Abstract
Context
Road expansion has raised concerns regarding road effects on wildlife and ecosystems within the landscape. Salamanders, critical ecosystem components and bioindicators, are vulnerable to road impacts due to habitat loss, migrations, and reliance on stream health. Systemic reviews considering the effects of different road types on salamanders are lacking.
Objectives
We summarize 155 studies of road effects on salamanders, including paved, unpaved, and logging roads, hiking trails, railroads, and powerlines. We examine trends in road type, study area, and impacts on salamanders; summarize current knowledge; and identify knowledge gaps.
Methods
We used Web of Science for literature searches, completed in January 2023. We reviewed and summarized papers and used Chi-squared tests to explore patterns in research efforts, research gaps, and impacts on salamanders.
Review
Roads had negative effects on salamanders through direct mortality, damaging habitat, and fragmenting populations. Traffic and wetland proximity increased negative impacts in some studies; abandoned logging roads showed negative effects. Positive effects were limited to habitat creation along roads. Habitat creation and under-road tunnels with drift fencing were effective mitigation strategies. Non-passenger vehicle roads were critically understudied, as were mitigation strategies such as bucket brigades and habitat creation along roads.
Conclusions
With road networks expanding and salamander populations declining, managers must account for road effects at landscape scales. The effects of non-paved roads on salamanders are poorly understood but critically important as such roads are frequently located in natural areas. Managers should incorporate mitigation strategies and work to reduce road impacts on vulnerable wildlife.
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Introduction
Road networks have expanded greatly across the world, particularly in the past century as the proliferation of vehicles has necessitated higher quality, paved and expanded road designs and higher connectivity of population centers (van der Ree et al. 2015a). In 2003, 83% of the continental United States was within 1 km of a road (Riitters and Wickham 2003), and global road networks have continued to expand to exceed 64 million km in length as of 2013 (van der Ree et al. 2015a). The proliferation of roads has raised concerns regarding their effects on the landscape, with the subdiscipline of “road ecology” emerging to explore and understand the biotic and abiotic environment (van der Ree et al. 2011).
Roads and associated vehicular traffic can have strong effects on the landscape and associated wildlife populations through direct mortality from vehicles, habitat loss and fragmentation from roads, and indirect negative effects of runoff from impervious surfaces (van der Ree et al. 2015b). Road mortality is an obvious effect. One of the first published articles on the effects of roads on wildlife reported 225 vertebrate road-kills from 29 species on a 1017 km length of highway in the Midwestern United States (Stoner 1925). Road mortality has been extensively studied in a variety of taxa, including deer (Steiner et al. 2014) and moose (Child et al. 1991; Joyce and Mahoney 2001; Rolandsen et al. 2011), insects (Muñoz et al. 2015), small mammals (Fahrig and Rytwinski 2009), reptiles (Andrews et al. 2008), and amphibians (Beebee 2013). Beyond direct mortality, roads may also fragment landscapes by acting as physical or behavioral barriers to wildlife (Jaeger et al. 2005; Eigenbrod et al. 2008). Fragmentation can lead to genetic isolation (Holderegger and Di Giulio 2010), which in turn can make these isolated populations more susceptible to future declines. Habitat lost to road construction is also notable, as the increasing extent of these cleared areas erodes the surrounding habitat. Furthermore, road effects can reach into otherwise intact landscapes, with verges introducing habitat edges along their lengths (Eigenbrod et al. 2009) and further degrading important habitats, referred to as the “road effect zone” (Forman 2000). Additionally, runoff from impervious surfaces of paved roads can contain damaging chemicals from road deicers and vehicle emissions (Kayhanian et al. 2012).
Herpetofauna are particularly vulnerable to road mortality due to their varied habitat use during different and complex life stages, low dispersal ability and slow movement rate, lack of behavioral responses to traffic, and strong site fidelity (Rytwinski and Fahrig 2012; Lesbarrères et al. 2014). Salamanders are critical components of natural ecosystems (Wyman 1998; Mathewson 2007) and are useful as bioindicators of forest, stream, and wetland health (Welsh and Ollivier 1998; Welsh and Droege 2001; Welsh and Hodgson 2013). Unfortunately, salamanders are threatened by habitat loss and changes to the landscape, and many semi-aquatic salamander species make large migrations that could be interrupted by the presence of roads or lead to direct mortality from vehicles on roads (Carr and Fahrig 2001). Furthermore, stream salamanders and aquatic larvae may be strongly affected by sedimentation due to runoff from impervious road surfaces (Lefcort et al. 1997; Welsh and Ollivier 1998). Collectively, roads are considered a major threat to salamander populations.
Prior literature examining road effects on salamanders appears to focus on paved roads, which may leave managers of other road types uninformed or obscure less obvious drivers of road effects. Herein, we comprehensively review published studies of the effects of roads on salamanders, with a primary objective of describing road characteristics that may mitigate or exacerbate effects on salamanders, and assessing effectiveness of mitigation efforts. We also examine the potential for road surface type, road location, and traffic to influence salamanders, expecting that higher traffic, road paving, and proximity to wetlands will increase impacts on salamanders. We begin with a systematic review of the literature examining trends in results, road type, and area of study, and end with a subjective review of findings from the literature.
