If you (as a species) get trapped on an island, there is really one of three things that can happen. The first thing, which seems to be like it would be the most common of the three things, is that you and your species would go extinct on the island. Perhaps your species still survives on the mainland, but the island group has died off. The second, and second most likely to happen (in my opinion) is you and your species, over many generations, shrink, to match the food supply. If there is a limited supply of food, then the smallest of your species, not the largest, are much more likely to survive. And the third happenstance is that you and your species grow in size over many generations. Say you and your species are rats. On the mainland, you are preyed upon by dogs, foxes, coyotes, cats, and the like. When you and other of your rat buddies became trapped on the island, there was an abundnace of food there, but no predators to prevent you from growing bigger. So grow you did. We will talk about a very interesting occurrence of this later on. This shrinking and growing, called "Foster's Rule," is often simply known as "The Island Rule."
For now, however, we are going to focus on the more common of the two, and the more interesting (both in my opinion); island dwarfism. This has occurred many, many times throughout history, and even to humans! However, today we are going to look at one particular occurrence of this dwarfism. The Channel Island fox of the Channel Islands off of the coast of California.
Scientists believed that the ancestors of these foxes "rafted" to the northernmost islands in the island chain sometime between 10,400 and 16,00 years ago. These ancestors would have been the gray fox, very similar to the ones we see today. The foxes rafted over during the last Ice Age. This would have dramatically lowered the sea levels, and much of the water that is in today's oceans would be locked away in the ice caps at the poles, or in glaciers.
As you can see in the map below, the four islands of San Miguel, Santa Rosa, Santa Cruz, and Anacapa are all located with only shallow water separating them. During the Ice Age, with the lower sea levels, these four islands were all one island, called Santa Rosae. Also at this time, the distance from the mainland to the "mega-island" would be much smaller, making the crossing for these gray foxes much easier. It has been theorized that Native Americans then brought the fox to the four southern islands, as hunting dogs, or perhaps even pets.
|The mainland gray fox. Photo credit Zack Neher, taken at Brookgreen Gardens in South Carolina.|
Even smaller than the mainland kit and swift foxes, the Channel Island is the smallest of all of North America's foxes. Like many island animals, the Channel Island fox is labeled as "Critically Endangered" by the IUCN, as they don't have much territory to spread into when humans influence them. One influence was indirect, but still devastating for the foxes. Prior to the 1990s, the golden eagle was a rare visitor to these islands. The bald eagle, already well established in the area, was apparently a large deterrent for the golden eagle, preventing them from settling on the island. DDT helped to eradicate the bald eagle on the Channel Islands, and with very few bald eagles in the area, the golden eagle moved into the gap: nature abhors a vacuum!
Anyways, the golden eagle, unlike the bald eagle (who is primarily piscivorous) would, and did, hunt the Channel Island foxes; at four times the foxes size, they were most definitely a force to be reckoned with. That, coupled with diseases brought over from the mainland by domestic dogs, such as canine distemper, have also wreaked havoc upon the fox populations. Conservationists are currently working on a solution, and tracking the foxes with radio collars seems to be helping them learn more about the foxes, in an attempt to prepare for the future.