Influence of ecology and landscape on snake road mortality in a sagebrush‐steppe ecosystem

For depleted wildlife populations, understanding and effectively mitigating the direct and indirect impacts of roads can be a key component of recovery efforts. The ocelot is a federally endangered wild felid at risk of local extinction in south Texas, where vehicle collisions are their largest known source of mortality. To support road mortality mitigation, we evaluated mortality risk across the south Texas road network. We also assessed whether a sympatric carnivore, the bobcat, could serve as a surrogate to inform mortality risk. We used several presence-only species distribution models to evaluate a suite of candidate landscape and road attributes and identify those associated with elevated road mortality risk using a 35-year wild felid road mortality dataset. Consensus predictions among models identified high-risk road segments for ocelots and bobcats. We compared predictions between species to determine overlap of high-risk areas. Areas where core ocelot habitat intersected roadways represented the greatest mortality risk, reflecting ocelot reliance on intact habitat and avoidance of degraded and fragmented landscapes. Bobcat road mortality risk had similar relationships to landscape features, with nearly all areas classified as high-risk for ocelots also identified as high-risk for bobcats. Ocelots are vulnerable to road mortality in areas where roads intersect their intact scrub habitat. Bobcats, with appropriate data filters in place, are an acceptable surrogate for informing ocelot road mortality, particularly when ocelot mortality data are limited and for time-sensitive conservation decision-making. For ocelots and other fragmentation-intolerant carnivore species, wildlife underpasses and fencing in focused areas where roads and intact habitat intersect may be feasible options to reduce road mortality.

Regional analysis of road mortality in Europe

Road-associated mortality can lead to local declines of wildlife populations, and management agencies are actively implementing mitigation measures, especially focused on potential road mortality hotspots. In this study we used a spatially-explicit simulation modeling approach to estimate the hotspots of road mortality for the Eastern Hermann’s tortoise ( Testudo hermanni boettgeri ) within its distribution range in Romania. Using a field experiment, we first evaluated velocities while crossing roads. Adult male tortoises moved faster than females (3.98 m/min vs. 2.51 m/min) which led to higher individual probabilities for females being killed on high-traffic roads (0.61 for females vs. 0.44 for males at traffic levels of 7000 vehicles/day). Both males and females had similar road mortality probabilities for traffic levels <1000 and >35 000 vehicles/day. Our spatially explicit model suggests that, within the entire Romanian distributional range, the tortoises have an overall risk of road mortality 1.6%, which may have a negative impact on tortoise populations. Using the Getis-Ord Gi statistic, we identified road mortality hotspots with mortality rates of 5-30%, in areas bisected by high-traffic national and European-level roads. Our research is timely in that many low-traffic roads are predicted to have increased traffic associated with tourism activities, thus increasing the overall risk of mortality. We suggest that mitigation measures such as signage and roadside fences associated with underpasses have the potential to limit road mortality of this threatened species within predicted current mortality hotspots.

The Bayesian intrinsic conditional autoregressive convolution model was used to study the spatial variations in road mortality in the regions of Belgium, France and Germany. In all three countries, the spatial structure behind relative risk is significant and spatial heterogeneity predominates over unstructured heterogeneity. The maps of spatially-structured component of random effect enable the spatial structures of risk to be identified and highlight the zones and areas where the mortality risk exhibits spatial dependency. Hence, a west–east gradient in risk level is found in Germany and a north–south gradient in Belgium. In areas with high road network density (and relatively high population density), there is generally less heterogeneity in road mortality across neighbouring regions, while in areas with high regional disparities, there are significant spatial variations in mortality risk. Furthermore, a model was produced for 272 regions in 13 European continental countries, making it possible to investigate whether national borders have any specific effect on the distribution of road mortality risk compared to what happens within countries’ administrative borders. Cross-border regions were found not to be particularly likely to have similar road mortality risk levels where they shared a common national border. National borders have no specific effect on the distribution of road mortality across Europe.

