Herpetological Conservation and Biology 14(1):31–40. Submitted: 16 November 2017; Accepted: 12 February 2019; Published: 30 April 2019. CHANGES IN SNAKE ABUNDANCE AFTER 21 YEARS IN SOUTHWEST FLORIDA, USA DEAN A. CROSHAW1,3, JOHN R. CASSANI2, ELI V. BACHER1, TAYLOR L. HANCOCK2, 2 AND EDWIN M. EVERHAM III 1Florida Gulf Coast University, Department of Biological Sciences, Fort Myers, Florida 33965, USA 2Florida Gulf Coast University, Department of Marine and Ecological Sciences, Fort Myers, Florida 33965, USA 3Corresponding author, e–mail: [email protected] Abstract.—Global population declines in herpetofauna have been documented extensively. Southern Florida, USA, is an especially vulnerable region because of high impacts from land development, associated hydrological alterations, and invasive exotic species. To ask whether certain snake species have decreased in abundance over recent decades, we performed a baseline road survey in 1993–1994 in a rural area of southwest Florida (Lee County) and repeated it 21 y later (2014–2015). We sampled a road survey route (17.5 km) for snakes by bicycle an average of 1.3 times a week (n = 45 surveys) from June 1993 through January 1994 and 1.7 times a week from June 2014 through January 2015 (n = 61 surveys). Snake mortality increased significantly after 21 y, but this result may be due to increased road traffic rather than expanding snake populations. The snake samples were highly dissimilar in the two periods, suggesting changes in species composition. For example, one species showed a highly significant decrease in abundance (Rough Greensnake, Opheodrys aestivus) while another showed substantial increases (Ring-necked Snake, Diadophis punctatus). Because of uncertain differences in traffic volume between 1993 and 2015, other species offered ambiguous results in their abundance trends. Nevertheless, four additional species contributed at least 12.1% dissimilarity (North American Racer, Coluber constrictor; Red Cornsnake, Pantherophis guttatus; Eastern Ribbonsnake, Thamnophis saurita; and Southern Watersnake, Nerodia fasciata). Increases in human land-use due to development and agriculture along with associated loss of wetlands and native habitats may have contributed to the changes we documented. Future work should seek to understand fully these causes and the conservation needs of declining species. Key Words.—change in abundance; conservation; ecology; reptile; road mortality; road survey; traffic; wildlife INTRODUCTION that a new ecological sub-discipline in road ecology has emerged (Forman et al. 2003). An estimated 22% of The massive influence of human populations over land in the United States is affected ecologically by roads the status and trajectory of life has prompted scientists (Forman 2000). In addition to immediate mortality from to name the current geologic epoch the Anthropocene vehicle strikes (Hels and Buchwald 2001; Lode 2000), (Steffen et al. 2007). Unfortunately, the Anthropocene roads cause habitat fragmentation that inhibits natural may be historically remarkable because of associated animal movements (Clark et al. 2010; Herrmann et al. increases in rates of biodiversity loss, and many scientists 2017; Marsh et al. 2005; Shepard et al. 2008b), increase now recognize this as the sixth great global extinction environmental pollution (Karraker et al. 2008; Reeves event (Barnosky et al. 2011). Most taxonomic groups et al. 2008; Sanzo and Hecnar 2006), and facilitate have been affected, but among vertebrates, amphibians the spread of exotic species (Urban et al. 2008). Most have received extra attention over the years (Beebee studies show a negative impact of roads on animal and Griffiths 2005; Wake and Vredenburg 2008). More populations (Fahrig and Rytwinski 2009). Amphibians recently, reptiles have been recognized as experiencing and reptiles are especially susceptible, with a majority considerable declines (Gibbons et al. 2000) and snakes of studies showing a negative effect of roads on species in particular are under threat in many regions (Sullivan richness (Findlay and Houlahan 1997; Houlahan and 2000; Brodman et al. 2002; Hanson and McElroy 2015). Findlay 2003) and individual populations (Fahrig and Herpetologists have called for increased monitoring Rytwinski 2009). In fact, several studies documenting and vigilance regarding worldwide snake populations the impact of roads on animal populations found that (Filippi and Luiselli 2006; Reading et al. 2010). amphibians and reptiles represent the majority of all Roads are an anthropogenic factor that can contribute observed mortality, sometimes > 90% (Colino-Rabanal to declines of herpetofauna (Forman and Alexander and Lizana 2012; Glista et al. 2008). Road mortality 1998; Trombulak and Frissell 2000) and have such is sometimes exceedingly high with as many as 10,000 important and widespread effects on natural ecosystems or more frogs per km killed each year (Goldingay and Copyright © 2019. Dean A. Croshaw 31 All Rights Reserved. Croshaw et al.