Regional diversity
• What factors operating at regional scales account for local patterns of species diversity?
– Dispersal
• Range expansion/contraction
• Movement into new areas
– Speciation
• Vicariance, ecological speciation, speciation by polyploidy, etc.
– Extinction

The Equilibrium Theory of
Island Biogeography (ETIB)
• Robert H. MacArthur and E. O.
Wilson (1963)
• What is an island?
• What is biogeography?
• What is dispersal?

What is an island?
• Ecological island: habitat surrounded by an inhospitable matrix
– Oceanic islands
• surrounded by saltwater
– Continental islands
• e.g. patches of habitat
– Other islands
• “sky islands” = montane habitats surrounded by desert or forest
• Lakes and ponds surrounded by terrestrial habitat
• Rivers and streams, separated by dry drainage divides

Alexander von Humboldt
(1769-1859)
 Russian naturalist & explorer
 Father of biogeography
 1799-1804 travelled throughout Latin and S.
America
 Documented several key biogeographic patterns

The species-area relationship (SAR)
 The SAR is one of the
“laws” of ecology.
 What could cause this pattern?
 Discuss.

General interacting explanations
• Habitat diversity hypothesis
• The likelihood of encountering new habitat types and thus new niches for different species increases as area increases.
• Dispersal hypothesis
• Large areas are likely to receive more colonists than small areas.
• Area-Extinction hypothesis
• The population sizes of species are greater in large areas. Thus, fewer species go extinct than in small areas where there are more rare species.

ETIB immigration and extinction
• How are I and E modeled in the ETIB?

ETIB immigration and extinction
• How are I and E modeled in the ETIB?
Immigration
I
P
No. species on island (S)

ETIB immigration and extinction
• How are I and E modeled in the ETIB?
Immigration
I
P
No. species on island (S)
P = total no. of species in the regional species pool

ETIB immigration and extinction
• How are I and E modeled in the ETIB?
Immigration Extinction
I E
P
No. species on island (S)
P = total no. of species in the regional species pool
P
No. species on island (S)

ETIB: equilibrium species number
• What is the equilibrium species richness for an island?

ETIB: equilibrium species number
• What is the equilibrium species richness for an island?
• This is produced by the interaction of the I and E curves, determine equilibrium number species (S’)
I E
P
S’
No. species on island (S)
P = total no. of species in the regional species pool

ETIB: theory on area and isolation
• Large islands should hold larger populations than small islands, so large islands should have reduced extinction rates.

ETIB SUMMARY
• Lowest diversity: small-distant islands
• Intermediate diversity: small-near and largedistant islands
• Highest diversity: large-near islands

How does island isolation affect extinction and immigration?
• The distance of an island from the mainland (a source of colonists) should affect the immigration rate and thus, species diversity.
I n
I f
P
S f
S n
No. species on island (S)
E
• Isolation should also affect how frequently declining populations are “rescued” from extinction .
“The rescue effect”

How does island size affect extinction and immigration?
• Extinction rates on small islands would be higher because small islands have smaller populations than large islands.
I
S s
S l
No. species on island (S)
P
E s
E l
• Larger islands may accumulate more species by chance just because they are larger targets .
“The target effect”

Extending area and distance effects in the ETIB
Area Distance
Immigration (I)
Extinction
(E)
Target
Effect
ETIB
ETIB Rescue
Effect

Ultimately, however, the ETIB predicts the SAR
I n
I f
P
E s
E l
S sf
S lf
S sn
S ln
S sf
S sn
S ln
S lf
No. species on island (S)
Island area (A)
… even without target and rescue effects.

Species turnover
• Definition: change in species composition from one community to another through time
 Island biogeography theory makes 2 general predictions:
1. Species diversity is affected by size and isolation
2. Community composition is dynamic

Tests of island biogeography theory
• Dan Simberloff and E.O. Wilson’s test on mangrove islands in the
Florida Keys
• Jared Diamond’s test on
California’s
Channel Islands
Dan Simberloff
(1942-present),
University of
Tennessee
Jared Diamond (1937-present)
University of California at Los Angeles
Guns, Germs, and Steel: The Fate of Human Societies
Collapse: How Societies Choose to Fail or Succeed

Tests of isolation determining immigration and richness:
• Insects and spiders in the
Florida Keys
Results:

Tests of turnover and equilibrium prediction:
Extinctions were balanced by immigration despite turnover of species
Results:

Assumptions of island biogeography theory?
 No speciation
 Equilibrium dynamics prevail
 Problems?
 Both are unrealistic
 Near, large islands may have the same number of species or fewer species than small, distant islands .

Non-equilibrium dynamics
 Should we always expect an equilibrium or steady balance between immigration and extinction?
 Non-equilibrium in this context means that the slopes of the colonization-extinction curves for an island varies over time. Thus, where the two lines cross varies and the island diversity varies.