Methods
Literature search
We searched the Web of Science™ for studies on the effects of roads on salamanders. Search terms included road, logging trail, trail, street, firelane, lane, powerline, highway, path, interstate, paved, impervious surface, impervious cover, gravel, and dirt, in combination with the search terms salamander, ambystoma, plethodon, and salamandra. Searches were made in early 2022 in April and June and updated in January of 2023. We collected and examined papers, and removed any that did not consider the effects of roads on salamanders. Conference proceedings were also removed to avoid double-reporting published and presented studies and to limit analyses to finished and peer-reviewed works. We separately searched review papers with no primary data for additional literature, but did not include them in the analyses. Papers examining mitigation efforts rather than the impacts of the roads themselves were considered separately.
Data collection
For each paper we recorded authors, year, country and continent of study, ecoregion of the United States (if applicable), and species studied. We classified studies based on response variable type, road type, land-use context, salamander habitat association, and overall impact of roads on salamanders (Table 1). The unspecified roads category included both papers that examined paved and unpaved roads without distinguishing between them in the analysis and roads that used data such as GIS layers that provided a “road” variable that did not differentiate between road surfaces. These studies were included in the analysis due to their relatively high number and potential influence on the literature. Impacts were deemed to be positive or negative if the paper either reported a statistically significant result or reported a conclusion with positive or negative impacts on salamanders, while impacts were deemed to be neutral if the paper reported finding no effects on salamanders or did not have sufficient data to draw such conclusions. If papers reported multiple impacts, all impacts were recorded for analysis. If available, we recorded quantitative mortality data and traffic data. As logging roads may have very different use patterns than other unpaved roads such as residential roads, they were considered separately from unpaved roads. While powerlines are not designed for traffic like other forms of roads and trails, they are often used by utility and recreational vehicles and by hikers, and so are included here as a potential low-impact form of road.
If traffic data from studies in the United States were not reported, we first searched the U.S. Department of Transportation Federal Highway Administration’s travel monitoring database for traffic data, followed by state Department of Transportation databases. When traffic data were not available for the road being studied, data from an adjoining road of the same type was recorded from the year closest to that of the study if available. We standardized traffic data into units of vehicles per day. After examining histograms of available traffic data, traffic was sorted for analysis into no traffic, low traffic (≤2000 vehicles per day), medium traffic (2000 to 5000 vehicles per day), and high traffic (>5000 vehicles per day).
Statistical analysis
We visually examined changes in the number of papers on road effects on salamanders produced over time using a graph with an exponential best-fit line to see when research on the topic began, how interest has grown, and whether research has continued to increase or has begun to decline. We then tested for changes in the probability of negative, neutral, or positive effects over time using multinomial regression. We also examined trends in the area of study and road type studied over time using multinomial regression to examine where research has focused over time and whether some study areas or road types remain understudied.
We attempted a formal meta-analysis of effect sizes, but the variety of study methods used and dearth of data sufficiently reported to calculate effect sizes made it infeasible (Supplementary Table A1). Instead, we used Chi-squared tests with p-values computed by Monte Carlo simulation to assess whether impacts (negative, neutral, or positive) were independent of road type and area of study, assessing significance with randomization tests (alpha = 0.05). When papers recorded multiple or mixed impacts (e.g., negative impacts on one species, neutral impacts on another), each outcome was counted independently. Likewise, for analyses of road type, outcomes comparing different types of roads were split into multiple rows for each individual road type, and results, if multiple or mixed by road type, arrayed accordingly. Additional Chi-squared tests were used to assess whether impacts on salamanders were independent of land-use context (urban, ex-urban, rural, natural area), traffic (none, low, medium, high), habitat association (stream, terrestrial, both), and continent and ecoregion of study. Continent and ecoregion of study were included due to potential differences in salamander species and abundance, road prevalence and quality, road treatments, land use prevalence, research funding, and interest in the topic that may affect the outcome of research and the amount of research conducted. All analyses were done using R for Statistical Computing 2022.12.0 (R Core Team 2022) and R package MASS (Venables and Ripley 2002).
Results
We collected 155 papers on the effects of roads on salamanders. Nineteen papers examined only mitigation strategies, resulting in 136 papers to analyze for effects of roads on salamanders. Study locations in these papers included 17 countries spread across North America (n = 122), Europe (n = 20), Asia (n = 3), and the Middle East (n = 3), but not South America, Africa, or Australia, where salamanders occur more rarely and with less native diversity, or are not native (Wells 2007). In the United States, papers covered 33 states across the contiguous United States. The number of papers produced on this topic has increased over time, possibly with some leveling out in recent years (Fig. 1), which may be expected with the general growth of the field of ecology (McCallen et al. 2019). There was no significant effect of year on the proportion of studies addressing each response metric (X2 = 5.321, p = 0.503) or type of road (X2 = 1.495, p = 0.960).
