Distribution of Reptiles and Amphibians on the Islands of Eastern Lake Michigan: Summary and Analysis

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ABSTRACT

We studied the island biogeography of reptiles and amphibians in the Lake Michigan Archipelago, which is composed of twelve major islands in eastern Lake Michigan. We created an updated distributional record of reptiles and amphibians in the Lake Michigan Archipelago, quantitatively compared our distribution data with a commonly used data set published in 1948, analyzed the distributions in light of the theory of island biogeography, and compared species diversity of the Archipelago to that of the mainland and other island groups in the Great Lakes. A total of 10 reptile and 11 amphibian species are documented in the Lake Michigan Archipelago. Our updated distribution data are a qualitative improvement over the 1948 data set. There is a positive relationship between island area and the number of both amphibian species and reptile species present, but no relationship between the latter variables and island distance to the mainland. The Lake Michigan Archipelago appears to be less diverse both in general and in relation to the adjacent mainland than other island groups in the Great Lakes. Possible reasons for some of our findings are discussed. This research underscores the incompleteness of commonly used distribution data for the Lake Michigan Archipelago, and raises several questions for future study.

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INTRODUCTION

The study of insular flora and fauna has played a dominant role in shaping biological theory. In effect, modern biological study began with the work of Darwin (1859) and Wallace (1869) whose work on insular organisms helped lead them independently to the theory of evolution by natural selection. The subdiscipline of biogeography, formalized by Wallace (1880), has more recently stimulated a great deal of research. This began with the equilibrium theory of island biogeography (MacArthur and Wilson 1967). The equilibrium theory led indirectly to metapopulation biology (Hanski and Simberloff 1997) and directly to the unified neutral theory of biodiversity and biogeography (Hubbell 2001). In general, it is hard to overemphasize the importance of islands to the study of biology.

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The island group focused upon in this study is the Lake Michigan Archipelago. This archipelago is composed of twelve major and several minor islands located in eastern Lake Michigan. The islands vary widely in surface area and distance to the mainland (Table 1; Figure 1). The Archipelago is adjacent to the Leelanau Peninsula in the south and the Straits of Mackinaw in the north, a straight north-south distance of ca. 90 km. The islands with the highest elevation (Beaver, High, South Fox, and North and South Manitou) were first present in glacial Lake Algonquin ca. 9000-10,000 yr. B.P. (years before present). From 4000 to 8000 yr. B.P. (Lake Chippewa stage), water levels were lower. The northern islands (Beaver, Garden, High, Hog, Squaw, Trout, and Whiskey) were all exposed and connected to the mainland via Waugoshance Point, while the Manitou Islands were connected via the Leelanau Peninsula. Whether or not the Fox Islands were ever connected to the mainland is unknown. Lake levels then rose again during the Lake Nipissing stage (3000-4000 yr. B.P.), and all but the highest areas (parts of Beaver, Garden, High, North and South Fox, and North and South Manitou) were inundated. Water levels then gradually fell to that of modern-day Lake Michigan (Dietrich 1978; Hatt et al. 1948).

Land vertebrates likely reached the islands via land bridges during the Lake Chippewa stage or by rafting across the Lake since that time (Hatt et al. 1948). Because the islands were connected to the Lower Peninsula of Michigan and because surface currents run from the Lower Peninsula to the islands (Harrington 1895), land vertebrate populations on the islands are likely derived from those on the Lower Peninsula (but see Placyk and Gillingham 2002). The land vertebrates (amphibians, reptiles, birds, mammals) of the islands have long attracted the attention of biologists, but the only attempt at a comprehensive survey of the entire Archipelago was that of Hatt et al. …