History of Islands: Connection and Isolation

Catalina Island

Not all islands are created equal.

You can't tell at first glance, but the history of an island is one of the key factors that affects biodiversity on the island.



One important aspect of island history is how it formed -- for that we have to review a bit of basic geology. The earth's crust can be divided into roughly two layers: oceanic crust and continental crust. Continental crust is thinner than oceanic crust and less dense, so it "floats" on top of oceanic crust. Continents, obviously, are formed from continental crust. Depending on sea level, this crust is also found underwater and sometimes extends hundreds of miles from the coast as the continental shelf. Beyond the continental shelf, the ocean floor is oceanic crust.

Some islands form directly from the ocean floor -- these are called oceanic islands. The Hawaiian and Galapagos Islands in the Pacific and the Azores in the Atlantic are examples of oceanic islands. Typically the result of volcanic activity on the ocean floor, these islands are the peaks of undersea mountains.

Continental islands are found on the (you guessed it!) continental shelf and include the Florida Keys in the Caribbean and the Indonesian Islands in the Pacific. The Channel Islands are continental islands, though they are found in an area where things are jumbled, making it hard to clearly separate continental and oceanic crusts.

Part of Indonesia and Australia/Papua-New Guinea
 Gray areas show land masses at low sea level.
(Figure from Butler)

Another part of an island's history that affects biodiversity is its relation to the mainland. Some islands are separate from continents throughout their history. All oceanic islands and some continental islands fit this description.  Some continental islands, called land-bridge islands, were connected to the mainland in the past. Examples include the Florida Keys, some (but not all) of the Indonesian islands, and Papua-New Guinea. Many land-bridge islands have experienced repeated connections and separations from the mainland as sea levels have risen and fallen throughout the earth’s history.

California's Channel Islands are not land-bridge islands; they have never been connected to the mainland. Every living thing on the islands has arrived by traversing the ocean -- flying, swimming, floating, or hitch-hiking their way to the safety of land. Partly because of this barrier to colonization, diversity on the Channel Islands is lower than it is on the mainland.

Some of the Channel Islands, however, have been connected to each other. In particular, when sea level was lower during Pleistocene ice ages, the Northern Channel Islands formed one large island, Santarosae, that was only 2-5 miles from the mainland. In contrast, the Southern Channel Islands (Santa Barbara, Santa Catalina, San Clemente, and San Nicolas Islands) have always been isolated from the mainland and from each other.

The Northern Islands coalesced into Santarosae (in brown)
during the Pleistocene.
(Figure adapted from Erlandson, et al. 2011)

These historical differences over geological time scales has consequences for biological diversity on the Islands -- they affect dispersal from the mainland to the Islands and the similarity of the flora and fauna found on the Islands.

Next week's exciting continuation...

• The Island scrub jay -- a poster child for evolution on islands
• How do organisms get to islands?

References:
My lecture materials about island biogeography.

Butler, Rhett A. Diversities of image - rainforest biodiversity. Mongabay.com/A place out of time.: Tropical rainforests and the perils they face. Retrieved 24 March 2012.
[Yes, the author's name IS Rhett Butler!]
Go directly to the page with the image.

Erlandson, J. M. et al. 2011. Paleoindian seafaring, maritime technologies, and coastal foraging on California's Channel Islands. Science 331: 1181-1185. (used for figure of Santarosae only)

Gary Jacobson's website (Grossmont College, El Cajon, California)

Junger, A. and D. L. Johnson. 1980. Was there a quaternary land bridge to the Northern Channel Islands? In: The 2nd California Islands Multidisciplinary Symposium. pp. 33-39.