Eastern foxsnake (Elaphe gloydi) (Carolinian)COSEWIC assessment and status report: chapter 9
Population Sizes and Trends
In 1972 and 1973, D. Rivard made 155 observations of E. gloydi at 14 localities along the Lake St. Clair-Lake Erie waterway during surveys to examine the distribution of the species in North America (Rivard 1976). Since that time, several researchers have conducted capture-recapture studies within Ontario: northeast of Amherstburg (Essex-Kent) (Freedman and Catling 1978), Point Pelee (Essex-Kent) (M’Closkey et al. 1995; T. Linke unpubl. data), GBI during the early 1980s (Georgian Bay Coast) (Parks Canada, unpubl. data), Pelee Island (Essex-Kent) (Brooks et al. 2000, Willson 2002), Killbear Provincial Park and surrounds (Georgian Bay Coast) (Chora et al. 2001, Lawson 2004, Paleczny et al. 2005), Awenda Provincial Park and surrounds (Georgian Bay Coast) (T. Tully, pers. comm., Coxon 2002), East Sister Island (Essex-Kent) (D. Jacobs unpubl. data), (Honey Harbour-Port Severn (Georgian Bay Coast) (MacKinnon 2005, Row and Lougheed 2006), and Point Pelee-Hillman Marsh (Essex-Kent) (Row and Lougheed 2007, ongoing). Surveys for foxsnakes have also been conducted within the Haldimand-Norfolk regional population by M. Gartshore et al. and most intensively and consistently by S. Gillingwater et al. These efforts, along with other observations compiled in the OHS database, form the basis of our current understanding of the Eastern Foxsnake’s abundance in Ontario.
Simple ground searches were carried out on Long Point from 1996-99 and again from 2003-04 (S. Gillingwater pers. comm.). The number of foxsnakes observed was consistent across years, but the search effort increased markedly over the years from about 20 person days in 1996 and 1997 to about 85 person days with a larger search area in 2003-2004. These results suggest a decline in population on Long Point. Similar sorts of surveys for both turtles and snakes in the Big Creek area (See Figure 3) indicate that few snakes occur east, west or north of Big Creek where there is intensive farming and land use (S. Gillingwater pers. comm.).
Despite the number of capture-recapture and survey studies conducted on Eastern Foxsnakes since Rivard’s (1976) pioneering surveys in the early 1970s, reliable population estimates have been elusive. For example, although Freedman and Catling (1978) estimated a population size of 128 Eastern Foxsnakes at a 40-ha site in Amherstburg, Ontario, their estimate was based on 16 captures and only 1 recapture. Similarly, Rivard (1976) marked 135 individuals at Point Pelee but had a low recapture rate of 6.7%. Recapture numbers below a certain threshold are problematic because they invalidate most capture-recapture models. In response to low recapture rates with opportunistic sampling, and because of the need to acquire snakes early in the active season for telemetry studies, researchers at Point Pelee National Park (M’Closkey et al. 1995; T. Linke unpubl. data) and on Pelee Island (Porchuk 1996) encircled known hibernacula with perimeter-funnel traps. This methodology yielded substantially better foxsnake recapture rates at both sites. Unfortunately, just as the potential for sufficient data collection was being realized on Pelee Island, hostilities towards snakes became strong enough to make vandalism of hibernacula, made readily visible by perimeter fencing, likely (Willson 2002). Consequently, the trapping methodology was deemed too risky and only opportunistic sampling continued. However, based on these experiences, and also the success of researchers monitoring Gray (Eastern) Ratsnake hibernacula (e.g., Blouin-Demers et al. 2002), A. Lawson and C. MacKinnon attempted to trap foxsnakes hibernating at suspected sites in Georgian Bay (Brooks et al. 2003). Although some of the trapping efforts were successful, much of the terrain was not conducive to perimeter fence setup. Nevertheless, even at sites where trapping was not effective, the island-based hibernacula facilitated relatively thorough sampling--although sites where traps were ineffective required considerable persistence and commitment by the researchers. Consequently, A. Lawson was able to monitor several communal hibernacula and C. MacKinnon was able to thoroughly monitor a large hibernaculum from 2003–2005. Monitoring of this latter hibernaculum is continuing through a partnership between Parks Canada, Ontario Ministry of Natural Resources, and collaborating researchers (Row and Lougheed 2006), and offers the best chance to document demographic characteristics of a foxsnake hibernaculum.
