Long-term population estimates and synchronous variation in two populations of black rat snakes (Elaphe obsoleta) in Eastern Ontario

Synchronous variation describes the pattern that occurs when variables, such as population size, from several separate populations increase or decrease together. It is common in animal populations and is relevant when studying species of conservation interest because synchrony prevents population rescue. We used 23 years of mark-recapture data to test for the presence of synchronous variation in population size for two separate populations of the threatened black rat snake (Elaphe obsoleta) in Eastern Ontario. The Jolly-Seber method was used to estimate population size. The population sizes tended to vary synchronously from 1982 to 2005. Synchronous variation in population size can be attributed to the Moran effect, where two separate populations become synchronous due to common environmental perturbations. Evidence of synchrony has serious conservation implications; this population of black rat snakes is more vulnerable to environmental perturbations because all subpopulations would become rare simultaneously. Introduction Long-term population monitoring plays an essential role in conservation. Examining population trends is the only means to assess species status and to evaluate the efficacy of management actions. Synchronous variation is common in animals (Ranta et al. 1995; Hudson and Cattadori 1999). Subpopulations that vary asynchronously can rescue neighbouring populations through dispersal (Koenig, 1999), lessening the risk of local extinctions. In contrast, subpopulations that vary synchronously may cause the entire population to be more susceptible to extinction. We used 23 years of mark-recapture data to test for population size synchrony among two genetically distinct populations of the threatened black rat snake (Elaphe obsoleta) in Eastern Ontario, thus extending the work of Weatherhead et al. (2002). The black rat snake is limited in Canada to a few disjunct populations along the north shore of Lake Erie and one continuous population on the Frontenac Axis of Eastern Ontario.