Lake Erie watersnake COSEWIC assessment and status report: chapter 4

Distribution

Global range

The Lake Erie Watersnake has one of the smallest distributions of any snake on the continent (Campbell et al. 1991), having a global range of less than 40 km in diameter (King 1998) (Figure 1). The Lake Erie Watersnake is only found on islands in the western end of Lake Erie between Catawba/Marblehead Peninsula in Ohio and Point Pelee in Ontario. Historically, N. s. insularum had a range that included 22 or more offshore islands and rock outcrops of western Lake Erie (USFWS 2003). Reports dating back to 1893 suggest that the Lake Erie Watersnake was observed on all islands in western Lake Erie (King 1998). Recently, Lake Erie Watersnakes have been observed on Ballast, Gibraltar, Gull, Green, Kelleys, Middle Bass, North Bass, Rattlesnake, South Bass, Starve, and Sugar Islands in the U.S. (USFWS 2003). Excluding Starve and Gull islands (reportedly too small to provide hibernacula), the U.S. islands in the western end of Lake Erie provide approximately 68 km of critical shoreline habitat (King 1998).

Figure 1. Global distribution of the Lake Eriewatersnake (Nerodia sipedon insularum) (indicated with a tiny arrow).

Figure 1. Global distribution of the Lake Eriewatersnake (Nerodia sipedon insularum) (indicated with a tiny arrow).

Canadian range

In Canada, the Lake Erie Watersnake has been observed on the following nine islands: Big Chicken, Chick, East Sister, Hen, Little Chicken, Middle, Middle Sister, North Harbour, and Pelee (USFWS 2003; Figures 1, figure2). All nine islands are located in Ontario in the western end of Lake Erie near the U.S. border (Figure 1). Although historical evidence indicates Lake Erie Watersnakes once existed on all nine islands, only six of the islands (East Sister, Hen, Middle, Middle Sister, North Harbour, and Pelee) have been extensively or casually surveyed since the early 1980s (King 1987a; King et al. 1997). Lack of observation of watersnakes during recent short searches suggests that watersnakes no longer exist on North Harbour and Middle Sister Island (King et al. 1997). However, since researchers have been denied access to North Harbour and Middle Sister Islands (they are privately owned), these observations are based on only one or a few visits and the absence or presumed extirpation of the Lake Erie Watersnake on these islands is not conclusive. The last observation records for North Harbour and Middle Sister Islands, the two most westerly Canadian islands, were in 1939 and 1941, respectively (King et al. 1997). The Ontario islands account for 41 km (or approximately 38%) of the global 109 km of critical shoreline habitat (King 1998). Pelee Island is the largest island (47 km²/4091 ha) in the Lake Erie Watersnake’s range and also supports the largest population in Canada (Campbell et al. 1991; NatureServe 2005).

Figure 2. The island region of western Lake Erie, showing the four islands (East Sister, Hen, Middle, and PeleeIsland) where the Lake Erie watersnake (Nerodia sipedon insularum) is known to be extant in Canada.

Figure 2. The island region of western Lake Erie, showing the four islands (East Sister, Hen, Middle, and PeleeIsland) where the Lake Erie watersnake (Nerodia sipedon insularum) is known to be extant in Canada.

The extent of occurrence of the Lake Erie Watersnake in Canada is estimated to be 188 km². The area of occupancy of the Lake Erie Watersnake in Canada is estimated to be 24 km². The area of occupancy accounts for all shoreline habitat and possible hibernacula on the four islands in Canada (East Sister, Hen, Middle, and Pelee) where Lake Erie Watersnakes are known to be extant.

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