Eastern Hog-nosed Snake (Heterodon platirhinos) COSEWIC assessment and update status report: chapter 7

Population Sizes and Trends

Search effort

Only a handful of areas in Ontario have been extensively and actively searched for the presence of Eastern Hog-nosed Snakes. Areas which were thoroughly and regularly checked include Wasaga Beach PP, an area 30 km south of Parry Sound that has recently been bisected by the Hwy 400 extension, Long Point PP, Rondeau PP, St. Williams Forest Station, and the Upper Thames River Watershed. Many areas in Norfolk County are also sporadically, but usually annually, searched by naturalists and herpetologists for species at risk, including the Eastern Hog-nosed Snake. Over the past couple of decades, Eastern Hog-nosed Snakes have been observed occasionally by people conducting surveys for Spotted Turtles (Clemmys guttata) and Massasaugas (Sistrurus catenatus) on islands and on the mainland in the area of Twelve Mile Bay, south of MacTier, ON (J. Litzgus pers. comm.).

The most extensive sampling occurred within the town limits of Wasaga where a 5-year capture-mark-recapture study took place (Cunnington, 2004a,b, 2006, G. Cunnington pers. comm.). Search efforts enabled the researcher to estimate the size of the population and the number of snakes per hectare of suitable habitat.

During the summer 2005, a survey near the Trent-Severn Waterway was conducted through Parks Canada. This survey was performed by 4-5 people experienced with searching for H. platirhinos; they visited a total of 32 element occurrences from the NHIC database. No Eastern Hog-nosed Snakes were found but 7 of the 32 element occurrences were described as good habitat. The rest of the element occurrences were in habitat which was poor to fair at best (G. Cunnington pers. comm.).

Abundance

It is difficult to estimate the abundance of Eastern Hog-nosed Snakes in Canada because so few studies have been done on this species in Canada, and because there are few data available regarding population trends and abundance. However, it appears that Eastern Hog-nosed Snakes are almost always found in low densities wherever they occur (Schueler 1997). In Ontario, Schueler (1997) cited numerous reports from Georgian Bay region, Peterborough County, “Central Ontario generally” and Oxford County stating that the species was rare. However, it was reported to be common in PPNP and other areas in southwest Ontario in the early 20th century (Schueler 1997). An estimate of density from a 5-year study of a Canadian population in Wasaga Beach PP. has been calculated at approximately 0.04 individuals per hectare (G. Cunnington pers. comm., which is an order of magnitude less than densities reported by Platt (1969) from the central U.S.A. The number of individuals within the Wasaga town limits is approximately 58 adults (95% C.I. = 42-67; G. Cunnington pers. comm.).  In a study conducted over three years (2003-2005) south of Parry Sound, approximately 20 Eastern Hog-nosed Snakes were found within an area roughly 10 x 5 km (J. Rouse pers. comm.), which translates to a snake density of 0.004 individuals per hectare, an order of magnitude less than reported by Cunnington. These three estimates may reflect a latitudinal trend but obviously are not strong evidence of such.

An estimate of the number of Canadian adult Eastern Hog-nosed Snakes will unavoidably be crude. There are 126 recently verified element occurrences in Ontario and many of them are based on single observations (Oldham and Ben-Oliel 2001). This suggests that some occurrences represent small populations (Oldham and Ben-Oliel 2001) or remnants of ecologically extinct populations. In the northern part of their extent of occurrence, the average range length (the two points furthest apart within the home range) for H. platirhinos was determined to be 2180 m (116-4971; Rouse 2006). So, to estimate the abundance of Eastern Hog-nosed Snakes, all known observations from the NHIC database (only element occurrences confirmed from 1990 onward were used) were overlain with a 3x3 km grid (based on home range sizes reported by Cunnington 2204b, and Rouse 2006), and all squares that contained observations were compiled and a total area of occupancy (AO) was calculated. The total AO was then multiplied by the density of 0.04 individuals per hectare. According to these calculations there are approximately 7,524 Eastern Hog-nosed Snakes in Ontario. This is clearly a rough estimate, but there are several considerations that suggest it is not unreasonable.