For the most part, foxsnakes are easier to find in southwestern (SW) Ontario than they are along the Georgian Bay coast, and this is likely the result of several factors. First, anthropogenic refuse with favourable thermal and shelter properties for a foxsnake is far more common in SW Ontario. These shelter features are far easier to check (i.e., flip or lift) than table rocks along the Georgian Bay coast. Second, overall abundance of the species in the areas where it remains in SW Ontario is likely higher because of the decreasing reproductive output or success of oviparous snakes at increasing latitudes; specifically, clutches of eggs will hatch later in Georgian Bay than in SW Ontario. Third, the high spatial dispersion levels exhibited by individual foxsnakes in Georgian Bay--levels that are not possible in SW Ontario--will further decrease the species’ density at any one site. Despite these factors, monitoring of Eastern Foxsnake population demographics may be more effective in the Georgian Bay region simply because the number of individuals attending any one hibernaculum is higher.
Froom (1972) speculated that Eastern Foxsnake numbers were rapidly declining, and an anecdotal survey of population trends in 1973 suggested that the majority of Ontario populations did indeed appear to be on the decline (Rivard 1976, Rivard 1979). Many naturalists and biologists believe that large snake species (e.g., Eastern Foxsnakes, Gray Ratsnakes, Eastern Hog-nosed Snakes, and Massasaugas) have significantly declined in southern Ontario in the last few decades. Although it is not possible to demonstrate a recent range contraction with data contained in the OHS, the putative decline of Eastern Foxsnake numbers in Ontario is undoubtedly real if loss of suitable habitat (e.g., the conversion of the vast majority of wetlands in Essex-Kent) and the high density of roads in southern Ontario can be shown to have a negative impact on foxsnake populations. Evidence for negative effects of the first factor, wetland habitat loss, comes from the less frequent observations of foxsnakes in areas completely devoid of wetland features, in comparison to the number of foxsnake observations in areas with marshland habitat (e.g., Point Pelee, Hillman Marsh). That these observation differences are not merely the result of differing sampling regimes (e.g., more researchers and naturalists looking for wildlife in natural areas) comes from the fact that despite the extensive road network covering all SW Ontario, snakes are reported more often on roads in or adjacent to natural areas such as Point Pelee and Rondeau. Indeed, consider that there are few areas in SW Ontario where foxsnake populations would not come into contact with roads using even the most conservative space use requirements documented for the species. In light of the extensiveness of this road network and the number of studies that have documented substantial foxsnake mortality on small stretches of road (e.g., Ashley and Robinson 1996, Brooks et al. 2000, MacKinnon et al. 2005, Farmer 2007), it logically follows that the number of foxsnakes that have been killed on roads in Ontario has been considerable. Although population-level effects were not demonstrated by these studies of road mortality, the regular loss of mature individuals from a population has been modelled for other reptiles and shown to result in population decline (Brooks et al. 1991, Garber and Burger 1995).
The rescue effect is defined by COSEWIC as the “Immigration of gametes or individuals that have a high probability of reproducing successfully, such that extirpation or decline of a population, or some other Designatable Unit, can be mitigated. If the potential for rescue is high, the risk of extirpation may be reduced.”
Eastern Foxsnake populations on the Canadian islands of the Lake Erie Archipelago are within movement distance of foxsnake populations residing on American islands, and snakes may be able to cross from the USA to Ontario at the Detroit River and the north end of Lake St. Clair (to Walpole Island) (Figure 1). Although migration between islands is likely rare, the species definitely shows the capacity to move long distances over water. However, such rescue, if it ever occurs, would only provide some possible genetic benefit as the USA populations are also at risk and could not provide sufficient migrants for recruitment. Given the vast distances between the three regional populations in Ontario, interchange of individuals between them is not possible (Figure 2).
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