Number of studies on the effects of roads on salamanders by year. Black line indicates an exponential regression best-fit line of all papers collected
Of response types, studies of population metrics (population) were most common (n = 74), followed by mortality (n = 25), genetics (n = 18), and occupancy (n = 16); habitat studies were the least common (n = 2; Table 2, Fig. 2). Overall, 71.1% of documented road effects on salamanders were negative, whereas only 4.7% were positive (Table 2). The relative frequency of negative, neutral, or positive results reported by the study was independent of the response variable examined (Table 2; X2 = 17.81, p = 0.138). However, the relative frequency of negative versus non-negative results varied with response type (X2 = 14.24, p = 0.025). Specifically, frequencies of negative effects were higher than expected for mortality and behavior studies but lower than expected for occupancy studies.
Graph of proportion of results of studies by area of study. n = number of outcomes for each area
Effects on salamanders were independent of traffic levels (X2 = 9.148, p = 0.174, Table 3), which was quantitatively reported by 28 papers, with an additional 11 papers supplemented by online data. Collapsing neutral and positive results for traffic into a single category yielded similar results (X2 = 5.59, p = 0.136).
Across all road types, and counting each road type in studies comparing multiple road types, 69.6% of documented effects were negative (Table 4). Only seven positive effects were reported (4.3% of total), found disproportionately in studies on hiking and, to a lesser degree, unpaved roads (Table 4, Fig. 3). Impacts tended to vary by road type (X2 = 27.17, p = 0.048 for all road types; X2 = 5.28, p = 0.086 for paved and unpaved alone), with examination of standardized residuals indicating that negative effects were proportionately more common for paved roads (78% of 97, Table 4) and positive effects more frequent than expected for hiking trails (33% of 6, Table 4). Collapsing neutral and positive results into a single “non-negative” category yielded similar results (X2 = 14.09, p = 0.021). Results were independent of whether the study was in an urban, ex-urban, rural, or natural area (X2 = 8.65, p = 0.504, Appendix Table 5), whether the study examined stream, terrestrial, or both salamander groups (X2 = 1.696, p = 0.801, Appendix Table 6), and where the study took place (by continent: X2 = 7.777, p = 0.259, Appendix Table 7; by ecoregion (USA): X2 = 2.453, p = 0.982, Appendix Table 8).
Graph of proportion of impacts of studies by road type. n = number of outcomes for each road type
Twenty-one papers reported a numerical estimate of salamander mortality. Reports were in a variety of units that precluded a common analysis: number of salamanders killed per kilometer surveyed, number of salamanders killed per total observed, number of salamanders killed per salamander marked for capture-recapture, number of salamanders killed per day or per survey, and number of salamanders killed per total population estimate. The largest single category was number of salamanders killed per kilometer surveyed with nine papers, all of which reported negative results.
Discussion
Research on the effects of roads on salamanders has increased exponentially in the past few decades. Despite this increase, many regions with salamanders remain understudied, obscuring any potential region-specific effects. Most studies have examined paved roads, likely due to their potential for high, long-term impacts on wildlife. Many papers did not specify the type of road studied or considered multiple types of roads together, often relying on GIS or satellite data that were not differentiated by road type. Papers regarding impacts of gravel and dirt (unpaved) roads on salamanders are less frequent, with only a few studies offering comparisons to paved roads. The impacts of other kinds of roads, including logging roads, skidder trails, railroads, and hiking trails, are distinctly understudied. Given that unpaved roads had mostly negative effects on salamanders and may occur frequently in otherwise natural, even protected areas (DeMaynadier and Hunter 1995), the lack of research on commercial, recreational, and other non-passenger vehicle roads represents a significant gap in understanding road effects on salamanders.
Direct mortality
The earliest papers on the effects of roads on amphibians reported on direct mortality, with Duellman (1954) finding 274 tiger salamanders (Ambystoma tigrinum) dead and 46 alive on a paved road. Later, Williams and Gordon (1961) described two dead green salamanders (Aneides aeneus), the first observations of that species on a paved road. Other studies have considered the effects of direct salamander mortality along roads, with mortality estimates ranging from 1.4 to 18.8% of populations and 3.3 to 37.5% of observed individuals (Appendix). However, mortality may be higher than reported, and mass mortality events more frequent, as detection of such events required at least seven repeated surveys for a certainty of 85% (Hallisey et al. 2022). Unfortunately, a mortality rate over 10% may lead to local extirpations, and 22–73% of the populations in central and western Massachusetts may exceed this threshold (Gibbs and Shriver 2005). Applying this criterion, several of the populations we reviewed are likely imperiled by mortality on roads (Appendix).
