Do wildlife crossing structures mitigate the barrier effect of roads on animal movement? A global assessment

Abstract

The widespread impacts of roads on animal movement have led to the search for innovative mitigation tools. Wildlife crossing structures (tunnels or bridges) are a common approach; however, their effectiveness remains unclear beyond isolated case studies. We conduct an extensive literature review and synthesis to address the question: What is the evidence that wildlife crossing structures mitigate the barrier effect of roads on wildlife movement? In particular, we investigated whether wildlife crossing structures prevented an expected decline in cross‐road movement, restored movement to pre‐construction conditions, or improved movement relative to taking no action. In an analysis of 313 studies, only 14% evaluated whether wildlife crossing structures resulted in a change in animal movement across roads. We identified critical problems in existing studies, especially the lack of benchmarks (e.g. pre‐road, pre‐mitigation, or control data) and the use of biased comparisons. Wildlife crossing structures allowed cross‐road movement in 98% of data sets and improved movement in ~60%. In contrast, the decline of wildlife movement was prevented in fewer than 40% of cases. For most structure types and species groups there was insufficient evidence to draw generalisable conclusions. Synthesis and Applications: The evidence to date suggests that wildlife crossing structures can mitigate the barrier effect of roads on wildlife movement, but in many cases have been poorly implemented or evaluated. The most supported measures were the addition of ledges and vegetation cover to increase movement for small mammals; underpasses to prevent the decline in movement of ungulates following road construction; and improving road‐crossing for arboreal mammals using canopy bridges and vegetated medians. We strongly recommend that future use of crossing structures closely adheres to species‐specific, best‐practice guidelines to improve implementation and be paired with a thorough evaluation that includes benchmark comparisons, particularly for measures and species that lack sufficient evidence (e.g. invertebrates, amphibians, reptiles, birds, and overpasses).

Crossings and collisions – Exploring how roe deer navigate the road network

“…A common strategy to mitigate the impacts of roads on wildlife is to install wildlife crossing structures, i.e., structures designed to allow movement of an animal from one side of a road to another without travelling across the road surface, ultimately promoting or restoring movement across the landscapes (Smith et al 2015). We argue, that installing crossing structures in areas of high WVC risk only addresses one part of the problem and ignores the issue of barrier effects all together (Soanes et al 2024;Zimmermann Teixeira et al 2017). Our approach to compare WVC risk to crossing activity is an appropriate tool to inform the correct implementation of effective measures to reduce WVC and enhance landscape connectivity, especially when demographic data of individuals involved in WVC, or when GPS-collar data across multiple populations, are available.…”

Anthropogenic, environmental and temporal associations with vertebrate road mortality in a wildland–urban interface of a biodiverse desert ecoregion

“…An understanding of when, where and why wildlife use roads can guide mitigation planning for management activities, identify spatio-temporal patterns and inform conservation strategies [ 29 , 33 , 53 , 54 , 116 ]. Wildlife–vehicle collision mitigation and road effect reduction strategies are discussed elsewhere [ 2 , 54 , 117 ], but we note that some roadkill mitigation measures (e.g. overpasses) have been initially effective in the Sonoran Desert ecoregion [ 117 , 118 ].…”

“…Wildlife–vehicle collision mitigation and road effect reduction strategies are discussed elsewhere [ 2 , 54 , 117 ], but we note that some roadkill mitigation measures (e.g. overpasses) have been initially effective in the Sonoran Desert ecoregion [ 117 , 118 ]. Here, we focused on anthropogenic, environmental and temporal factors associated with roadkill across biodiverse vertebrate groups in an aridland wildland–urban interface.…”

Editorial: Amphibian and reptile road ecology

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Editorial on the Research TopicAmphibian and reptile road ecology Roads have complex negative impacts on biodiversity and may threaten the persistence of wildlife populations by acting as barriers to movement or sources of increased and sometimes substantial mortality (e.g., van der Ree et al, 2015;Moore et al, 2023). Amphibians and reptiles (herpetofauna) are known to be particularly susceptible to negative road impacts (e.g., Beebee, 2013;Andrews et al, 2015).

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“…However, progress in knowledge of their effectiveness has been hampered by a lack of postmitigation research focused on individual and population-level responses to passagebarrier systems. Examples of critical knowledge gaps include quantified understandings of the proportions of individuals that successfully cross via passages or changes in population abundance over time (e.g., Soanes et al, 2024).This Research Topic aimed to increase our understanding of both the effects of roads on amphibians and reptiles and the effectiveness of mitigation structures. We sought research from a diversity of regions, landscapes, and species that addressed meaningful road ecology science questions to help inform conservation.…”