Gray ratsnake (Elaphe spiloides) COSEWIC assessment and status report: chapter 9
Population Sizes and Trends
There have been approximately 650 sightings of Gray Ratsnakes reported to the Ontario Herpetofaunal Summary since 1905 (Oldham and Weller, 2000). A majority (430) of the records are from the Great Lakes/St. Lawrence population, while the rest are spread across the 4 southwestern populations. The sightings are reported by researchers, naturalists and wildlife managers and do not represent a systematic quantification of the presence or absence of ratsnakes. There have been no attempts to accurately quantify the distribution of ratsnakes on the Frontenac Axis or in the Carolinian region.
Research and monitoring efforts across the Frontenac Axis have identified hibernacula and directly established the presence of populations in and around the Queen’s University Biological Station (QUBS), St. Lawrence Islands National Park, Murphy’s Point Provincial Park, Charleston Lake Provincial Park and Frontenac Provincial Park. Currently, population monitoring efforts are being continued at QUBS, St. Lawrence Islands National Park and Murphy’s Point Provincial Park.
There has been virtually no research conducted on the Carolinian populations and most of the information on the distribution is from NHIC records and anecdotal information. From 2001 – 2003 two individuals were radio tracked from the Oriskany population. During this study some effort was expended to search for individuals and to enclose and monitor the hibernaculum used by the two radio-tracked individuals. Aside from 1 road-killed individual in 2006, no other individuals were captured during this study (Yagi and Tervo, 2006).
Great Lakes/St. Lawrence Population
On the Frontenac Axis, Blouin-Demers and Weatherhead (2002a) estimated the density of ratsnakes in the QUBS study area to be 0.261 mature adults/hectare. The habitat quality is not uniform across the Frontenac Axis, however, and it is likely that the density is also variable. The quality of habitat across the Frontenac Axis was ranked using habitat suitability predictors, road density and patch size (See Habitat – Habitat Requirements) (Figure 3) and the quality of habitat at QUBS was relatively high (mean = 0.70 in a scale between 0 and 1). To estimate the abundance of the entire Great Lakes/St. Lawrence population, the area of land with similar habitat quality to QUBS (rank of > 0.70) was multiplied by the QUBS density, which gave an estimate of 25 000 adults. Because ratsnakes likely also occur outside these high quality habitat areas, this was considered to be a lower estimate. The upper estimate was determined by multiplying the total extent of occurrence by the density, which gave an abundance of 85 000 adults. These are very rough estimates of abundance and because there have been no efforts to systematically and accurately quantify the extent of Gray Ratsnakes within this population, it would be difficult to make a more accurate estimate at this time. There are no estimates of density or habitat use patterns for any of the Carolinian populations, making estimates of abundance impossible.
Great Lakes/St. Lawrence Population
No demographic data exist to allow for an estimation of population trends for the entire population. Weatherhead et al. (2002), however, examined the population trends from long-term (1981-1998) monitoring programs at 4 hibernacula in 2 subpopulations (QUBS and Hill Island in the St. Lawrence River). All of these hibernacula were located in protected areas. Over the study, the overall population size at both QUBS (slope of the regression of the log of population size on year = -0.013, p = 0.05) and Hill Island (slope of the regression of the log of population size on year = -0.009, p = 0.34) showed a slight decrease. This decrease, however, was only significant at QUBS (Weatherhead et al. 2002). At QUBS, the negative population growth was attributed to a declining recruitment rate (slope of the regression of the log of recruitment rate on year = -0.27, p = 0.09), which also caused a shift in the age structure towards more mature individuals. Population monitoring at more locations and for longer time periods is required to gain more insight into the factors behind this apparent decline (anthropogenic factors vs. natural fluctuations) and also to determine if this trend is consistent across the entire Great Lakes/St. Lawrence population.
Carolinian Population
The lack of current or past demographic data from these populations makes it impossible to accurately estimate population trends. Because 80 - 95% of the forest cover has been removed from this region since European settlement (see Habitat – Habitat Trends), it is inevitable that populations of Gray Ratsnakes have been drastically reduced in this region.
Gray Ratsnakes are widespread and common throughout the eastern-central United States. Both the Great Lakes/St.Lawrence and Carolinian populations, however, are genetically distinct and geographically disjunct from continuous portions of the species’ range in the United States. A small portion of the Great Lakes/St. Lawrence population ranges into upper New York State and it is likely that these populations exchange individuals, but the majority of this population resides within Canada. Rescue is unlikely because of the St. Lawrence River and highway 401, which are major barriers.
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