While our systematic review failed to find an association between traffic levels and road effects on salamanders, several studies have found that traffic and location are strong contributing factors to road mortality, suggesting that factors such as vehicle velocity and timing may be additional important contributors to road mortality. More cars at higher speeds increased salamander mortality in several studies (Hels and Buchwald 2001; Howard and Schlesinger 2013; Rassati 2016; Sinai et al. 2020; Hallisey et al. 2022; Wilkinson and Romansic 2022), although effects were greater on frogs than salamanders (Mazerolle 2004; LeClair et al. 2021) due to differences in animal movement speed or local abundance (Hels and Buchwald 2001; Brehme et al. 2018). Salamander species that undergo seasonal migrations attempt crossings during these periods and rarely encounter roads otherwise; salamanders are also nocturnal, when fewer cars are on roads, and travel on warm, rainy nights, with associated increases in mortality during such weather events (Taub 1961; Madison and Farrand 1998; Clevenger et al. 2001; Briggler et al. 2004; Homan et al. 2008; Titus et al. 2014; Bieri and Leonard 2019; Mestre et al. 2019). Along with traffic, location also contributes to road mortality, with higher measured mortality in areas near wetlands (Glista et al. 2007; D’Amico et al. 2015; Shin et al. 2020; Hallisey et al. 2022) and more ‘hot-spots’—locations with unusually high mortality—near wetlands in models (Langen et al. 2007; Patrick et al. 2012; Schuett-Hames et al. 2019; Hallisey et al. 2022).
Effects on populations and occupancy
Effects of roads on salamander population metrics was the most common area of study, with mostly negative results, while occupancy had a lower proportion of negative to non-negative results than expected (Fig. 2). Higher road density corresponded with lower abundance of salamanders (Houlahan and Findlay 2003; Chen and Roberts 2008; Veysey et al. 2011; Aldridge 2020) and declines in salamander diversity (Lehtinen et al. 1999; Chen and Roberts 2008; Jacobs and Houlahan 2011), occurrence (Rubbo and Kiesecker 2005; Blank and Blaustein 2014), stream salamander density (Ward et al. 2008) and embryonic survival (Brady 2012). On a more positive note, some studies concluded that effects of roads on salamander abundance and occupancy were outweighed by forest cover and other nearby natural features (Ficetola et al. 2009; Holzer 2014; Cassel et al. 2019). As roads may exhibit negative effects on salamander populations over a large area along their length, extirpation—a negative effect on occupancy—may only emerge in extreme cases or small populations, while population density, and survival may be more sensitive metrics of negative effects of roads. However, even if roads have weak direct impacts on salamander populations, road construction in forested areas invariably involves a loss of forest cover and other changes to the environment, which can be substantial along wider roads and thus exert indirect negative effects on salamanders (van der Ree et al. 2015b).
Paved roads can greatly influence salamander populations, with several papers raising concerns about potential extirpation of populations due to road and urban development (Barry and Shaffer 1994; Gibbs and Shriver 2005; Stranko et al. 2010; Bieri and Leonard 2019). Terrestrial salamanders were lower in number, stream salamanders occurred less frequently, egg masses were smaller, and overall salamander diversity was lower near paved roads (Diller and Wallace 1994; Gibbs 1998; Foster et al. 2004; Porej et al. 2004; Bowles et al. 2006; Miller et al. 2007; Price et al. 2010; Stranko et al. 2010; Karraker and Gibbs 2011a; Gravel et al. 2012; Guderyahn et al. 2016). Some studies reported no effects of paved roads on salamander populations, albeit with caveats. The proportion of impervious surfaces alone had little effect on stream salamander survival and colonization (Price et al. 2010), and an additional two studies found no effect of road proximity on egg mass counts (Petranka et al. 2003; Skidds et al. 2007). Two other studies found no effects of road construction on salamander occupancy in nearby wetlands, but noted that man-made wetlands constructed to mitigate damage from road construction negatively impacted salamanders by drying faster than natural wetlands (Swartz et al. 2020; Oja et al. 2021). Given that wetlands near roads, including roadside ditches, dry faster and may thus impact successful recruitment, counts of egg masses alone may not reflect the full impacts of roads on salamander recruitment. These impacts may, therefore, be stronger than some studies have reported.
Our systematic review found that unpaved roads also had predominantly negative effects on salamander populations, although less so than paved roads, which was supported by individual studies. Unpaved roads often were associated with lower area of cover objects salamanders may use as refuges, lower soil moisture, higher soil temperature, and increased sedimentation of nearby streams (Marsh and Beckman 2004; Jackson et al. 2022), all of which can contribute to declines in salamander abundance (Marsh and Beckman 2004; Marsh 2007; Semlitsch et al. 2007). Slimy salamander (Plethodon glutinosis) populations, however, were not affected by gravel roads (Marsh and Beckman 2004; Karraker and Gibbs 2011a). Notably, the effects of unpaved roads on spotted salamander egg mass size were also much smaller than effects of paved roads (Marsh and Beckman 2004; Karraker and Gibbs 2011a). As with paved roads, traffic may be a contributing factor, as gated unpaved roads and ATV trails had no effect on salamander abundance and diversity, while ungated roads had significant negative effects (Marsh 2007; Hunkapiller et al. 2009).
