Gray ratsnake (Elaphe spiloides) COSEWIC assessment and status report: chapter 6

Distribution

Global Range

The Gray Ratsnake is widely distributed throughout the forested areas of eastern and central United States, but occur in only two small disjunct regions in Ontario. The rest of its distribution occurs relatively continuously from southwestern New England, south along the western edge of the Appalachian Mountains to the Gulf of Mexico, west to the Mississippi River, and north to southwest Wisconsin (Fig. 1).


Canadian Range

The Canadian distribution of Gray Ratsnakes is limited to southern Ontario and represents a small proportion of the global distribution (<5%; Fig. 2). Within Ontario, ratsnakes are associated with two regions which are discussed separately below.


Figure 2: Canadian Distribution of Gray Ratsnakes (Elaphe spiloides)

Figure 2. Canadian distribution of Gray Ratsnakes (Elaphe spiloides).

 


Carolinian Population

In southwestern Ontario, ratsnakes currently inhabit small parts of the Carolinian forest running along the northern shore of Lake Erie. Prior to European settlement, large tracts of deciduous forest intermixed with open savanna would likely have provided an abundance of suitable habitat in this region. Records of Gray Ratsnakes extend from Point Pelee (Logier, 1925) east to Fonthill (Lindsay, 1931), and it is likely that ratsnakes once extended continuously across most of the Carolinian zone of southwestern Ontario. As discussed by Prior and Weatherhead (1996), Gray Ratsnakes most likely migrated into Canada around both sides of Lake Erie following the retreat of the Laurentide ice sheet (~7000 years ago) and subsequent northward advance of deciduous forest (Smith, 1957; Pielou, 1991).

The current extent of occurrence of the Carolinian population is relatively large (7300 km2), but this distribution is severely fragmented and the area of occupancy is only around 320 km2. The area of occupancy for all of the Carolinian populations was determined by outlining the post-1985 occurrence records and the extent of occurrence was calculated by outlining the areas of occurrence with a convex polygon. The previous COSEWIC status report (Prior and Weatherhead, 1996) recognized the persistence of only 4 very small, isolated Carolinian populations (Fig. 2) and each of these is discussed below.

  1. The Skunk’s Misery population is generally located north of the Thames River between Wardsville and Bothwell. The most recent verifiable observations for this population are from 1984 (Prior and Weatherhead, 1996; Oldham and Weller 2000) and 1997 (A. Woodliffe pers. comm. 2006); however, there are also recent reports (<10 years) of snakes resembling Gray Ratsnakes from this region (D. Martin pers. comm. 2006). Records near the towns of Rodney and New Glasgow (1987) south of Highway 401 may represent a separate population associated with 16 Mile Creek (Prior and Weatherhead, 1996; Oldham and Weller 2000). Because of the limited number of observations for this population the 1984 observations were also used in the area of occupancy calculation.
  2. The Big Creek population seems to be the largest of the Carolinian populations and ranges approximately from the base of Long Point to the town of Langton in the north, Turkey Point P.P. in the east, and Port Burwell in the west. The total area of occupancy is approximately 260 km2 and is based on approximately 25 relatively recent (post-1985) NHIC observations.
  3. The Oriskany Sandstone population is confined to a relatively small area (<10 km2) in Cayuga and Oneida townships and is roughly bound by the towns of Nelles Corners, Cayuga, and Decewsville. Up until 2000, the most recent confirmed sighting was 1990 (Prior and Weatherhead, 1996). Recently, two individuals from this population were located and radio-tracked and one other road-killed individual was found during the study (Yagi and Tervo, 2006). Both radio-tracked individuals hibernated in close proximity, but despite efforts to fence and trap this area, no other individuals were captured and the size of the hibernaculum is unknown.
  4. As discussed in the previous status report (Prior and Weatherhead, 1996), the Niagara population is poorly defined and most likely consists of a few small disjunct populations. Verifiable occurrences are located around the towns of Fonthill and Ridgeway. Another recent occurrence record from the town of Winger could represent another disjunct population.


Great Lakes/St. Lawrence Population

In southeastern Ontario, the Gray Ratsnake population is typically associated with the Frontenac Axis, which is a southeast extension of the Canadian Shield that connects with the Adirondacks in northern New York State. The bulk of this population lies in Frontenac, and Leeds and Grenville Counties; however, a small portion of the range crosses over the St. Lawrence River into Jefferson and St. Lawrence Counties in upper New York State. Using the NHIC records and knowledge from experts in the field (S. Thompson pers. comm. 2005; J. Leggo pers. comm. 2005; T. Norris pers. comm. 2005), the extent of occurrence of the Canadian portion of this population was determined to be roughly bordered by Highway 7 in the north, the St. Lawrence River in the south, Highway 38 in the west and Highway 29 in the east.

The extent of occurrence of the Great Lakes St.Lawrence population in Canada spans approximately 4000 km2 and is separated from the populations in southwestern Ontario and southern New York State by approximately 300 and 150 km respectively. The isolation of this population has been recognized since the early 1900s (Lindsay, 1931; Toner, 1934; Logier, 1957), and the absence of historic records of Gray Ratsnakes along the northern shore of Lake Ontario and upper New York State (Weber, 1928) suggest that the separation between these populations may have preceded European settlement (Prior and Weatherhead, 1996). Recent genetic and morphological evidence (Gibbs et al. 2006) suggest that the Great Lakes/St.Lawrence population is a hybrid between the central and eastern ratsnake clades (as proposed by Burbrink, 2001; see Species Information - Name and Classification), implying that this population was colonized by an eastern expansion of the central clade along the northern shore of Lake Ontario, and a northward expansion of the eastern clade along the eastern edge of Lake Ontario.

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