River valleys and snake genetics

Pituophis catenifer sayi

The genetic population structure of snakes can vary markedly based on a number of intrinsic and extrinsic factors. Some snake species show only very modest levels of subdivision or none at all, while others show a high degree of differentiation over small spatial scales. The variation among species is likely due to a wide variety of biological traits (e.g. natal philopatry, home range size, specific habitat requirements), as well as the ability of individuals to disperse. Dispersal and associated gene flow among breeding groups may also be influenced by extrinsic factors such as natural or man-made barriers.  It is only recently that snake populations at range peripheries or in extreme environments have become the focus of conservation genetics studies. Conservation challenges may be exacerbated for populations at range peripheries, where extreme environments and naturally sparse distributions interact with anthropogenic activities to generate additional risk factors.

Snake populations at the northern limits of their range face ecological challenges that may affect dispersal, gene flow, and ultimately genetic population structure. For example, snakes at higher latitudes in North America and Eurasia rely heavily on a limited number of suitable hibernacula to survive harsh winters, and they often exhibit high fidelity to these sites (e.g. Elaphe obsoleta obsoleta, Gloydius halys). In addition, landscapes containing both suitable hibernacula and summer habitat for northern snakes may be patchily distributed at range margins, a situation that is exacerbated by human activities that cause habitat loss and fragmentation. Thus, gene flow may only occur among northern snake populations when individuals travel long distances away from dens to breed; however, successful dispersal may be uncommon, resulting in highly subdivided populations. Interestingly, several recent studies have shown that some snake species have much larger home ranges and travel long distances from hibernacula at northern range limits. In principle, these behavioral traits may partially counteract the barriers to gene flow discussed above. Understanding this situation is of key interest in Canada, where a variety of North American snakes, some of which are of conservation concern, reach their northern range limits.

On the North American Great Plains, several snake species reach their northern range limit where they rely on sparsely distributed hibernacula located in major river valleys. Independent colonization histories for the river valleys and barriers to gene flow caused by the lack of suitable habitat between them may have produced genetically differentiated snake populations. In a recent paper Somers et al (2017) test this hypothesis, using 10 microsatellite loci to examine the population structure of two species of conservation concern in Canada: the eastern yellow-bellied racer (Coluber constrictor flaviventris) and bullsnake (Pituophis catenifer sayi) in 3 major river valleys in southern Saskatchewan. Fixation indices (FST) showed that populations in river valleys were significantly differentiated for both species (racers, FST = 0.096, P = 0.001; bullsnakes FST = 0.045–0.157, P = 0.001). Bayesian assignment (STRUCTURE) and ordination (DAPC) strongly supported genetically differentiated groups in the geographically distinct river valleys. Finer-scale subdivision of populations within river valleys was not apparent based on their data, but is a topic that should be investigated further. The findings highlight the importance of major river valleys for snakes at the northern extent of their ranges and raise the possibility that populations in each river valley may warrant separate management strategies.

Somers CM, Graham CF, Martino JA, Frasier TR, Lance SL, Gardiner LE, et al. (2017) Conservation genetics of the eastern yellow-bellied racer (Coluber constrictor flaviventris) and bullsnake (Pituophis catenifer sayi): River valleys are critical features for snakes at northern range limits. PLoS ONE12(11): e0187322. https://doi.org/10.1371/journal.pone.0187322

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