Although comparatively understudied (n = 5), logging roads are reported to reduce nearby salamander abundance (deMaynadier and Hunter 2000; Semlitsch et al. 2007). Jefferson (Ambystoma jeffersonianum) and marbled salamanders (Ambystoma opacum) also selected breeding ponds further from logging roads (Chambers 2008). However, pond use by eastern spotted newts in the same study was not affected by proximity to logging roads (Chambers 2008), demonstrating interspecific differences in tolerance to road presence. Similarly, slimy and mole salamanders were captured in lower rates along 3–4-year-old skidder trails, but marbled salamander captures were only slightly lower and spotted salamanders were unaffected (Cromer et al. 2002).
Intensity of use and elapsed time since use of logging roads may influence the magnitude of their effects on salamanders, as wider, higher-use logging roads had stronger effects on salamanders than skidder trails, which are temporary and used less (DeMaynadier and Hunter 2000). Although smaller, older logging roads and skidder trails had less effect on salamanders, their presence can continue to impact nearby populations for decades, with observed negative effects of skidder trails 3 years after last use (Cromer et al. 2002) and of logging roads abandoned 80 years previously (Semlitsch et al. 2007).
Hiking trails negatively affected salamanders disproportionately less often than expected in our review, with the highest proportion of positive effects reported on salamanders of any road category. Nonetheless, slope, ground and leaf cover can be lower on hiking trails, which may affect salamander populations (Milanovich et al. 2015). The lack of vehicular traffic may reduce the effects of hiking trails on salamanders, although the low number of studies precludes a definitive assessment. While powerlines also lack vehicular traffic and also had a low proportion of negative effects on salamanders in our review, the very low number of studies again makes any assessment difficult. Similarly, we could draw no conclusions from the lone study including railroads as a category.
Indirect effects of roads
Roads indirectly affect salamanders by changing habitats, altering behavior, and reducing population connectivity. Road effects on salamanders may result from habitat fragmentation, canopy loss, and sedimentation and increased flow rates of nearby waterways (Welsh and Ollivier 1998; Willson and Dorcas 2003; Ward et al. 2008; Barrett et al. 2010b, a; Welsh et al. 2019; Jackson et al. 2022). Habitat loss and fragmentation is particularly notable as a major threat to salamanders (Ficetola et al. 2015). While hiking trails had less negative effects than expected, heavy recreational hiking may still significantly damage critical habitat both on- and off-trail, particularly around campsites (Wood et al. 2006). For stream salamanders, passable culverts may provide some salamander habitat and connectivity across roads (Adcock et al. 2022). However, impassable culverts may limit salamander movement (Ward et al. 2008). Unpaved roads may present further barriers to salamander movement as they lose water at high rates compared to forested habitat, which may prevent salamander crossings and fragment populations (Peterman et al. 2014).
Salamanders may be able to adapt to roadside conditions, as one study found that salamanders collected from roadside pools were more able to survive roadside conditions than those collected from wetlands further from roads (Brady 2012). However, other indirect effects of roads may present further dangers. One study found that bottles and similar road refuse, more common along high-traffic roads, may fatally trap salamanders (Benedict and Billeter 2004). Another study examined disease transmission and found that it was higher along hiking trails than either paved or unpaved roads (Beukema et al. 2021). With road mortality a major concern, and disease threatening many salamander populations (Martel et al. 2014), such indirect effects should be given greater attention to understand better the full extent of road impacts on salamanders.
Salamander behavior can influence their susceptibility to roads. Salamanders have exhibited a freeze response to oncoming vehicles (Carpenter 1955; Mazerolle et al. 2005), increasing their chances of being hit, but may move faster and in a straighter line on both paved and unpaved roads, which may reduce the time they spend in danger (Semlitsch et al. 2012). Paved and unpaved roads may limit dispersal, as they were less permeable to ambystomid salamander movements than forested habitat (Madison and Farrand 1998; Homan et al. 2008; Titus et al. 2014) and reduced the rate of return of displaced red-back salamanders (Marsh et al. 2005; Snyder et al. 2021). The lack of tree canopy over many roads discourages movements, as salamanders preferred closed-canopy habitat and avoided powerline corridors and other narrow canopy gaps (Demaynadier and Hunter 1999; Alix et al. 2014; Cecala et al. 2014). Fewer studies have examined indirect effects of logging roads, but one study found that salamander movements, including home range movements and adult dispersal, were significantly curtailed by logging roads, with consequences for population connectivity (DeMaynadier and Hunter 2000). Overall, all types of roads can affect salamander behavior and habitat in ways that may adversely affect local populations.
