MacArthur and Wilson II: Archipelagos

Last weekend, my post outlined MacArthur and Wilson's Theory of Island Biogeography for one island. Recall that we discussed on how islands gained species, through colonization, and how they lost species, through extinction.

But islands often occur as part of an archipelago, a collection of islands. Islands in the archipelago vary in a number of ways, including topography, shape, size, and distance to the mainland. MacArthur and Wilson focused on the effect that two characteristics affect diversity: distance between the island and the mainland, and island size.

The Island Scrub Jay -- Cause for Concern

Painting by Eli W. Blake Jr.
December 1886
(From Collins 2009)

The Island scrub jay (Aphelocoma insularis) is found only on Santa Cruz Island, the largest of the eight California Channel Islands. It is the only bird island endemic in the continental United States. (An endemic is a species unique to one location and found nowhere else.) Estimates of the population size for Island scrub jays has varied over time and between studies. Generally speaking, older studies estimated a larger population size than more recent studies, suggesting that the population has declined.

Wildlife biologists studying the birds cannot confirm a population decline, but the present-day estimate (2500) is small enough to concern them for several reasons. First, of course, this population is restricted to a single island, increasing the likelihood that one catastrophic event (like a fire) could jeopardize the entire species. Quite literally, their eggs are all in one basket! Second, a population size of 2500 is smaller than that of other bird species designated as endangered by the US federal government (e.g. the Red-cockaded woodpecker). A third reason for concern is that when island populations of other species have declined, they have often declined very rapidly, leaving little time for addressing, or even detecting, the decline. 

Island Biogeography & the Channel Islands

E. O. Wilson (top)
Robert H. MacArthur
(below)
(from bioquest.org)

I anticipated posting this weekend about how species get to islands. I will get to that, but I thought that first I should talk about island biogeography. When I was first introduced to this model of island biogeography, I was fairly baffled. I didn't get the swooshing lines. I didn't understand why their intersection was interesting. Over the years, I've had to teach about the model to undergraduates, as both a graduate student and a faculty member. As soon as I understood it, it became one of my favorite concepts to teach!

The Theory of Island Biogeography, published by Robert H. MacArthur and Edward O. Wilson in 1967, is a classic ecological model of island biodiversity. An abbreviated explanation of this model was in the most recent Catalina Island Conservancy online newsletter.

Glidden's Archaeological Archives Uncovered!

Unearthing Catalina History Using Skeleton Clues

This story appeared in the LA Times today about Ralph Glidden, an archaeologist/huckster who lived on the island in the first half of the 20th century.

One of Glidden's notebooks
(Photo by Christina House, from LA Times story)

John Boraggina, the recent curator of the Catalina Island Museum found Glidden's archives in a storage room at the Museum. He was shocked to find that the records point to the locations where Glidden's archaeological finds were made.

Archaeologists have long been troubled by Glidden's collection because he was not above supplementing bona fide artifacts and human remains from Catalina from those he purchased elsewhere. This complicates the historical interpretation of his finds. He operated a "museum" on the island until 1950 and tried to sell the collection after his business closed. The pieces were eventually purchased by the Wrigley Family who in turn donated them to the Catalina Island Museum. The Museum has since placed some of the more important artifacts and remains with the Fowler Museum at UCLA.

Glidden's Catalina Museum of Island Indians
(Photo from tongvapeople.com)

Full citation:
Sagahun, Louis. 1 April 2012. Unearthing Catalina history using skeleton clues. Los Angeles Times. http://www.latimes.com/news/local/la-me-catalina-bones-20120402,0,5531466.story?

Photo credits:
House, Christina -- photo included in the LA Times story.
http://www.tongvapeople.com/island-of-indians.html