regional diversity what factors operating at regional scales account for local patterns of species...

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

• Robert H. MacArthur and E. O. Wilson (1963)

• What is an island?

• What is biogeography?

• What is dispersal?

The Equilibrium Theory of Island Biogeography (ETIB)

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.

• 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.

General interacting explanations

• How are I and E modeled in the ETIB?

ETIB immigration and extinction

• How are I and E modeled in the ETIB?

ETIB immigration and extinction

Immigration

No. species on island (S)

Rate

P

I

• How are I and E modeled in the ETIB?

ETIB immigration and extinction

Immigration

No. species on island (S)

Rate

P

P = total no. of species in the regional species pool

I

• How are I and E modeled in the ETIB?

ETIB immigration and extinction

Immigration Extinction

No. species on island (S)

Rate

No. species on island (S)

Rate

P

P = total no. of species in the regional species pool

P

I E

• What is the equilibrium species richness for an island?

ETIB: equilibrium species number

• What is the equilibrium species richness for an island?

ETIB: equilibrium species number

• This is produced by the interaction of the I and E curves, determine equilibrium number species (S’)

No. species on island (S)

Rate

P

P = total no. of species in the regional species pool

I E

S’

ETIB: theory on area and isolation

• Large islands should hold larger populations than small islands, so large islands should have reduced extinction rates.

• Lowest diversity: small-distant islands

• Intermediate diversity: small-near and large-distant islands

• Highest diversity: large-near islands

ETIB SUMMARY

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.

• Isolation should also affect how frequently declining populations are “rescued” from extinction.

No. species on island (S)

Rate

P

In E

Sn

If

Sf

“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.

•  Larger islands may accumulate more species by chance just because they are larger targets.

No. species on island (S)

Rate

P

I Es

Ss Sl

“The target effect”

El

Extending area and distance effects in the ETIB

Area Distance

Immigration (I)

Extinction(E)

TargetEffect

ETIB

ETIB Rescue Effect

No. species on island (S)

Rate

P

I Es

SsnSsf

El

Ultimately, however, the ETIB predicts the SAR

In

If

Sln Island area (A)

Log

no.

specie

sSlf

Ssn

Ssf

Sln

Slf

… even without target and rescue effects.

• 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

isolation2. Community composition is dynamic

Species turnover

• 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 AngelesGuns, Germs, and Steel: The Fate of Human SocietiesCollapse: How Societies Choose to Fail or Succeed

Tests of island biogeography theory

• Insects and spiders in the Florida Keys

Tests of isolation determining immigration and richness:

Results:

Extinctions were balanced by immigration despite turnover of species

Tests of turnover and equilibrium prediction:

Results:

No speciation

Equilibrium dynamics prevail

Problems?Both are unrealisticNear, large islands may have the

same number of species or fewer species than small, distant islands.

Assumptions of island biogeography theory?

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.

Non-equilibrium dynamics