Effects on genetics
Changes in salamander movement and behavior may increasingly isolate populations, which can be monitored through genetic differentiation. Effects of roads on salamander genetics have been variable, with unspecified road cover tending toward negative effects on gene flow and genetic diversity (Noël et al. 2007; Trumbo et al. 2013; Emel et al. 2019), but one study finding no effect of road cover on spotted salamanders (Purrenhage et al. 2009). Paved roads have resulted in either strong negative effects (Marsh et al. 2008; Van Buskirk 2012; Munshi-South et al. 2013; Bani et al. 2015; McCartney-Melstad et al. 2018; Homola et al. 2019) or little to no effects (Compton et al. 2007; Richardson 2012; Straub et al. 2015; Feist et al. 2017; Schmidt and Garroway 2021). As with mortality, factors such as road size and traffic intensity may play a role, as larger roads likely have stronger effects than smaller roads with fewer vehicles (Marsh et al. 2008; Van Buskirk 2012; Bani et al. 2015; Homola et al. 2019). However, other factors may be in play, as dwarf salamander genetic richness and isolation were significantly negatively affected by low-traffic unpaved roads (Mckee et al. 2017). Landscape configuration is likely a factor in the strength of road effects, as it had stronger influences on salamander population connectivity than unpaved roads in models (Compton et al. 2007).
Road age may also play a role in the strength of road effects. Salamanders have relatively long lifespans compared to other amphibians and may occur at high local abundance, both of which may buffer them against genetic differentiation (Straub et al. 2015). Moreover, they also move relatively little, so dispersal may be impacted less rapidly by a newly constructed road, causing a lag between road construction and detectable genetic drift (Richardson 2012).
Effects of other types of roads on salamander genetics are severely understudied. We found no studies on the effects of logging roads on salamander population connectivity. Railroads were considered in one study, and had little effect on salamander genetic connectivity (Richardson 2012). Powerlines also had little effect on genetic differentiation in Webster’s salamanders (Plethodon websteri, (Feist et al. 2017). Additional research is needed to examine the effects of logging roads and other unpaved roads and trails on both salamander behavior and, ultimately, population connectivity (Semlitsch et al. 2007).
Runoff
Salamanders are often used as indicator species due to their high sensitivity to pollutants (Welsh and Droege 2001), with declines serving as a warning of adverse impacts to habitats even if more tolerant taxa are unaffected. Road runoff is one type of pollutant that strongly affects salamanders. In particular, the effects of road salt may cause mass mortality events (Duff et al. 2011) and can increase salt and chloride concentrations in nearby waterways above levels tolerated by adult or larval stream salamanders (Karraker et al. 2008a; Brady 2012; Tornabene et al. 2020; Izzo et al. 2022). Laboratory studies have determined that exposure to road salt increased stress levels of Jefferson salamanders, increased rates of developmental deformities in rough-skinned newts (Taricha granulosa), decreased larval survival of spotted salamanders (Ocampo et al. 2022), and resulted in both adult and larval mortality at high concentrations (Collins and Russell 2009; Duff et al. 2011; Terui et al. 2018); Appendix). The amount of sodium chloride in road runoff that reaches waterways depends on the amount of impermeable surface, but concentrations from 36 to 1390 mg/L have been observed in watersheds near urban areas, and up to 3731 mg/L with additional brief spikes up to 22,000 mg/L in stormwater management ponds (Izzo et al. 2022; Szklarek et al. 2022). Some salamander species showed higher tolerance to chloride concentrations than others (Appendix), but may struggle with indirect impacts from exposure. In field experiments, road salt lowered salamander prey abundance (Ozeri et al. 2021), which may have caused the reduced growth rates observed in spotted salamanders (Petranka and Francis 2013). Road salt runoff also lowered salamander abundance, mass, survival, and breeding activity, produced lower egg mass densities, and weakened eggs in natural ponds, culminating in negative effects on population size (Fukumoto 1995; Karraker et al. 2008b; Collins and Russell 2009; Karraker and Ruthig 2009; Karraker and Gibbs 2011b; Eakin et al. 2019; Wineland et al. 2019).
Road runoff is not limited to road salts, and other forms of runoff can strongly affect salamanders. Coal tar sealant runoff decreased spotted salamander growth rates and swimming ability (Bommarito et al. 2010), while fire retardant runoff reduced fire salamander survivorship and prey consumption and increased the length of the larval period (Ozeri et al. 2021). Increased levels of chemical contaminants occurred in salamander tissue from ponds near impervious surfaces such as roads, particularly in older developed sites (Diaz et al. 2020), and ponds with declining spotted salamander populations had higher aluminum, copper, and lead levels (Blem and Blem 1991). Additionally, runoff and soil erosion from roads and nearby construction can cause scouring in streams, which removes both salamanders and important habitat features and has contributed to salamander declines in urban areas (Orser and Shure 1972; Kiss et al. 2022).
