff & Wilson
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Metapopulations ! Collection of subpopulations of 1 species
! Proportion of sites occupied determined by colonization and extinction rates at each site
And now for a big mental leap... from diversity to individual population
dynamics
Rana cascadae
Proportion of sites occupied
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Metapopulations ! Collection of subpopulations of 1 species
! Proportion of sites occupied determined by colonization and extinction rates at each site
! Connected by individual movement (dispersal between sites provides colonists)
! Individual sites may be colonized in one year, and extinct the next
! Individual site dynamics are variable, but overall “metapopulation” is stable
And now for a big mental leap... from diversity to individual population
dynamics
Rana cascadae
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Metapopulation dynamics
brown=occupied white=empty
Given a fixed colonization rate and extinction rate…
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Metapopulation dynamics
red = went extinct since last time step
Given a fixed colonization rate and extinction rate…
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Metapopulation dynamics
Given a fixed colonization rate and extinction rate…
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Metapopulation dynamics
Given a fixed colonization rate and extinction rate…
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Metapopulation dynamics
Given a fixed colonization rate and extinction rate…
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Metapopulation dynamics
Given a fixed colonization rate and extinction rate…
What is the DYNAMIC EQUILIBRIUM here? Proportion of sites occupied by this single species ~ 6 of 11 sites
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Classic metapopulation
! Governed by fixed colonization & extinction rates
! Subpopulations have independent dynamics and are connected by dispersal
! All patches of identical quality (not realistic)