Roads have numerous negative ecological effects on terrestrial fauna, and vehicular mortality can have significant demographic consequences for some species. We studied road mortality of reptiles around Carlyle Lake, Clinton County, Illinois, USA, from April 2000 through November 2002, to assess the impact of vehicular traffic and identify influential factors. Carlyle Lake, a popular tourism/recreation area, is situated in a larger agricultural landscape and is home to the largest Illinois population of the endangered Eastern Massasauga (Sistrurus catenatus). We documented 321 cases of reptile road mortality (84 individuals of six turtle species and 237 individuals of nine snake species) while driving our approx. 46 km study route roundtrip daily. Turtle road mortality was highest in May and June, and positively associated with precipitation and minimum daily temperature. Colubrid snake road mortality was highest in April and October, and positively associated with minimum daily temperature. We recorded 42 cases of road mortality of S. catenatus with the highest number occurring from mid-August to mid-September. Road mortality in S. catenatus was biased toward adult males, which show an increase in movement in August, coinciding with the peak of the mating season and a period of high tourist visitation. The traffic intensity on a road segment did not significantly affect the level of road mortality, but segments through high quality habitats had higher levels of mortality than segments through lower quality habitats. Based on our study on the ecology of S. catenatus, we make recommendations to reduce road mortality that should aid in the conservation of the Carlyle Lake population.

The influence of thermal biology on road mortality risk in snakes

Bobcats (Lynx rufus) were extirpated from many midwestern states in the mid‐1800s owing to habitat loss and overharvesting. Recently, bobcats have recolonized Ohio, USA, and neighboring states and given their furbearer status elsewhere, there is interest in opening a harvest season; however, demographic factors and viability of this population are currently unknown. We developed a spatial population simulation model to assess the long‐term viability of the bobcat population in Ohio in the face of 2 human‐induced factors limiting population growth: potential harvest and road mortality. We combined habitat suitability and road mortality risk data with vital rates for bobcats from Ohio and other populations to simulate possible scenarios for Ohio's population. Our baseline scenario simulations showed no risk of extinction for Ohio's bobcat population in the next 40 years, but population trajectories were lower and exhibited greater uncertainty when we modeled the population with a lower maximum density of animals per cell. At low harvest intensity (αh = 0.05), the bobcat population also exhibits low risk of extinction. When harvest intensity increases (αh = 0.1, 0.15) or when adults (≥2 yr) are targeted by harvest, simulations show declining populations, greater uncertainty in projections, and possible risk of extirpation. Our models indicated that if harvest and road mortality are additive, then the bobcat population could withstand marginal increases in road mortality (αr = 0.1) at low harvest intensity (αh = 0.05). Future increases in road mortality with higher harvest intensity (αh = 0.1, 0.15) were unsustainable. Our results can be used by wildlife managers to assist with decisions on population‐level management. This simulation model can easily be adapted for other large mammals and can be modified to assess a variety of ecological and anthropogenic influences to wildlife populations.

Many conservation strategies focus on reducing the risk of road mortality. Turtles, which are highly susceptible to road mortality, exhibit life history traits that increase their vulnerability to population declines because of road mortality. Here, we use Akaike's Information Criterion to identify road mortality hot moments and hot spots for 5 turtle species (Chelydra serpentina, Pseudemys concinna, Terrapene carolina, Terrapene ornata, and Trachemys scripta). We tested 2 hypotheses: 1) Hot moments coincide with the breeding season of each species and 2) hot spots are associated with the habitat where each species is most commonly observed. As predicted, breeding season (May–Jun) explained most of the temporal variation in road mortality for all 5 species. Over 98% of all observed road mortality occurred during this 2‐month span. For C. serpentina and T. ornata, spatial variation in road mortality was best explained by the distance to the nearest water body, whereas distance to the nearest forest was the best model for T. scripta. The null model ranked best for P. concinna and T. carolina, suggesting none of our spatial models sufficiently explained mortality patterns of these 2 species. These results, coupled with previous studies, suggest that hot moments may be consistently associated with the breeding season. However, hot spots may be species‐specific, requiring conservation efforts to be targeted at certain species. © 2012 The Wildlife Society.