Potentially positive effects of roads
Only seven papers reported positive effects of roads on salamanders. Two studies found higher numbers of salamanders along hiking trails due to increased coarse woody debris from trail clearing, with no effect along unmaintained trails (Davis 2007; Fleming et al. 2011). In France and Switzerland, fire salamander (Salamandra salamandra) abundance was higher along roads and railways, likely due to their use of refuges in stone walls and roadside ditches and ponds (Tanadini et al. 2012; Trochet et al. 2016). Moreover, road-rut ponds on unpaved roads could serve as important breeding habitats for salamanders (Adam and Lacki 1993; Feldmann 2007). Finally, salamander body growth rates increased in urban streams, likely due to higher temperatures and flow rates (Barrett et al. 2010b).
Although habitat loss remains a major driver of amphibian declines, some argue that habitat features created during road construction may bolster struggling populations. However, salamanders confined to pools on roads may be more at risk of human disturbance, and road-rut ponds may serve as vectors for disease (Mitchell 2005). Such habitats, rather than serving as quality salamander habitat, may instead act as ecological traps by attracting breeding salamanders and serving as sinks instead of sources to the population (Hale and Swearer 2016). Given the overwhelmingly negative effects of roads on salamanders through direct mortality, indirect effects, road runoff, and overall greater human disturbance, the observed positive effects of habitat creation appear incremental and are likely to be overwhelmed unless high-quality habitat is created effectively along with other mitigation efforts to reduce negative road effects.
Mitigation
Mitigation strategies have been increasingly developed with the rising awareness of the effects of roads on salamanders. Effectiveness of most mitigation strategies such as bucket brigades, habitat creation, and under-road culverts is understudied. Under-road tunnels are an exception and have demonstrated positive effects in lowering salamander mortality on roads, especially combined with drift fencing to direct salamanders to the entry point (Jackson and Tyning 1989; Aresco 2005; Pagnucco et al. 2012; Fitzsimmons and Breisch 2015; Boyle et al. 2021). However, adverse effects on salamander behavior highlight the need for more effective tunnel designs. For instance, 75% of spotted salamanders observed at tunnel entrances crossed through (Jackson and Tyning 1989). However, in follow-up studies tunnel use declined over the subsequent 30 years and only a small proportion of salamanders reached the tunnels and completed the crossing (Hedrick et al. 2019; Brehme et al. 2021). Salamander movement is affected by tunnels and tunnel design: long-toed salamanders (Ambystoma macrodactylum) were observed to pause at tunnel entrances, and both fire salamanders and northern crested newts (Triturus cristatus) moved more continuously through shorter tunnels (Pagnucco et al. 2011; Testud et al. 2022). Such changes to behavior may indicate an unwillingness to enter a tunnel and make salamanders more susceptible to predation or desiccation along roads and drift fencing (Aresco 2005; Pagnucco et al. 2011; Boyle et al. 2019).
Suggestions for tunnel improvement include using angled drift fences to direct salamanders to tunnels, widening tunnel entrances to > 0.4 m, and placing tunnel entrances 12.5 m apart (Bain et al. 2017; Hedrick et al. 2019). Tunnel entry lights and platforms did not improve tunnel use (Hedrick et al. 2019), but frog calls near tunnels may increase chances of tunnel crossing completion for salamanders (Testud et al. 2022). Weather is also a factor for managers to consider, as precipitation can increase chances of tunnel use, and tunnel moisture may decrease crossing time (Bain et al. 2017). Overall, tunnels and drift fencing have the potential to greatly reduce road mortality of salamanders and other amphibians and should increase in effectiveness with continued improvements to design.
Unlike under-road tunnels, culverts were not effective at mitigating road effects for stream salamanders, spotted salamanders, or larval coastal giant salamanders (Dicamptodon tenebrosus); salamanders did not appear to utilize them for road crossings (Sagar et al. 2007; Patrick et al. 2010; Anderson et al. 2014), although spotted salamanders were more tolerant of different culvert designs (Patrick et al. 2010). Rather than facilitating movement, culverts may present an isolating barrier for salamander populations (Anderson et al. 2014). The potential for better-designed culverts to support salamander road crossings will require additional research.
Only a few studies examined salamander habitat improvement along roadways, despite the potential for positive effects. Coarse woody debris inputs, particularly larger pieces, on abandoned logging roads may support salamander populations (LeGros et al. 2014, 2017), while adding branch frames to improve breeding pond quality along a highway may increase salamander activity (Yumei et al. 2022). Urban management may have the greatest potential in protecting imperiled urban habitats. Lee et al. (2022) created a framework for urban management planning that combined community engagement and habitat modeling, although future monitoring will be required to assess its success.
Other mitigation strategies such as bucket brigades are crucially understudied, despite their frequent use to ferry migrating salamanders across roads. A single modeling study concluded that efforts focused on metamorph rather than adult migrations would have higher positive impacts on populations and that volunteer distribution along roads was more important than number or frequency of brigade nights (Sterrett et al. 2019). Unfortunately, the model only considered consistent, long-term volunteer effort and did not include metapopulation dynamics. Thus, additional research including field studies are needed to assess effectiveness of bucket brigades.
