Island biogeography I:
the idea
Bio 415/615
Questions
1. What are the opposing forces of island
biogeography?
2. Why do islands have fewer species than
continental areas of the same size?
3. How has IBT been tested?
4. What is the rescue effect?
Log Species Richness
Recall species-area relationships
Log Area
Consider islands
• Bigger islands should have more species
than smaller islands. Why?
– More habitats (or env circumstances)
– Support larger populations
– Can hold larger animals (e.g., big ranges)
• What else determines how many species
are on islands?
Richness as a f(area, distance)
Island Observations
Islands have fewer species than
samples within contiguous
continental areas as a function of
size
Islands have ever fewer species as
they get smaller (z is higher)
Isolated islands have fewer species
than less isolated islands of the
same size
Beyond Islands: Habitat Loss &
Fragmentation
• Area decreases (Grain decreases)
• Isolation increases (Distance
increases)
• Fewer species expected
• Insularization
Occam’s principle of parsimony
with a warning from Einstein
One should not increase
beyond what is
necessary the number
of entities required to
explain anything
William of Ockham
Everything should be
made as simple as
possible but not
simpler
Albert Einstein
IBT
The Theory
• Immigration, extinction
– Straight to concave
• Turnover
• Near, Far
• Large, Small
Immigration Rate
Rate of New Species
Immigration
High
Low
0 Number of Resident Species Many
Extinction Rate
Low
Rate of
Extinction
Rate of New Species
Immigration
High
0 Number of Resident Species Many
Equilibrium
Low
Equilibrium
Turnover
0 Number of Resident Species Many
Rate of
Extinction
Rate of New Species
Immigration
High
Rate of New Species
Immigration
Far from Mainland
Number of Species
Rate of
Extinction
Isolation
Rate of New Species
Immigration
Large Island
Number of Species
Rate of
Extinction
Area
Rate of New Species
Immigration
Small, Far Island
Number of Species
Rate of
Extinction
Equilibrium
Small, Close Island
Number of Species
Rate of
Extinction
Rate of New Species
Immigration
Equilibrium
Large, Close Island
Number of Species
Rate of
Extinction
Rate of New Species
Immigration
Equilibrium
Rate of New Species
Immigration
Large, Far Island
Number of Species
Rate of
Extinction
Equilibrium
IBT
Extensions of Theory
• Target effect
• Rescue effect
I
E
Distance
MW
Rescue
Area
Target
MW
Rescue effect
Brown & Kodric-Brown 1977
Isolation influences extinction rates of
extant species too, by ‘rescuing’ them
from extinction through continuous
supply of more individuals
Target Bigger islands are
Effect bigger ‘targets’
for colonization
Rescue
Effect
Number of Species
Rate of
Extinction
Rate of New Species
Immigration
Area and Isolation
Extensions of Theory
• Target effect
• Rescue effect
• Landscape ecology: matrix, patch
quality, corridor
Simberloff: Experimental Test
Simberloff’s mangrove islands
Land bridge islands
Barro Colorado Island, Panama
1. Hilltop = 15.7 km2 of lowland tropical forest.
2. Isolated in 1914 when Lake Gatun was formed
by construction of the Panama Canal.
3. Knowing area and period of isolation, can
model extinction.
4. 108 species of breeding birds in 1938.
5. Terborgh used land bridge model to predict
17 would be lost in 50 years; really 13 = 12% of
108.
So why does insularization
lead to species loss?
The 3 Step Process of Species Loss and
Extinction Debt
The 3 Step Process of Species Loss and
Extinction Debt
Instantaneous
Sampling
Fast
Isolation
Slow
Area
The 3 Step Process of Species Loss and
Extinction Debt
Instantaneous
Fast
Faster & Greater
Loss as Area
Slow
The 3 Step Process of Species Loss and
Extinction Debt
Instantaneous
Fast
Faster & Greater
Loss as Area
Slow
The 3 Step Process of Species Loss and
Extinction Debt
Instantaneous
Fast
Faster & Greater
Loss as Area
Slow
Extinction debt
Steeper z
Next: beyond islands