Given that many NHIC element occurrences represent single observations and that some of these observations were of juvenile Eastern Hog-nosed Snakes, it is unlikely that the population exceeds 10,000 adult individuals. Conversely, if all NHIC element occurrences were included (i.e. we assume that there are still snakes at all places where they have not been reported since before 1990), then the estimated population would exceed 10,000 snakes. Most of the element occurrences that are “historic” (i.e. not seen since 1990) are in the northern Georgian Bay region (see Figure 2) where density (0.004/ha) appears to be much lower than further south. Many of these sites probably still have Eastern Hog-nosed Snakes, but this lower density if incorporated into the abundance estimate would consequently add relatively few snakes to the total. In contrast, most “historic” sites in the Carolinian region probably are truly historic (i.e. the snake has been extirpated), given ongoing habitat loss and decline, the dense network of roads, the small size of protected areas and remaining patches of suitable habitat (see Figure 5) and a relatively large number of observers and the remarkable display of this species which makes its discovery memorable. The same assessment can be applied to many EOs east of and around the Wasaga/Port Severn/Honey Harbour area on the Shield. This supposition gains support from the failure of a search team to find any Eastern Hog-nosed Snakes in 32 EOs on the Trent-Severn Waterway in 2005, and that many EOs were no longer suitable habitat (G. Cunnington, pers. comm.). Finally, if we ignore these “uncertainties” and look at the NHIC map (Figure 2), we see that the hog-nosed snake recently seems largely restricted to a 3-4 isolated areas of southwest Ontario and an area around the fast-growing Honey Harbour-Wasaga Beach region of Muskoka, and the heavily used south-west edge of the Shield ( see also Schueler 1997). Thus, it seems reasonable to infer that the total Ontario population would be less than 7500 adults (see next section).

Fluctuations and trends

All available information points toward a decline in number of animals based on decline in available habitat and disappearance or drastic decline of Eastern Hog-nosed Snakes from several areas including parts of the Trent-Severn, Algonquin Park (Brooks et al. 2003), parts of Long Point (R. Bolton pers. comm.. 2007), Pinery PP (A. MacKenzie pers. comm. 2007), Pelee Island and Point Pelee NP. Schueler (1997) cites several other areas in which this snake seems to have gone from being common to being rare or extirpated including Boshkung Lake, Haliburton County, Big Creek, parts of Haldimand–Norfolk County, Essex County and the Toronto area. The NHIC has determined that Eastern Hog-nosed Snakes have been extirpated from a minimum of 8% of element occurrences in Ontario (Oldham and Austen 1998). Furthermore, 35% of element occurrences are ranked as historic, or unconfirmed in the last 20 years (Oldham and Austen 1998). However, the species undoubtedly persists at many northern sites because searches have been limited and the species is cryptic and uncommon there. Ultimately, however, the decline in available habitat, the increase in road construction and subsequent vehicle traffic in south-central Ontario, and the threat of human persecution can only mean a decline in number of snakes (S. Gillingwater pers. comm.).

In the Carolinian region, it seems likely that the hognose exists in viable numbers only around the Pinery PP area and parts of Haldimand-Norfolk County (Figures 2, figure 55). Outside these places, the species exists only in isolated small populations.The Eastern Hog-nosed Snake is never found in high densities and is highly vagile, which when considered together increase its susceptibility to urbanization, habitat fragmentation and mortality when crossing roads.  In the latter case, increased mortality is an inevitable consequence of increased vagility, all else being equal (Bonnet et al. 1999; Shine et al. 2004; Rouse 2006). Roads increase this risk because snakes are small and slow. Thus a vagile snake is not mobile compared to, for example, a vagile canid. For a vagile snake, roads represent linear barriers, either snakes attempt to cross and are killed or they avoid crossing and are confined and isolated by the roads (Gibbs, 1998; Cunnington 2004ab, 2006; Hawbaker et al. 2006; Rouse 2006). Vagility and high dispersal capability are traits that have been previously implicated in road mortality of herpetofauna (Gibbs 1998; Bonnet et al. 1999; Carr and Fahrig 2001). According to Roe et al. (2004, 2006), the notion that animals that regularly migrate or disperse long distances may be at greater risk of decline in fragmented landscapes is in contrast to the widely held belief that such animals may be most resistant to habitat fragmentation. However, if costs (e.g., mortality) incurred during dispersal or migratory movements are considered, what may be a valuable trait for colonizing new habitat patches or utilizing widely dispersed resources may become a handicap in landscapes modified by road networks.

Rescue effect

Although H. platirhinos is found in the U.S. directly south and west of Ontario, the populations at the southern part of its Canadian range are separated from the American populations by the Great Lakes. Given that Eastern Hog-nosed Snakes have only rarely been reported to swim (Tyning 1990), there is little evidence that they would cross large water bodies, and any rescue by American snake populations would be unlikely. Also, H. platirhinos is currently designated as Vulnerable (S3) in Michigan, Ohio, and Pennsylvania, which further decreases the likelihood of exchange.

Rescue effect from one Ontario population to the next is also highly unlikely because Eastern Hog-nosed Snakes have shown reluctance to cross roads and those that do venture on roads, are likely to be killed (Rouse 2006).

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