While awareness of the public health burden of road insecurity is recent, the idea that it is developing countries, particularly in Africa, that experience high road deaths is older. Tunisia is an example of this. In this context, our article proposes recommendations through the study of the road mortality rate in Tunisia based on population density and belonging to a geographical unit. To do so, we used the Bayesian ecological regression model whose parameters are adjusted by Gibbs sampling. The analysis shows that the variation in road mortality risk is highest at the delegation level but lowest at the district and governorate levels. An estimated elasticity of −0.25 at the district level means that a 10% increase in population density can lead to a 2.5% decrease in road deaths. Bayes Relative Risk Mapping could help identify areas with high road mortality and strengthen road safety decision making.

BackgroundUnderstanding the ecological consequences of roads and developing ways to mitigate their negative effects has become an important goal for many conservation biologists. Most mitigation measures are based on road mortality and barrier effects data. However, studying fine-scale individual spatial responses in roaded landscapes may help develop more cohesive road planning strategies for wildlife conservation.Methodology/Principal FindingsWe investigated how individuals respond in their spatial behavior toward a highway and its traffic intensity by radio-tracking two common species particularly vulnerable to road mortality (barn owl Tyto alba and stone marten Martes foina). We addressed the following questions: 1) how highways affected home-range location and size in the immediate vicinity of these structures, 2) which road-related features influenced habitat selection, 3) what was the role of different road-related features on movement properties, and 4) which characteristics were associated with crossing events and road-kills. The main findings were: 1) if there was available habitat, barn owls and stone martens may not avoid highways and may even include highways within their home-ranges; 2) both species avoided using areas near the highway when traffic was high, but tended to move toward the highway when streams were in close proximity and where verges offered suitable habitat; and 3) barn owls tended to cross above-grade highway sections while stone martens tended to avoid crossing at leveled highway sections.ConclusionsMortality may be the main road-mediated mechanism that affects barn owl and stone marten populations. Fine-scale movements strongly indicated that a decrease in road mortality risk can be realized by reducing sources of attraction, and by increasing road permeability through measures that promote safe crossings.

Wildlife road mortality tends to aggregate spatially at locations commonly referred to as road mortality hotspots. Predictive models can be used to identify locations appropriate for mitigation measures that reduce road mortality. However, the influence of imperfect detection (e.g., false absences) during road mortality surveys can lead to inaccurate or imprecise spatial patterns of road mortality hotspots and suboptimal implementation of mitigation measures. In this research, we used amphibians and reptiles as a case study to address imperfect detection issues when estimating the probability of road mortality hotspots using occupancy detection modeling. In addition, we determined the survey effort needed to achieve a high probability of detecting large roadkill events. We also assessed whether vehicle travel reductions associated with the COVID-19 pandemic travel restrictions led to reductions in road mortality. We conducted surveys at 48 sites throughout Rhode Island, USA, from 2019–2021. In total, we observed 657 carcasses representing 19 of Rhode Island’s 37 native species. Of the 19 native species, eight species of frogs, four species of salamanders, four species of snakes, and three species of turtles were observed. We documented a reduction in roadkill density and the proportion of dead versus live amphibians and reptiles in pandemic years (2020 and 2021), but we were unable to link reductions in roadkill density to reductions in traffic volume. Our model results indicated that large roadkill events were more likely to occur on roads near wetlands and with low traffic volume and were more likely to be detected as daily precipitation increased. We determined that there was a low probability of detecting large roadkill events, suggesting that imperfect detection influences detection of large roadkill events, and many were likely missed during our surveys. Therefore, we recommend using occupancy modeling to account for the influence of imperfect detection when estimating road mortality hotspots. This approach will more effectively guide the implementation of mitigation measures.

