Lake Erie watersnake COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
Similar to many reptiles in Canada, Lake Erie Watersnakes are declining as a result of habitat loss, human persecution, mortality on roads and predation. Contamination is not currently a demonstrated factor in decline; however, it may be a potential threat to the Lake Erie Watersnake. These threats are amplified because of the watersnake’s extremely small geographic range on islands, their high visibility, aggressive defensive behaviour, and limited population size.
Habitat Loss/fragmentation
The islands in the western portion of Lake Erie are becoming increasingly popular for recreation and tourism. These activities are focused on shoreline development for cottages, retirement homes, and tourist accommodations. Loss and degradation of suitable and critical shoreline habitat due to development have detrimental effects on watersnakes, although there is evidence that watersnakes have adapted well to some anthropogenic structures (King 2001; USFWS 2003). Inland habitat, used for hibernation by some watersnakes, is similarly threatened by increasing development, particularly construction of homes, roads, commercial structures, and the infrastructure that accompanies community development.
Road Mortality
Increasing tourism-based development leads to more road construction and human usage resulting in higher road mortality rates of snakes. Although Lake Erie Watersnakes travel only short distances over land on islands, they are still affected by road mortality (King 1998; Willson et al. 2002). Although there are no roads on three of the Canadian islands where Lake Erie Watersnakes occur; there are a large number of roads on Pelee Island, some of which run parallel to most of the shoreline. Furthermore, more human inhabitants increase the likelihood of snakes being killed by other machines, such as boats and lawn mowers (USFWS 2003). A survey of road mortality of reptiles on Pelee Island in the mid-1990s showed that the roads that run parallel and close to the entire island’s shoreline take a high toll on Lake Erie Watersnakes (Willson et al. 2002). In 1993 and 1994 combined a total of 81 Lake Erie Watersnakes were recorded dead on the road. In 1998 and 1999, only 22 were found dead despite increased traffic, which may indicate that numbers of snakes had declined.
Persecution
The most significant factor in watersnake population decline may still be persecution by people (USFWS 2003). In the past, people have attempted to rid their islands of watersnakes through "campaigns of extermination" (King 1998; USFWS 2003). Since the Lake Erie Watersnake’s designation by the Fish and Wildlife Service (1999) and the Ontario Endangered Species Act (1977), watersnakes have been protected by legislation from persecution in some areas. However, such designations have been known to reduce private landowner cooperation (USFWS 1999) and some researchers have experienced a lack of cooperation and have been denied access onto certain properties (King 2003; D. Jacobs pers. comm. November 2004; R. Brooks pers. comm. May 2005).
Contamination
The threat of contamination in Lake Erie Watersnakes has increased with the dietary switch to round gobies as their primary food source. Gobies feed on Zebra and Quagga mussels, both of which are filter feeders, having the potential to bioaccumulate toxins. Hence, there may be a greater likelihood of bioaccumulation of toxins in present watersnake populations than in the past (King 2004a). Comparative analyses of blood samples taken from Lake Erie Watersnakes (pre- and post-goby invasion) are currently underway in order to establish levels of PCBs and related toxic compounds (King 2004a), though preliminary analyses suggest that contaminants in Lake Erie Watersnakes have not increased as a result of consuming round gobies (USFWS 2005).
Predators
Certain predators of watersnakes, such as raccoons and foxes, thrive close to human activity. Additionally, domestic and feral cats and dogs frequently kill watersnakes (USFWS 2003). Furthermore, some of the western Lake Erie islands are now occupied by large colonies of Double-crested Cormorants and there is evidence of mortality caused by these birds (D. Jacobs pers. comm. November 2004).