III. Island Biogeography

III. Island Biogeography

Biogeography: The study of the distribution of organisms in space and time.

Biogeography looks at four fundamental processes:

1. Dispersal: Movement of organism(s) from a point of origin (= location of source, or ancestral, population) to a new location.

2. Colonization: Organism reaches new location, survives, reproduces, and establishes new population.

3. Extinction: Species is eliminated from a particular area (i.e., no more reproducing individuals present); species may survive elsewhere, and may re-colonize area where it went extinct.

4. Evolution: Surviving population in a particular area undergoes change(s) in frequency of gene alleles; may result in altered phenotype, and, given sufficient time, possibly the formation of new species (= speciation).

Islands are important natural laboratories for the study of biogeography, ecology, population

genetics, evolutionary biology, etc.

Early naturalists (e.g., 16th-18th centuries) exploring isolated islands noted new types of plants and animals, which were often distinctive for each island or island group.

For several centuries, scientific focus was on cataloging the diversity of island organisms.

Darwin observed dozens of animal species unique to the Galapagos

…including 13 species of Galapagos Finches

1859 - Publication of “On the Origin of Species”

Darwin speculated on possible means by which organisms colonized islands and evolved into new species (e.g., Galapagos finches)

1883 - Eruption of Krakatau (Krakatoa), a volcanic island in Indonesia (Aug. 26-27).

Half of Krakatau was blown away; remaining portion, Rakata (a volcanic cone), plus neighboring islands, left covered with 30-60 m of pumice and ash (= sterile landscape?).

Rakata and adjacent islands formed a laboratory for study of island colonization and tropical succession:

• May 1884 - first researchers reach the islands; find only a spider in a crevice on the south side of Rakata.

• October 1884 - grass shoots growing on Rakata.

1886 - Botanists, and later zoologists, begin monitoring colonization of Rakata:

• nine species of flowering plants present on beaches;

1897 - 23 species of flowing plants present;

• development of coastal forest provided seeds and fruits for colonizing bats and birds;

• ferns (with spores that can be dispersed by wind) were first colonizers away from the coast;

1908 - 46 species of flowing plants and 13 species of birds present;

1934 - 30 species of birds present; but, at least 5 bird species present in early 20th century were now extinct on Rakata;

• ~ 50% of inland plant species on Rakata in 1897 have become extinct; however,

• since 1934, 16 additional families of higher plants have colonized.

• colonization by new plant species was initially high, then dropped as available space became occupied by pioneer species; • immigration rate then increased as developing forests created new habitat (= potential new ecological niches); • as forests replaced grasslands, grasses, and insects and birds dependent on grasses, became extinct on island.

Biogeographical lessons from Krakatau:

Composition of plant and animal communities at any given time reflect

• colonization• local extinction• succession• disturbance

Recent studies* have re-evaluated ecological succession and extinctions on Rakata and adjacent islands since 1883:

• Most plant extinctions have been species introduced by people, and rare or ephemeral species;

• Few naturally colonizing and established species have become extinct.

*e.g., Whittaker, R.J. et al. 1992 GeoJournal 28.2: 201-211. Whittaker, R.J. et al. 2000, J. Biogeograpy 27(5):1049-1064

More recently, island biogeographers have begun focusing on patterns and mechanisms of evolution of island flora and fauna.

Dr. Koning will discuss this…stay tuned

Types of Islands

A. Continental Islands: Formed on continent; may have formerly been connected to mainland by land bridge:

Current Sea Level

Continental Shelf

ContinentSubmerged Land Bridge

Former Sea Level

Island

Examples of Continental Islands

1. British Isles

2. California Channel Islands

3. Block Island, Nantucket, Martha’s Vineyard

British Isles: Land mass is part of European continent. During the last ice age, Britain was connected to Europe by a plateau called Doggerland.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Doggerland

Source: New Scientist, 8 Nov. 2008

As Ice Age ended, rising sea level flooded Doggerland and formed English Channel.

Dogger Bank, an upland area of Doggerland, outlined in red.

England

France

North Sea

California Channel Islands: Group of eight islands

off the California coast; during last ice age, some were connected to mainland by land bridge.

Block Island, Nantucket, Martha’s Vineyard: Coastal wedge sediment islands formed by glacial deposits (terminal/recessional moraines); probably no dry, passable connection to mainland since last Ice Age. Long Island is also of this type.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

http://www.uwsp.edu/gEo/faculty/lemke/glacial_processes/MoraineMovie.html

Click to Play Animation

Continental Islands: Two Unusual Cases

1. San Salvador’s offshore cays: Rising sea level caused erosion of San Salvador, leaving many small, erosion-Resistant islands, or cays (“keys”).

Cays on the horizon (arrow) were once part of San Salvador.

Continental Islands: Two Unusual Cases

1. San Salvador’s offshore cays: Rising sea level caused erosion of San Salvador, leaving many small, erosion-resistant islands, or cays (“keys”).

2. Terrestrial habitat islands: Isolated region on larger land mass, such as • mountain top; • forest remnant surrounded cleared land; • forest remnant on island in river or lake;• water-filled tree hole in forest

Barrow Colorado Island (BCI): A 1500 hectare remnant of lowland moist forest in the middle of the Panama Canal; it is managed by the Smithsonian Institute as a tropical research site.

B. Oceanic Islands: Never connected to continent; usually formed by volcanic activity and isolated from continent by deep ocean.

Continental Shelf

Current Sea Level

Former Sea

Level

Oceanic Island

Sea Floor

UnderseaVolcano

Examples of Oceanic Islands

• Iceland• Japan• Aleutians• Bermuda• Caribbean Islands• Hawaiian Islands• South Pacific Atolls• Et al.

Many Caribbean islands were formed by volcanic activity at subduction zone.

Oceanic Islands: Two Unusual Cases

New Zealand

New Zealand: Landmass represents the highlands of a submerged continent called Zealandia. South Island straddles two lithospheric plates and subduction zone.

Oceanic Islands: Two Unusual Cases

New Zealand

Bahamas

Bahamas Banks: No dry land connection to continent?

LandSat Image of San Salvador Island

• San Salvador sits on isolated portion of Bahamas Platform

• Surrounded by deep ocean

• Never connected to other Bahamian Bank islands, or to continent

•Qualifies as an oceanic island

End of Slide Show

4/13/09

Refer to Handouts for Remainder of Lecture