Dolichophis caspius is frequently affected by road mortality, with many victims recorded also in Romania. Although some regions in the country have a presumptive high risk of road mortality, there is no certain data from all regions. In 2021 and 2024, we analyzed this species’ road mortality at its northern distribution range limit (lower Olt River valley, southern Romania). We identified 66 road-killed D. caspius over the course of eight days. Most victims (62.12%) were juveniles. The high number of road-killed juveniles is a consequence of the road`s features, which crosses many small human settlements (villages) and has numerous curves and speed enforcement cameras; therefore, the speed is reduced to 50 km/h. Consequently, the small-sized road-killed juveniles were easier to observe, as compared to roads of higher speeds, where they can be easily missed. The high number of juveniles indicates that in the region there are large populations that reproduce, as they are probably favored by climate warming. Although the absolute number of road-killed juveniles was higher at the end of summer and autumn, the ratio of juveniles and adults was the same in spring. The differences in road mortality on different days, even of the same month, indicate the importance of local microclimate conditions for the activity and road mortality of D. caspius. The lower Olt River valley region has a high risk of road mortality for this species, as it shows the scale of this phenomenon by highlighting the impact on juveniles.

Snakes on a lane: Road type and edge habitat predict hotspots of snake road mortality

Road networks pose many well‐documented threats to wildlife, from fragmenting habitats and restricting movement to causing mortality through vehicle collisions. For large, wide‐ranging mammals, home range requirements and seasonal migrations often necessitate road crossings, posing threats to human safety, property, and animal survival. Artificial nightlight, emanating from light posts and urban sky glow, is ubiquitous on and around road networks worldwide; however, its effects on road crossing behavior and the associated mortality risk for wildlife are not well understood. By integrating the latest NASA nightlight products with GPS collar data collected from 67 mule deer (Odocoileus hemionus) over a 7‐year period (2012–2018), we used a resource‐selection framework to assess factors influencing seasonal crossing behavior and road mortality in Salt Lake City, Utah, an expanding metropolitan area in the United States. We found deer preferred to cross the road where surrounding artificial nightlight was lower in both summer and winter seasons, especially during crepuscular and nighttime periods. However, lower nightlight levels also increased the risk of road mortality. Areas with more shrub cover and lower speed limits increased the likelihood of crossing as well as lowered the risk of road mortality. There were five times as many mortality events in winter as in summer, likely because of the combination of deer preference for dark roads mixed with proximity to both higher speed roads and increased human activity. Better understanding how a pervasive and expanding environmental pollutant like artificial nightlight may attract or repel human‐tolerant wildlife species from roadways presents an opportunity to mitigate collision risk while improving population management strategies for this abundant, generalist herbivore and many other economically and ecologically important species.

BackgroundDespite considerable research on nest site selection of diamondback terrapins and the risks that coastal roads pose to populations of this obligate estuarine turtle, there are gaps in our understanding of movements and habitat use by female terrapins during the nesting season. In particular, movements within salt marshes, shallow water habitats, and nest site-adjacent developed habitats remain relatively understudied. To investigate habitat use and risk exposure of this semiaquatic species, we employed an automated radio telemetry system featuring 31 passive receivers. In 2021, concurrent with annual nest surveys, we tracked 60 telemetered female terrapins near a nesting area in southern New Jersey subject to impacts from road mortality. We triangulated terrapin locations based on the relationship between radio signal strength at each receiver and distance, generated utilization distributions using kernel density estimation, and classified habitat using multispectral imagery.ResultsWe detected differences in habitat use versus relative availability within general (95% KDEs) and core (50% KDEs) use areas at a population level. Core use areas suggested more frequent use of salt pools, marsh, and upland habitats and less frequent use of developed and wide tidal creek habitats than expected based on availability alone. To further characterize variability in habitat use, we compared selectivity between terrapins grouped by proximity of their capture location to a road. Terrapins nesting closer to the road showed relatively more frequent usage of developed habitats and less frequent usage of narrow creeks and regularly flooded marsh throughout the nesting season.ConclusionsPatterns of habitat selection across all terrapins underscore the importance of shallow water habitats such as salt pools near nesting areas. Individuals tended to spend more time in habitat types close to their nesting locations, such that terrapins initially captured near roads spent more time in high-risk developed areas throughout the study period. Road mortality risk may not be homogenous within this nesting population, which may be a critical consideration for demographic modeling. This study expands our understanding of terrapin movements while demonstrating the effectiveness of a novel radio telemetry approach, and contributes to conservation planning in a rapidly changing salt marsh landscape.