Gray ratsnake (Elaphe spiloides) COSEWIC assessment and status report: chapter 7
Habitat
Active Season
The Gray Ratsnake is semi-arboreal and typically associated with a wide range of woodland and scrub habitats across its distribution (Ernst and Barbour, 1989; Durner and Gates, 1993).Within Canada, Gray Ratsnakes inhabit two regions in Ontario with significantly different habitats. The Frontenac Axis is dominated by rolling terrain of mature, second-growth deciduous forest, intermixed with numerous lakes, wetlands, abandoned agricultural fields and bedrock outcrops (Beschel, 1962), whereas the Carolinian forest region is dominated by agricultural land mixed with smaller patches of open deciduous forest (Kelly, 1990) and a dense network of roads.
On the Frontenac Axis, detailed assessments of habitat use have been undertaken at two different scales (home-range scale; location scale) and for four reproductive classes (males; non-gravid females; gravid females; juveniles), using data collected from a long-term radio-telemetry study (1996 – 2004) conducted at the Queen’s University Biology Station (QUBS). Because of the large differences in habitat availability between the two regions, it is unlikely that habitat patterns observed on the Frontenac Axis can be directly extrapolated to the populations in the Carolinian forest region. No assessments of habitat use have been conducted on individuals from any Carolinian populations and, therefore, the patterns discussed below relate to the Great Lakes/St. Lawrence population (Frontenac Axis).
Large-scale assessments of habitat use have revealed that ratsnakes use habitat non-randomly at the home-range scale, and that there is no significant difference in habitat use between any of the reproductive classes. All reproductive classes preferred home ranges containing >28% edge habitat (10 m buffer on either side of forest) and modest amounts of forest cover (41% - 53%), and avoided home ranges with >17% marsh habitat (Row, 2006). At the location scale, ratsnakes again used habitat non-randomly, but males and non-gravid females demonstrated habitat use patterns that significantly differed from gravid females (Blouin-Demers and Weatherhead, 2001a). At this scale, all adult reproductive groups preferred locations close to trees and edges, with a high ground cover of logs. Gravid females were distinguished from the other two reproductive classes by preferring locations closer to large trees and further from rocks and smaller trees (Blouin-Demers and Weatherhead, 2001a). Based on these results, ratsnakes seem to require a mosaic of forest and open habitat with a high edge to area ratio. These results are consistent with radio-telemetry studies conducted in other portions of ratsnake’s range (Durner and Gates, 1993).These habitat preferences were combined with road density and patch size estimates to rank (between 0 – 1) the suitability of habitat within 500 ha grid cells, overlaid across the Frontenac Axis (Row, 2006; Fig. 3). Although no extensive efforts have been made to systematically quantify ratsnake distribution across this area, higher-ranking cells correlated well with recent occurrence records.
Figure 3: Rank of habitat inside 500 ha grid squares overlaid across the Frontenac Axis
Habitat was ranked from least (0) to most (1) suitable.
Throughout the active season, the location of appropriate basking and retreat sites is necessary for thermoregulation, ecdysis, and predator avoidance. Retreat sites and basking sites are often used multiple times by an individual throughout the active season, and over multiple years. Retreat sites that are commonly used on the Frontenac Axis are the inside of hollow logs and trees, under rocks or in rock crevices; individuals were concealed in retreat sites in approximately 65% of radio telemetry locations (Blouin-Demers and Weatherhead, 2001a). Communal use of shedding sites is frequently observed and the same individuals will often show a high fidelity to these sites (Blouin-Demers and Weatherhead, 2001a). Snakes have impaired vision and maintain body temperatures closer to their preferred range prior to ecdysis (Gibson et al. 1989; Blouin-Demers and Weatherhead, 2001b), therefore, these locations must provide both higher thermal quality and increased protection from predators. The aggregation observed at these sites may be due to a limited number of suitable locations. On the Frontenac Axis, ratsnakes will typically use standing hollow snags as shedding sites (43% of the time), but have also been observed to use old buildings (20%), rock crevices (16%), hay piles (10%), and hollow logs (8%; Blouin-Demers and Weatherhead 2001a). These shedding sites are also more likely to be located in edges, most likely because of the higher thermal quality of this habitat type.
Throughout most of their range, Gray Ratsnakes hibernate during the winter months. In the more southern populations, hibernation only lasts a few months (November – February), but in the Canadian portion of their range, winters are much longer and ratsnakes typically hibernate for 6 months of the year (late October – early mid-April) (Weatherhead, 1989) making hibernation an important aspect of their ecology. Due to the extreme temperatures and long periods of inactivity, mortality can easily occur during hibernation or shortly after emergence if suitable hibernacula are not found (Prior and Shilton, 1996).