—Snake abundance change in southwest Florida, USA. Taylor 2006), and this alone can compromise population Here, we aim to document possible differences in viability (Beebee 2013). Snakes, because of their use snake relative abundance between two time periods of roads for behavioral thermoregulation, are often separated by 21 y in a rural area of southwest Florida, impacted by road traffic (Evans et al. 2011, Rosen USA, while simultaneously providing baseline inventory and Lowe 1994). Enge and Wood (2002) estimated data for future retrospective analyses. We collected conservatively that as many as 1.4 million snakes are snake road survey data in 1993–1994 and 2014–2015. killed each year on rural Florida, USA, highways. A comparison of the two sampling periods allowed us Despite their negative ecological consequences, to determine if the relative abundance of snake species roads can be effective as incidental sampling transects has changed during these two decades and, if so, which for snakes and other herpetofauna. Use of roads to species show evidence of abundance increases and collect data has a long history in herpetology (e.g., Fitch decreases. 1949; Campbell 1953) and can be employed to sample live and dead snakes by car, bicycle, or on foot (Bonnet et MATERIALS AND METHODS al. 1999; McDonald 2012). Such road surveys are often part of efforts to characterize a local fauna (Coleman et We established a road survey route (17.5 km) in al. 2008; Desroches et al. 2010; Enge and Wood 2002). Buckingham, Lee County, Florida, USA (Fig. 1) in the This method is a convenient, but underused, approach summer of 1993. This area offers a mixture of upland for long-term monitoring of threatened amphibian and and wetland habitats, the former being used mainly in reptile communities, and it can allow researchers to small agricultural operations and rural residential. We perform retrospective comparative studies to document conducted 45 surveys by bicycle from June 1993 to change (Sullivan 2000). It is also a useful way to assess January 1994 (mean = 1.29 ± 1.07 SD surveys per week) the need for and success of ongoing mitigation efforts to and 61 surveys from June 2014 to January 2015 (mean reduce animal mortality on roads (Aresco 2005; Shepard = 1.74 ± 0.70 surveys per week). We conducted surveys et al. 2008a; Shwiff et al. 2007). However, as with any without seasonal bias as shown by an analysis of their sampling method, there are important limitations and days of the year, with 1 January as 1, 2 January as 2, biases inherent to road surveys (discussed in Steen and and so on (t = 0.01, df = 104, P = 0.990; 1993–1994 Smith 2006). For example, small, cryptic, arboreal, and mean = 224.9 ± 79.36 SD, 2014–2015 mean = 224.7 ± fossorial individuals and species are likely not observed 102.5). We positively identified species and recorded as readily. Moreover, roads have the potential to attract GPS coordinates of all encountered snakes (living or or repel animals, and crossing behavior may differ due dead) before removing them from the roadway to ensure to sex, life stage, or condition of the animal (Jochimsen they were not counted in subsequent surveys. 2006). We analyzed snake observation data to ask whether In Florida, declines of snakes and other herpetofauna there were differences between the two sampling periods have been documented (e.g., Godley and Moler 2013) (1993–1994 and 2014–2015). Because our counts are and retrospective studies suggest that shifts in community not distributed normally, we used the nonparametric composition have occurred (Dodd et al. 2007; Cassani Wilcoxon signed-rank test for paired samples in the et al. 2015). Although the causes are complex, NPAR1WAY procedure in SAS (SAS Institute 2003) multifactorial, and sometimes enigmatic (as in Winne et to test for differences between sampling periods in al. 2007), they likely consist of a combination of habitat the mean number of individuals recorded during each loss (Enge and Marion 1986), habitat degradation (Delis sampling run for all species. We set the threshold for et al. 1996), habitat fragmentation (Pike and Roznik statistical significance a priori to 0.05. Traffic volume 2009), invasive exotic species (Forys and Allen 1999; often correlates positively with animal mortality Smith 2006), altered hydrology (Greenberg et al. 2015), (Mazerolle 2004; Fahrig et al. 2005; Coleman et al. and climate change (Catano et al. 2015). We know 2008), although the precise nature of the relationship is that such changes in animal