When are habitat patches really islands?

Here we present a method for determining whether habitat patches are truly functioning as habitat islands. We do so by assessing if the species distribution pattern across the islands, when compared to a mainland pattern, conforms to the predictions of the theory of island biogeography. The putative islands in our study are a series of hummocks in a swamp in the ES George Reserve, Pinckney, MI. We assess the tree flora on each hummock and find that hummocks that are at a distance from the mainland forest have fewer tree species than subsets of the mainland having the same area as the island. The four hummocks greater than 350 m from the mainland had significantly fewer species than mainland samples of the same size, while those less than 200 m away were not statistically different. These results suggest that at least some of the hummocks function as biological islands for trees.