Eastern Hog-nosed Snake (Heterodon platirhinos) COSEWIC assessment and update status report: chapter 8
Limiting Factors and Threats
Limiting factors for this species include the availability of suitable habitat, especially sandy soils for oviposition (in some areas) and hibernation sites, and the availability of prey. These soils were easiest to clear and preferred for agriculture (Armason 2001; W. Chesworth, pers. comm. 2007), and intensive agriculture is the land use with the strongest link to species’ endangerment in Ontario (Kerr and Cihlar 2004). Hence, the hog-nosed snake lost much of the habitats that are key to its survival. The lack of available oviposition sites is especially remarkable in Wasaga Beach PP where only 1.3% of the total available area is suitable for oviposition (G. Cunnington pers. comm.). There, communal nesting and clustering of oviposition sites have been observed (G. Cunnington pers. comm.).
Eastern Hog-nosed Snakes are prey specialists and in Canada have only been observed to feed on toads, so the disappearance of Bufo americanus and/or Bufo fowleri would likely cause these snakes to vanish completely from such areas. This effect may have occurred at Pinery PP recently where it appears that both toads and hog-nosed snakes were at one time commonly encountered and reported from the dune habitat but now seem to have declined sharply in this habitat (A. MacKenzie, pers. comm. 2007). Historically, the Eastern Hog-nosed Snake occurred at all sites along Lake Erie in Ontario where Fowler’s Toads (Bufo fowleri) were found (Schueler 1997). At Point Pelee and on Pelee Island, where these toads have apparently disappeared (Green 1989), so has H. platirhinos, even though the American Toad (Bufo americanus) remains common (Schueler 1997).
Road mortality, human persecution, nest predation, and other anthropogenic threats all negatively affect this species’ survival. Motor vehicles on paved roads, dirt roads, and trails may be second only to habitat loss as a cause of declines and losses of reptile populations (Wright 2007) and especially pose a major threat to vagile snakes like the eastern hog-nose (Gibbs, 1998; Bonnet et al. 1999; Rudolph et al. 1999; Carr and Fahrig, 2001; Webb et al. 2003; Crowley 2006; MacKinnon et al, 2005; Rouse 2006). Crowley (2006) demonstrated using NHIC Element Occurrences that road density is significantly higher where Hog-nosed Snakes have become extirpated than where they are still extant. A similar conclusion, albeit somewhat anecdotally based, was drawn by Wright (2007) concerning disappearance of Western Hog-nosed Snakes (Heterodon nasicus nasicus) in an area of Alberta bisected by a paved road. Recently, Clark (2007) has reported that in New York roads increase genetic differentiation among populations of Timber Rattlesnakes (Crotalus horridus) isolated by these roads. Radiotracking data show that some Eastern Hog-nosed Snakes avoid crossing roads and this avoidance restricts the size of their home range and further isolates/fragments populations (Rouse 2006). Recent studies of Prairie Rattlesnakes (Crotalus viridus) and Massasaugas (Sistrurus catenatus) suggest that mortality on roads may act as a selective pressure that reduces the mobility of the population by removing those individual snakes that are more vagile (Jorgenson and Gates, 2006, Rouse pers. comm., D. Jorgenson pers. comm.). Reluctance to cross roads might seem beneficial to the survival of this snake but in reality it illustrates a scenario where fragmentation of suitable habitat by expanding road networks will produce a number of small isolated populations which are unable to disperse and may suffer from inbreeding.
As noted earlier, even protected areas are dangerous to these snakes because most of these areas in the southern parts of the species’ Ontario range are small and intensively used by people and their vehicles (Crowley 2006, Kerr and Cihlar 2004). The Carolinian region which holds about half of the Eastern Hog-nosed Snake’s distribution in Canada is severely fragmented, has an extremely high density of roads (Taylor et al. 2001) and is highly modified into intensive agriculture and urban areas, and the conservation status of the entire region has been termed “critical” (Ricketts et al. 1999). Over 94% of upland forests have been cleared and ploughed (Larsen et al. 1999) and over 99% of its dry prairie habitat has been converted (Bakowsky and Riley 1994). In the snake’s range in the Georgian Bay region the expansion of Highway 69 and the related upgrades of surrounding roads (see Habitat Trends section) and the continued frantic development of other areas of the southern Shield will increase anthropogenic threats to hog-nosed snakes as well as increase the snakes’ mortality from increased, and higher speed traffic (Aresco 2005; Farmer 2006) and from associated landscape changes (Crowley 2006).
Human persecution is also a major threat because of the hog-nosed’s exaggerated and intimidating, although harmless, defensive display and the fact that these displays make it resemble venomous snakes such as “cobras” and “puff adders”. This persecution is especially significant in urbanized areas surrounding snake habitat (e.g. Wasaga Beach PP), although cottagers still kill snakes even in protected areas (S. Gillingwater pers. comm.). Nest predation by subsidized predators such as raccoons has not been quantified but is potentially a significant threat. Aside from the aforementioned threats, occasionally, human garbage might pose a small threat; there have even been two reported cases of H. platirhinos getting stuck in discarded pop cans (S. Gillingwater pers. comm.). Also, there is a growing demand for these snakes in the pet trade (S. Gillingwater, email, Nov. 2007; P. Catling, email, Nov. 2007). (Also, see Google “hognosed snake pets” for a large selection of relevant websites selling/trading/extolling this species as pets.)