Limitations
We have attempted to accurately summarize the available literature on the effects of roads on salamanders. However, vote-counting approaches have well-known limitations. Publication biases towards studies finding significant effects have been noted in the literature (Kimmel et al. 2023), which may have led to lower sample sizes of studies with “neutral” effects. Publication biases also favor unique results and discourage replicated studies and results (Fraser et al. 2020), which may weaken the generality of some of our conclusions. The literature also suffers from a geographic bias towards North American sites and studies from English-speaking countries (Mongeon and Paul-Hus 2015).
Roadkill studies such as those reviewed here also favor locations with known populations of the species or taxa of interest or road-kill hotspots, which may ignore areas where populations have been extirpated by factors such as road mortality (Ascensão et al. 2019). Furthermore, studies using counts of individuals killed on roads may suffer from biases in detection and carcass persistence, which may differ by survey methods (Teixeira et al. 2013). Our review examined a wide variety of studies using different survey methods, as well as different study methods in general along a variety of spatial and temporal scales, which may introduce further heterogeneity. Our review also suffered from a lack of studies available in some areas of research, particularly the effects of logging roads and hiking trails, which resulted in inconclusive effects likely attributable to low statistical power. The lack of available traffic data likewise weakened the strength of our examination of the effects of traffic on road effects on salamanders, and we suggest that future studies include traffic information in their data or analysis.
Conclusions
We review the effects of roads on salamanders as described by 155 studies and conclude that roads have strong negative effects on salamander populations. These effects threaten many salamander populations with extirpation, and may have long-lasting effects on habitat and population connectivity. Previous studies support the hypothesis that paved roads with high traffic tend to have stronger negative effects, whereas narrower roads with less traffic have fewer or weaker effects. While our traffic analysis did not find a relationship between traffic and negative effects, potentially due to lack of traffic data, some studies have suggested that even unpaved roads may negatively affect salamander populations, including logging roads abandoned for decades and skidder trails unused for years (Cromer et al. 2002; Semlitsch et al. 2007). Road placement is important; roads bisecting wetlands or close to salamander habitat showed stronger negative effects on salamander populations. Some salamander species, such as those more tolerant to disturbance, may withstand the effects of roads better than more vulnerable species. As salamander populations continue to decline and road networks continue to expand, consideration should be given to the effects of roads on vulnerable populations and mitigation efforts such as under-road tunnels that may protect them.
Our review revealed a distinct lack of research on roads closed to public vehicles including logging roads and skidder trails, railroads, hiking trails, and powerlines. The few available studies did document some notable negative effects on salamanders, particularly from logging roads and skidder trails, and also on some hiking trails. However, our systematic review of the proportion of these effects was inconclusive, and the lack of research makes it impossible to draw definitive conclusions. Additional research to help more fully understand and mitigate these putative effects is needed, particularly due to the predominance of unpaved roads and hiking trails in otherwise natural areas that provide important, supposedly intact salamander habitat. More research is also needed on mitigation strategies, particularly popular strategies such as bucket brigades, and best practices for common, long-term strategies such as under-road tunnels and culverts. Avoiding wetlands and salamander habitat, incorporating improved mitigation strategies such as under-road tunnels and drift fences into road construction, and taking steps to reduce road runoff and limit traffic during migration seasons, if necessary, may improve the outlook for nearby salamander populations, which will only grow in importance as salamanders continue to decline in the face of human expansion.
Data availability
The datasets generated and analyzed during the current study have been made available as supplementary files.
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Acknowledgements
We gratefully acknowledge the traditional homelands of the Indigenous People upon which Purdue University is built. We honor and appreciate the Bod´ewadmik (Potawatomi), Lenape (Delaware), Myaamia (Miami), and Shawnee People who are the original Indigenous caretakers. We appreciate the members of the joint Swihart-Zollner-Flaherty lab group who provided constructive comments on earlier drafts of this paper. Funding for this work was provided by McIntire-Stennis Cooperative Forestry Research Program (Project IND00008276MS), Purdue Forestry and Natural Resources’ Wright Endowment, and the Indiana Department of Natural Resources through the Hardwood Ecosystem Experiment—a partnership of the Indiana Department of Natural Resources, Purdue University, Ball State University, University of Illinois-Champaign, and Drake University to investigate forest management practices in southern Indiana.
Funding
Funding for this work was provided by McIntire-Stennis Cooperative Forestry Research Program (Project IND00008276MS), Purdue Forestry and Natural Resources’ Wright Endowment, and the Indiana Department of Natural Resources through the Hardwood Ecosystem Experiment.
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All authors contributed to study design. Literature searches and data collection were performed by AO. Data analyses were performed by AO with assistance from RS. The first draft of the manuscript was written by AO and all authors contributed to editing and preparation of the final version. All authors read and approved the final manuscript.
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Ochs, A.E., Swihart, R.K. & Saunders, M.R. A comprehensive review of the effects of roads on salamanders. Landsc Ecol 39, 77 (2024). https://doi.org/10.1007/s10980-024-01867-3
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DOI: https://doi.org/10.1007/s10980-024-01867-3