Urbanization is an increasingly pervasive form of land use, which drastically alters ecological communities (Blair 1996; McDonnell et al. 1997; Shochat et al. 2006; McKinney 2008). However, urbanized areas support complex ecological communities often including large numbers of non-native species, with heavily fragmented natural habitats within business/residential areas that vary considerably in their quality, structure, and function. Studies of urbanized systems have garnered attention in recent decades, shedding light on how organisms respond to anthropogenic habitat modification. Yet despite this increased attention, bias persists in the study of urban ecosystems, with vertebrate response to urbanization largely focused on birds and mammals (Shwartz et al. 2014; LaPoint et al. 2015).
Herpetofauna often contributes substantially to vertebrate biomass in terrestrial and semi-aquatic systems, and serve as important environmental indicators due to their often intermediate trophic position, sensitivity to environmental conditions, and formation of linkages between aquatic and terrestrial systems. As such, they have been used as model organisms to address a range of ecological hypotheses (e.g. Pianka 1973; Inger and Colwell 1977; Godley 1980; Fauth et. al 1989; Bell and Donnelly 2006). Within urban systems, herpetofauna exhibit broadly similar responses to urbanization as other vertebrate groups, with many species responding poorly, and a notable few thriving (Germaine and Wakeling 2001; McKinney 2008; Hamer and McDonnell 2010; Ljustina and Barrett 2018; but see Ackley et al. 2015; Winchell et al. 2018). Still, studies examining herpetofauna in these contexts remain few compared to other vertebrate groups (Brum et. al 2022). Efforts to remedy this disparity are particularly important as herpetofaunas have suffered substantial biodiversity loss, with habitat modification in particular suggested as a major contributor to these perceived declines (Alford and Richards 1999; Gibbons et al. 2000; Semlitsch 2000; Blaustein and Kiesecker 2002; Collins and Storfer 2003; Beebee and Griffiths 2005; Böhm et al. 2013). Within this understudied group, studies examining snakes are particularly lacking.
Though abundant in many ecosystems, snakes are often difficult to study because of their secretive behaviors and low detection probability. Despite these characteristics, some studies have successfully examined snakes in urbanized settings, where again, they generally follow broad, well described trends in vertebrate response to urbanization, with most species responding negatively to urbanization (McKinney 2008; Todd et al. 2016; Sullivan et al. 2017; Todd et al. 2017). Still, numerous snake species continue to persist within urbanized settings, though the precise manner in which they utilize urbanized environments is poorly understood.
Studies examining how snakes live within urbanized systems have often focused on remnant habitat fragments embedded within urbanized matrices, and have described assemblage compositions, demographics, movement ecology, and population genetics (e.g. Kjoss and Litvaitis 2001; Enge et al. 2004; Vignoli et al. 2009; Gangloff et al. 2017; Carrasco-Harris et al. 2020). Other studies examining snakes within what may be perhaps considered the more human-dominated portions of urban landscapes, have reported little impact on demographic structure in small fossorial species, occasional high densities, and reduced movement and foraging efforts in association with artificial habitat structures (Gaul 2008; Zappalorti and Mitchell 2008; Pattishall and Cundal 2008; Pattishall and Cundal 2009; Ackley and Meylan 2010; Cundal and Pattishall 2011; Dick and Mebert 2017). However, given the heterogeneity of anthropogenic landscapes and variable species responses, it is necessary to broaden the context in which snakes inhabiting urbanized areas are examined.
Studying snakes outside of remnant habitat fragments within a more developed matrix may be hampered by some of the common constraints of urban ecology studies, such as restricted access to private property, and destruction/theft of field equipment. However, certain features of urban areas provide practical remedies to some of these inherent problems, allowing for effective sampling given the appropriate model group. For example, flood control canals, often traversing highly urbanized matrices, are generally accessible and potentially support a rich herpetofauna, including semi-aquatic snake species (Ljustina 2017; Ljustina and Barrett 2018).
Watersnakes (genus: Nerodia) are relatively large, semi-aquatic snakes widely distributed throughout the eastern United States (Gibbons and Dorcas 2004). Unlike many other snake species, a comparatively rich literature exists exploring the life histories and ecologies of many species in this genus, encouraged by their often high population densities and conspicuous behaviors. Members of this genus feed on a variety of fishes and amphibians, though interspecific variation exists within the bounds of these generalizations. Throughout their range, several species appear to tolerate urbanization well, with some species attaining high densities within anthropogenically modified systems (Ackley and Meylan 2010; Todd et al. 2017; Ljustina 2017; Ljustina and Barrett 2018).
In southeastern Louisiana, four watersnake species may be encountered in freshwater/riparian ecosystems: Mississippi Green Watersnake (Nerodia cyclopion), Plain-bellied Watersnake (Nerodia erythrogaster), Southern Watersnake (Nerodia fasciata), and Diamond-backed Watersnake (Nerodia rhombifer) (Gibbons and Dorcas 2004). These species vary in their dietary and habitat preferences; Mississippi Green Watersnake and Diamond-backed Watersnake are primarily piscivorous and generally more aquatic (Hebrard and Mushinsky 1978; Mushinsky and Hebrard 1977a; Mushinsky and Hebrard 1977b; Mushinsky et al. 1982), while Plain-bellied Watersnake and Southern Watersnake feed preferentially on anurans, and tend to be more terrestrial in their habits (Kofron 1978; Michot 1981; Hyslop 2001; Roe at al. 2003; Gibbons and Dorcas 2004; Camper 2010).
Flood control canals are common features in many costal/low-lying cities, and vary in their physical structure, but how these structural variations influence herpetofaunal species richness and abundance is unknown. Canal depth may be especially important in structuring herpetofaunal communities, as this factor may impact the abundance and diversity of fishes, which correlates negatively with anuran abundance and diversity (Porej and Hetherington 2005; Petranka et al. 2007; Holbrook and Dorn 2016). Numerous fishes are preferred prey of different Nerodia spp., so they may adversely impact the anuran prey of species such as Plain-bellied Watersnake and Southern Watersnake, and also directly determine habitat quality for piscivorous species such Mississippi Green Watersnake and Diamond-backed Watersnake. As such, canal structural variation in urbanized systems may exemplify habitat and dietary preferences in these abundant, semi-aquatic mesopredators.
In this study, we examine a gradient of canal transects stratified by depth to determine whether depth influenced the presence and abundance of different watersnake species. We hypothesized that shallower canals would support a higher abundance of anurans, and so would have a higher abundance of Plain-bellied Watersnake and Southern Watersnake. In contrast, we hypothesized that deeper canals would have a higher abundance of fishes, and so would support greater abundances of the more piscivorous Mississippi Green Watersnake and Diamond-backed Watersnake. In addition to testing these hypotheses, we also include basic morphological and dietary data for each watersnake species.