Lack of detectable genetic differentiation between den populations of the Prairie Rattlesnake (Crotalusviridis) in a fragmented landscape

Abstract

Numerous studies have reported genetic fragmentation of species whose habitat has been modified by roads and other anthropogenic features, but it is still not clear how most species respond to roads and whether genetic effects can be detected over a limited number of generations. We used road-crossing models and population genetic analysis (based on microsatellite loci) to make inferences about functional connectivity between populations of the Prairie Rattlesnake (Crotalus viridis (Rafinesque, 1818)) on opposite sides of the Trans-Canada Highway near Medicine Hat (Alberta, Canada). The road-crossing model predicted a high probability of mortality while crossing the Trans-Canada Highway. However, model-based genetic clustering methods (STRUCTURE and BAPS) did not detect structure; a nonmodel-based clustering method (DAPC) found structure, but most groups consisted of individuals captured throughout the study area. Estimates of effective population size were immeasurably large and power to detect genic differentiation was diminished if the effective size exceeded 500; this reduction in power was intensified when the number of loci was reduced (from eight to five to account for null alleles). Our results corroborate accounts of long-distance migration by this species and indicate that genetic fragmentation may not be easily detectable over this spatial and temporal scale. Résumé :Si de nombreuses études ont fait état de la fragmentation génétique d'espèces dont l'habitat a été modifié par des routes ou d'autres éléments d'origine humaine, la réaction de la plupart des espèces à la présence de routes n'est pas bien circonscrite, ni la question à savoir si ses effets génétiques peuvent être détectés sur un nombre restreint de générations. Nous avons utilisé des modèles de franchissement de route et l'analyse génétique des populations (basée sur les marqueurs microsatellites) pour établir des inférences concernant la connectivité fonctionnelle de populations de crotales de l'ouest (Crotalus viridis (Rafinesque, 1818)) de part et d'autre de la route Transcanadienne près de Medicine Hat (Alberta, Canada). Le modèle de franchissement de route prédisait une probabilité élevée de mortalité durant le franchissement de la Transcanadienne. Toutefois, des méthodes de regroupement génétique basées sur des modèles (STRUCTURE et BAPS) n'ont détecté aucune structure; une méthode de regroupement non basée sur un modèle (DAPC) a décelé une structure, mais la plupart des groupes étaient composés d'individus capturés dans toute la région à l'étude. Les estimations de la taille effective des populations étaient trop grandes pour être mesurées et la puissance de détection d'une éventuelle différentiation génétique se trouvait réduite si la taille effective dépassait 500. Cette réduction de la puissance était amplifiée quand le nombre de marqueurs microsatellites était réduit (de huit à cinq pour tenir compte d'allèles nuls). Nos résultats corroborent les cas signalés de migrations sur de grandes distances par cette espèce et indiquent q...

A spatial genomic approach identifies time lags and historical barriers to gene flow in a rapidly fragmenting Appalachian landscape

“…Our results further emphasize an association between high-traffic roads and genetic differentiation in pit vipers (Clark et al, 2010, DiLeo, Row, & Lougheed, 2010, DiLeo et al, 2013, Bushar et al, 2015, Herrmann, Pozarowski, Ochoa, & Schuett, 2017; but see Weyer, Jørgensen, Schmitt, Maxwell, & Anderson, 2014). These findings are in addition to field studies that have suggested the outsized role played by road mortality in snakes, and herpetofauna more generally (Andrews & Gibbons, 2005;Row, Blouin-Demers, & Weatherhead, 2007;Shepard et al, 2008).…”

Landscape genetics in a changing world: disentangling historical and contemporary influences and inferring change

“…The early emergence of this pattern may reflect the strong interest of landscape geneticists in movement and dispersal, and a greater tendency for authors to report or comment on species' dispersal behaviour. Some studies have associated large population size, either historical or contemporary, with longer time lags (Ramirez- Barahona & Eguiarte 2014;Weyer et al 2014). For example, large effective population size was associated with an inability to detect genetic effects of a recent road barrier in the prairie rattlesnake (Crotalus viridus), even though an ecological model predicted high road mortality (Weyer et al 2014).…”

“…Some studies have associated large population size, either historical or contemporary, with longer time lags (Ramirez- Barahona & Eguiarte 2014;Weyer et al 2014). For example, large effective population size was associated with an inability to detect genetic effects of a recent road barrier in the prairie rattlesnake (Crotalus viridus), even though an ecological model predicted high road mortality (Weyer et al 2014). Zellmer & Knowles (2009) explained the lack of a detectable time lag in the wood frog by the occurrence of extinction-colonization dynamics.…”

Costs and benefits of straight versus tortuous migration paths for Prairie Rattlesnakes (Crotalus viridis viridis) in seminatural and human-dominated landscapes

“…It was surprising that road mortality was not more prevalent in our study, given that roads are a significant source of snake mortality (Bonnet et al 1999;Row et al 2007;Quintero-Ángel et al 2012;Weyer et al 2014). The low road mortality rates we observed may, at least in part, be explained by the observed differences in migration behaviour in the two study landscapes.…”

“…This is because, all else being equal, a snake that makes more tortuous movements is less likely to encounter a road than a snake that makes straighter movements. In support of this, Weyer et al (2014) found eight times more prairie rattlesnakes on a road near the hibernacula in the semi-natural landscape than on the road D r a f t 14 near the hibernacula in the human-dominated landscape. Actual road mortality rates relative to other sources of mortality (e.g.…”