Gray ratsnake (Elaphe spiloides) COSEWIC assessment and status report: chapter 10
Limiting Factors and Threats
Limiting Factors
A number of intrinsic life-history characteristics make Gray Ratsnakes particularly vulnerable to disturbances. Biennial reproduction, delayed age of maturity (~7 years), and slow growth rates can cause seemingly small increases in mortality to have significant population level impacts (Brooks et al. 1991; Congdon et al. 1993). Furthermore, suitable habitat for Gray Ratsnakes consists of a mosaic of forested and open habitats and large areas of suitable habitat are required to support viable populations. Individuals can travel at least 4 km from their hibernacula. Within Ontario, the Great Lakes/St. Lawrence and Carolinian populations face similar threats, but with different degrees of imminence and importance, and are discussed separately below.
Threats to the Great Lakes/St. Lawrence Population
This population has a relatively small extent of occurrence (~4000 km2), but there are no hard data defining the extent of existing populations within this area. The distribution of suitable habitat (Fig. 3) across the Frontenac Axis (see Habitat – Habitat requirements) suggests that although there are still large tracts of continuous suitable habitat, the total amount is much less than 4000 km2 and becoming increasingly fragmented. Recreational activities have been increasing in the heart of the Rideau Canal (Prior and Weatherhead, 1996) leading to much more development in the area (S. Thompson pers. comm. 2006). To maintain viable populations, continuous tracts of suitable habitat must be preserved to maintain connectivity between existing populations. The expansion of the road network, especially major paved roads like highway 401, and the loss of continuity between habitat patches has made the Great Lakes/St. Lawrence population increasingly fragmented or even severely fragmented in the case of populations away from the centre of the Frontenac Axis range (see Fig. 3). Because ratsnakes hibernate in communal hibernacula, development can also be particularly detrimental to populations when existing hibernacula are eliminated.
Another significant threat facing all populations of Gray Ratsnakes is road mortality. Previous studies have documented the negative effects roads can have on reptile populations, either through direct mortality (Rodda 1990, Ashley and Robinson 1996) or by fragmenting populations (Shine et al. 2004, Andrews and Gibbons 2005). Gray Ratsnakes have large home ranges and dispersal distances which allows local subpopulations to interchange individuals and genes, but also makes this species vulnerable to proliferating road networks, which in turn increases fragmentation of populations and habitat. Although no quantitative data exist on the direct effect of road mortality, across the Frontenac Axis, a mean of 6 dead ratsnakes (Range = 1 – 9) are found each year on a 10 km stretch of road inside the QUBS study area (Row et al. 2007). Similarly, at Murphy’s Point PP, 16 dead ratsnakes were found on 18 km of road over 10 y, and in Clara Lake PP there were 8 dead over 7 km in 2 y (S. Thompson, pers. comm. 2007).These rates likely represent a significant mortality for ratsnake populations given their life history. Indeed, a recent Population Viability Analysis (PVA) conducted on the study population in the protected QUBS study area within ideal habitat in the Frontenac Axis indicated that observed rates of adult mortality (estimated at 9 adults per year) on local (gravel) roads increased the probability of extinction from 7.3% to 99% in 500 years. Only as few as three adult females being killed on the roads each year raised extinction probability to > 90% per 500 years (Row et al. 2007). Many areas on the Frontenac Axis have a much higher road density and speed limits and poorer habitat quality than do these protected areas and hence their ratsnake populations should suffer even greater risk of extinction from road mortality.
Carolinian Population
Although no studies have quantified the patterns of habitat use by Gray Ratsnakes in the Carolinian region, studies in a variety of habitat types and regions have all shown that ratsnakes will rarely be found far from forest habitat, and greatly prefer a habitat mosaic of forest and open habitats (Fitch, 1963; Durner and Gates, 1993, Blouin-Demers and Weatherhead, 2001a). Across the Carolinian region, the landscape is dominated by intensive agriculture and a dense network of roads; any remaining suitable habitat is limited and severely fragmented. This lack of suitable habitat is reflected in the small and extremely fragmented populations of ratsnakes remaining in the Carolinian region. It is unknown if the remaining suitable habitat is sufficient for the long-term survival of any of the existing populations.
Population viability analysis on the Great Lakes/St. Lawrence population suggests that a population of at least 141 mature individuals (network of ~8 hibernacula) was needed to support a viable population (Tews, 2005). On the Frontenac Axis, this translated to an area of at least 540 ha of continuous suitable habitat. In the Carolinian region, density is likely much lower and mortality is likely higher (higher road density), and therefore a forested area much larger than 540 ha of continuous suitable habitat will be needed to support a viable population. The persistence of the remaining Carolinian populations is likely jeopardized by their small size and isolation and by the reduced, small areas of suitable habitat.
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