Editorial on the Frontiers Research Topic: Amphibian and Reptile Road Ecology Provisionally Accepted

Cheryl S. Brehme^1* Silviu O. Petrovan² Viorel D. Popescu³ Thomas E. Langton⁴ Kimberly M. Andrews⁵ Robert N. Fisher¹

• ¹USGS, Western Ecological Research Center, United States
• ²Department of Zoology, Faculty of Biology, School of Biological Sciences, University of Cambridge, United Kingdom
• ³Department of Ecology, Evolution and Environmental Biology, Columbia University, United States
• ⁴Herpetofauna Consultants International Ltd,, United Kingdom
• ⁵Marine Extension and Georgia Sea Grant, University of Georgia, United States

Road mortality hotspots are commonly used for prioritizing placement of fencing and passages; however, data available and approaches used can vary widely (Paemelaere, et al., 2023;Ribeiro, et al., 2023). Shin et al. 2022 compared citizen science (CS) collected roadkill data in the Republic of Korea to standardized published data and found advantages of widely available CS data in increasing both geographic and temporal breadth. CS data identified hotspots of mortality and captured observations on behavioral ecology of herpetofauna, such as temporal patterns and trends in breeding, hibernation, and habitat use. They concluded that the two types of data are complementary, and that recording spatial and temporal effort would benefit CS survey data in less-studied faunas. Gonçalves et al. 2023 published a standardized 6-step sampling and analytical framework for use in prioritizing mitigation actions for amphibians. The framework incorporates site selection, imperfect carcass persistence and detection probabilities, and higher priority values for natural areas with native cover types that are less prone to landscape transition. They then demonstrated the applicability of this novel framework along several roads in southern Brazil.The probability of populations being extirpated due to road impacts may affect decisions on mitigation implementation. Wilkinson and Romansic (2023) conducted population viability analysis for California newts along a 6.6 km stretch of road with high annual mortality. Annual monitoring by citizen scientists (Parsons, 2021) coupled with a road mortality and permeability study allowed estimation of future population size in the absence of mitigation. Results predicted population extirpation in <100 years indicating a strong need for safe crossings. Further development of these models to predict population viability with differing crossing designs and placements is currently underway (Gould et al., 2023).
Studies of species and individual movement patterns in relation to roads, barriers, and passages are paramount to informing connectivity and the design placement of these systems across the landscape. In this issue, Hromada et al. 2023 recorded Mojave desert tortoise movements using GPS loggers and found that they were generally more active and made longer movements near off-highway vehicle (OHV) areas, dirt roads, and road barriers, and were less active and made shorter movements near an unfenced highway. Similarly, using accelerometers, Tipton et al. 2023 found timber rattlesnakes also made longer movements over greater time periods when encountering dirt and low-traffic paved roads relative to their movements in surrounding habitats. Both studies suggest that increased energy expenditures of reptiles near roads and barriers may be related to direct interactions to these features (e.g., avoidance, pacing back and forth) or to responses to habitat and resource modifications associated with these linear features.Using temperature-sensitive transmitters, Sisson and Roosenburg (2023) were able to determine that timber rattlesnakes, particularly gravid females, easily breached an unmaintained barrier fence to access thermal refugia (open habitat, rock piles) available in the roadside right-of-way (ROW) area. In addition to fence maintenance, they suggested creating suitable thermal refugia away from the road to reduce risk of vehicle strikes and mortality from ROW maintenance. Testud et al. ( 2023) used PIT tags and multiple RFID antennas to monitor movements of great-crested newts within passages. They found that newts were more likely to move forward in the first meters of shorter passages, suggesting a need for research into the mechanisms responsible for this response (e.g., odor, brightness, temperature, ventilation, distance). These studies illustrate that understanding individual behavioral responses to roads, mitigation structures, and surrounding habitat may help to further understand broad-scale connectivity patterns and better inform mitigation strategies.
Once mitigation systems are constructed, it is vital to monitor their effectiveness, to verify their value, and improve future designs. Two studies in this research topic focus on wildlife barriers, intended to keep animals off roads and to lead them to safe passage(s). Conan et al. (2023) tested the efficacy of solid-panel permanent barriers of differing material, height, and shape (presence/absence of an overhang) with five amphibian species with different climbing and jumping abilities and in both dry and wet conditions. They found that a smooth 40-50 cm high concrete barrier equipped with a 10 cm overhang was effective in stopping the majority of amphibians. They also stressed the need for maintaining the vegetation near barriers for continued effectiveness. There is often high amphibian road mortality where barriers end (Helldin and Petrovan, 2019). Harman et al. ( 2023) tested the efficacy of experimental perpendicular and angled 'turnarounds' at fence ends in changing the movement trajectory of multiple amphibian species. They found that individuals of several amphibian species changed direction at the barrier turnarounds and oriented towards road passages, which supported their use for amphibian mitigation systems and corroborated their effectiveness in changing trajectories of snakes, lizards, and toads (Brehme, et al., 2020). The authors cautioned that length of barrier is important, and more studies are needed to inform the design and orientation of barriers.The permeability of under-road passages to amphibian movement can be widely variable based on biotic and abiotic passage characteristics, passage spacing, species, and location (Langton and Clevenger, 2017). In this Research Topic, enhancing the permeability of existing passages by modifying vegetation is suggested by the studies of Sisson and Roosenburg ( 2023 also showed that enhancing permeability of passages for amphibians migrating to aquatic breeding habitats may be achieved through acoustic enrichment (playing frog calls).Spacing passages in between long stretches of road lined with barriers can result in a large proportion of animals not finding passage entrances due to 'giving-up' (e.g., Ottburg and Van der Grift, 2019;Brehme, et al., 2021). Brehme et al. ( 2023) designed and tested a novel elevated road segment (ERS), similar to a low terrestrial bridge, that was placed on top of an existing road. The 20-cm high and 30-m long prototype was composed of road mats on top of billet support bars that were perpendicular to the road.The design negates or reduces the need for barriers as it creates open passages that are continuous across its length. Results of monitoring over four years showed this was effective for a large number of amphibian, reptile, and small mammal species and offered a new design option for crossings that can be deployed to any length.Finally, maintenance of mitigation structures is extremely important and short-term studies may not be reflective of future effectiveness (e.g., Sisson and Roosenburg, 2023;Hedrick et al., 2019) The long-term conservation of herpetofauna requires adequate planning for habitat connectivity to facilitate movement and allow adaptation. This includes designing, installing, and maintaining safe and effective crossing structures for linear transport infrastructure. Often, when a target species is documented using a crossing, it seems natural to consider the problem solved. However, when high connectivity is needed, installation of inadequate passage-barrier systems may reduce the proportion of animals successfully crossing the road and result in population decline (e.g., Ottberg and van der Grift, 2019). In addition, passage use may increase or decrease over time, but this pattern is infrequently captured as long-term studies are rare. The studies in this special issue contribute to enhancing our understanding of reptile and amphibian response to roads, barriers, and passage systems, and further inform mitigation planning, design, and maintenance. Well-designed and prioritized research is needed to address the importance of passage system attributes in enhancing crossing rates, as well as long-term population monitoring of all life stages, to assess the effectiveness of these systems for maintaining viable populations.