On the Frontenac Axis, 10 – 60 individuals typically aggregate for hibernation (Blouin-Demers et al. 2000) and individuals generally show strong fidelity to their hibernacula (98%; Weatherhead and Hoysak, 1989; Prior et al. 2001). Prior and Weatherhead (1996) measured a number of habitat variables at 10 communal hibernacula and paired random locations throughout the Frontenac Axis. Their analysis suggested that hibernacula were generally located on relatively rocky, south-facing slopes and could be distinguished from paired random locations based on these characteristics. They could not, however, be distinguished from potential hibernacula, intuitively identified by the researchers. There was also large variation in the habitat characteristics for the hibernacula, and several new hibernacula located since this study was conducted suggest that these trends do not extend to all hibernacula. It is more likely that subterranean structural features are more important than above-ground features in providing appropriate hibernacula. Although, subterranean features have not been quantified, ratsnakes are not freeze tolerant and, therefore, hibernacula must be below the frost line. The body temperatures of hibernating ratsnakes on the Frontenac Axis ranged from 3 to 7ºC (Weatherhead, 1989). Hibernacula would also have to be sufficiently humid to protect against dehydration (Costanzo, 1989).
Oviposition Sites
Incubation conditions can affect the phenotype (e.g. speed, aggressiveness, size) of offspring (Qualls and Andrews, 1999; Brana and Ji, 2000) in ways likely to affect their fitness. On the Frontenac Axis, Gray Ratsnakes have been observed to oviposit in the decaying matter inside standing snags, stumps or logs and also in compost piles (pers. obs.). In a nesting thermal gradient, females selected nest temperatures around 31ºC. Also, eggs incubated at 30ºC produced offspring more fit (e.g. faster, larger) than eggs incubated at 25ºC (Blouin-Demers et al. 2004). Both of these results suggest that the most ideal temperature for egg incubation is approximately 30ºC. Nests on the Frontenac Axis are often communal and are used for multiple years by multiple females (Blouin-Demers et al. 2004). Although some females will nest singularly, the thermal conditions in non-communal nests are generally not as ideal (Blouin-Demers et al. 2004). In non-communal nests, however, there has been no evidence of the nest parasite Nicrophorus pustulatus (see Biology –Interspecific Interactions) and the choice between communal and non-communal nests may represent a trade-off between superior thermal conditions and risk of parasitism (Blouin-Demers et al. 2004). Communal nesting may also result from a shortage of suitable nesting sites. The availability of appropriate nesting conditions is essential to the viability of ratsnake populations, and at the Queen’s University Biological Station (QUBS) efforts have been made to provide a number of ‘predator safe’ nests with little success in achieving the appropriate nest conditions or attracting females (Row, pers. obs.). OMNR and Leeds Stewardship have constructed and monitored nests in several areas in southeastern Ontario with slight success (9/13 eggs hatched from one clutch laid in an artificial nest box in 2005) (S.Thompson, pers. comm. 2007).
The marginal agricultural conditions on the Frontenac Axis have led to the abandonment of farmland over the last 60 years and allowed for large tracts of suitable habitat to remain (McKenzie, 1967). Suitable habitat mapped across the range of the Great Lakes/St. Lawrence population (Fig. 3), however, suggests that in the southern and northwestern portions of the range of this population the habitat is much less suitable. In these regions, cropland is more extensive and road density is much greater resulting in lowered suitability. Because of the poor agricultural quality of the remaining suitable habitat, further agricultural clearing is unlikely. The increasing recreational activity in the heart of the Rideau Canal (Prior and Weatherhead, 1996), however, is likely to increase development pressure and concomitant reduction and fragmentation of the remaining suitable habitat.
In contrast to the Frontenac Axis, more than 80% of the original forest cover in the Carolinian region has been removed (Kelly, 1990). Intensive agriculture and an extensive network of roads dominate the Carolinian landscape. These changes have resulted in major range reductions for a number of reptiles and amphibians in the area, including Gray Ratsnakes, presumably by causing a drastic reduction in the amount of suitable habitat for ratsnakes in this region. It is currently unknown whether the remaining habitat is sufficient to support viable ratsnake populations. The area surrounding the Big Creek population has some of the largest tracts of Carolinian forest remaining in Ontario, which is likely why this area seems to support the largest populations of Gray Ratsnakes in southwestern Ontario (see Distribution – Carolinian).
On the Frontenac Axis, the Great Lakes/St. Lawrence ratsnakes occur within numerous protected areas such as Murphy’s Point Provincial Park (13 km2), Frontenac Provincial Park (~50 km2), Charleston Lake Provincial Park (~25 km2), St. Lawrence Islands National Park (~24 km2) and the Queen’s University Biological Station (~30 km2). In addition, there are Gray Ratsnakes in some of Parks Canada’s Rideau Canal properties, and the Rideau Valley C. A. has ~700ha under protection that include records of Gray Ratsnakes. The Cataraqui C. A. has 400ha within the ratsnake’s range (S. Thompson, pers. comm. 2007). In total, these areas only protect approximately 4% of the 4000 km2 extent of occurrence on the Frontenac Axis. All of these protected areas are isolated from each other and the largest continuous tract of land (Frontenac Park) is approximately 50 km2.
The distribution of the Carolinian populations is less clearly defined and therefore, the amount of protection for these populations is more difficult to quantify. However, in Haldimand-Norfolk, numerous small tracts of land that may contain ratsnakes have been purchased and protected through the Nature Conservancy of Canada and local conservation groups (M. Gartshore, pers. comm. 2006).
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