Beyond the wetland border: Estimating the impact of roads for two species of water snakes
Introduction
The growing demands upon the landscape for development, as well as the management of aquatic resources to support human activities have resulted in the decline, disappearance, or alteration of freshwater wetlands and associated faunal communities worldwide (Richter et al., 1997, Finlayson and Rea, 1999). In response to such declines, conservationists have been challenged with developing habitat protection guidelines for wetlands, but when protection is provided (e.g., as a local or state government ordinance), it is often too narrowly focused on the wetland itself such that the functional linkages among wetlands and between wetlands and the surrounding terrestrial habitats are neglected (Amezaga et al., 2002, Gibbons, 2003). Such failures in considering areas extending beyond the delineated wetland border in conservation plans are a concern, as wetland-obligate animals dependent on terrestrial habitats, or those that regularly move about the wider landscape, may not be afforded the necessary level of protection.
Reptiles are a group for which many species have a complex interaction with both aquatic and terrestrial habitats. In addition to their dependence on aquatic habitats, many wetland reptiles require movement through, or extended refuge within terrestrial habitats for nesting, aestivation, hibernation, migration, dispersal, foraging, or other critical behaviors (Burke and Gibbons, 1995, Buhlmann and Gibbons, 2001, Gibbons, 2003, Roe et al., 2003, Semlitsch and Bodie, 2003, Roe et al., 2004). As reptiles are confined to moving through (as opposed to flying over) terrestrial habitats, they are likely to encounter considerable challenges and risks associated with movements out of the wetland, especially in areas heavily modified by humans. Consequently, in addition to threats imposed by wetland destruction, many wetland reptiles may also be sensitive to alteration of nearby terrestrial habitats.
One of the most significant anthropogenic modifications of terrestrial habitats in the past century is the network of roads that has become a pervasive component of landscapes worldwide (Trombulack and Frissell, 2000). For instance, over six million kilometers of roads traverse the United States, where the impacts of roads are estimated to extend to one-fifth of the area (Foreman and Alexander, 1998, Foreman, 2000). What is most concerning is that road densities, and especially the traffic volumes on them, continue to expand (Foreman and Alexander, 1998). Roads have been implicated in increasing mortality and restricting movements in populations of terrestrial (Rosen and Lowe, 1994, Guyot and Clobert, 1997, Hoff and Marlow, 2002) and aquatic (Bernardino and Dalrymple, 1992, Ashley and Robinson, 1996, Haxton, 2000) reptiles at both local and regional scales (Gibbs and Shriver, 2002). While evidence suggests that roads may represent a growing threat to reptiles, such information often goes unrecognized by wetland managers and policy makers, despite the possibility that roads can potentially act as demographic sinks or isolating barriers to wetland reptiles and other fauna that require movement through terrestrial habitats.
Little is known regarding the consequences of human landscape modifications for snake communities (Kjoss and Litvaitis, 2001), especially for aquatic snakes. Here, we investigate the potential for terrestrial habitat modification to impact two species of water snakes, the northern water snake (Nerodia sipedon) and the imperiled copperbelly water snake (Nerodia erythrogaster neglecta, a subspecies of the plainbelly water snake), by simulating movements on maps of wetlands and road networks to estimate road-associated mortality. The northernmost populations of N. e. neglecta are federally threatened and endangered in Ohio, Michigan, and northern Indiana (Pruitt and Szymanski, 1997), and the southernmost populations are protected by the Copperbelly Water Snake Conservation Agreement in Kentucky, Illinois, and southern Indiana (Garrison, 1996). While N. e. neglecta requires conservation action throughout its range, N. sipedon remains widespread and abundant in this region. Radio-telemetric investigations have revealed N. e. neglecta to be more vagile, use larger areas, and make more frequent use of terrestrial habitats compared to N. sipedon (Roe et al., 2003, Roe et al., 2004), all of which are traits that may influence encounters with roads. Consequently, we predict road mortality to occur more frequently in N. e. neglecta than N. sipedon, potentially at levels that have contributed to historic N. e. neglecta declines and that may represent a current impediment to their persistence and recovery.
Section snippets
Methods
We estimated annual mortality resulting from vehicular collisions using the following equation developed by Hels and Buchwald (2001) and modified by Gibbs and Shriver, 2002, Gibbs and Shriver, 2005:where Pkilled represents the probability of being killed by a vehicle during a road crossing, and n crossings is the predicted number of times a typical individual crosses roads during its annual movements.
To estimate n crossings, we simulated movements of N.
Results
The mean number of predicted road crossings stabilized around a constant value after approximately 70 simulations (Fig. 2). In the NE region, mean predicted road crossing frequency was higher for N. e. neglecta (6.4 ± 0.8) than for N. sipedon (1.5 ± 0.3; Kruskal–Wallis: X2 = 33.27, df = 1, p < 0.001). Movements among wetlands accounted for the majority of predicted road crossings (91% for N. e. neglecta, 94% for N. sipedon), while few snakes are predicted to cross roads during movements to and from
Discussion
Our models support the hypothesis that roads may be a more significant source of mortality for N. e. neglecta than for N. sipedon. Heavily trafficked roads (>1000 vehicles/lane/day) are predicted to inflict mortality rates greater than 60% for N. e. neglecta, while average road mortality rates in the three regions (142–235 vehicles/lane/day) are predicted to be between 14% and 21% for N. e. neglecta (Table 1, Fig. 4). In contrast, N. sipedon is predicted to have average road mortality rates of
Acknowledgments
We thank N. Herbert, A. Barun, and Mark Myers for assistance with this project. This work was supported by the University of Canberra’s Vice Chancellor’s Postgraduate Research Scholarship and the W.J. Weeden award to J.H.R., and Grants from the United States Fish and Wildlife Service and the Indiana and Ohio Departments of Natural Resources to B.A.K.
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