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Evaluating and elevating the role of wildlife road crossings in climate adaptation
Abstract
Beyond the well-established benefits of wildlife road crossings and associated infrastructure—improving driver safety, reducing animal mortality, reconnecting habitats—there is another important but often underappreciated benefit: supporting wildlife and ecosystems in adapting to climate change. We explore this potential by (1) synthesizing the literature surrounding climate adaptation and wildlife crossings, (2) presenting a case study on how crossings support shifting animal migrations, and (3) describing key considerations for incorporating climate information into crossing prioritizations. Among other climate-adaptive benefits, research suggests crossings can support species range shifts and protect access to resources even as drought and human development compromise that access. Our case study outlines an approach for prioritizing crossing locations most likely to support animal migration both today and into the future. By accounting for such dynamics, wildlife crossings can be a cost-effective tool that protects wildlife as well as motorists and enhances the resilience of infrastructure and ecosystems in a changing world.
In a nutshell:
- Wildlife crossings and associated infrastructure (eg fencing) are effective at reducing wildlife mortality and at connecting habitats, but their prioritization and siting rarely consider climate change
- Strategically located crossings protect access to key resources as warmer temperatures, droughts, and disturbances—along with expanding human land use—compromise that access
- Examining where animals are moving today and predicting where they may move under changing climatic conditions and land use can indicate potential crossing locations that support wildlife now and into the future
- Climate-informed wildlife crossings can be a cost-effective, win–win solution that leads to greater resilience for wildlife and for the infrastructure itself
The conservation community has long understood that roads—including 4.17-million miles in the US alone (USDOT 2021)—can imperil and impede wildlife and reduce landscape permeability (Forman et al. 2003; Torres et al. 2016). To date, however, investments in wildlife–highway mitigation measures—including crossings over and under roads, fencing, escape ramps, and wildlife guards—have largely been motivated by reducing costly wildlife–vehicle collisions and improving driver safety, or, in the case of underpasses and culverts, by managing surface water. Benefits to wildlife are often a secondary consideration (Newell et al. 2022).
However, this has begun to change, as several high-profile wildlife crossings and countless others have generated ample evidence that such investments benefit wildlife and ecosystems (Figure 1). Wildlife crossings and associated infrastructure like fencing (hereafter, collectively termed “wildlife crossings”) can reduce wildlife mortality by over 80% (Rytwinski et al. 2016); mitigate genetic isolation and population fragmentation (Sawaya et al. 2019); and protect access to breeding grounds, seasonal resources, and migratory corridors (Seidler et al. 2015), among other benefits. Now, construction of the largest wildlife crossing structure on the planet is underway in California's Santa Monica Mountains (Figure 1f), a project largely motivated by mountain lion (Puma concolor) conservation and designed as a holistic ecosystem crossing. There and across the globe, the needs of wildlife are increasingly driving wildlife crossing prioritizations and investments, a trend that is likely to continue given unprecedented federal funding and policy commitments, as in the US (Panel 1).
Yet, beyond the substantial benefits noted above that wildlife crossings offer to wildlife and society, there is another important and related but often underappreciated benefit: the potential to support wildlife and ecosystems in adapting to climate change. As effective as wildlife crossings can be, their siting and design too often fail to account for climatic changes and impacts, and there remains considerable opportunity for a more holistic integration of landscape-scale, climate-adaptive considerations in planning (Lister et al. 2015; Newell et al. 2022). In addition, climate change is rarely explicitly addressed in research on wildlife crossings and road ecology (eg via including climate projections in movement models). Of course, other concurrent efforts that stem habitat loss and fragmentation (like expanding reserves, protecting corridors, and restoring habitat) are imperative for enabling wildlife to adapt to a changing world (McLaughlin et al. 2022). But as a tangible, proven solution, wildlife crossings warrant a closer look as a tool that belongs in the climate adaptation toolbox.
We seek to address this gap and to advance the evidence of and opportunities for climate-informed wildlife crossings in three ways. First, we review and draw linkages between the literature on climate adaptation, connectivity, wildlife crossings, and road ecology to identify how wildlife crossings can provide important adaptation benefits. Second, we present an elk (Cervus canadensis) migration case study comparing the relative importance of proposed crossing structures under climate change, which underscores the importance of incorporating climate-change considerations into wildlife crossings. Finally, we describe key considerations for successfully advancing climate-informed wildlife crossings.
The climate adaptation potential of wildlife crossings: lessons from the literature
Enhancing connectivity is among the most frequently cited climate adaptation strategies for protecting biodiversity (McLaughlin et al. 2022). By increasing the effective area of accessible habitat and suitable climatic conditions and by counteracting the isolation of such patches, wildlife crossings represent an important tactic for pursuing this strategy. Unintended consequences of wildlife crossings may arise in some contexts—for example, by enabling the spread of invasive species or pathogens (Resasco et al. 2014; Rayl et al. 2021), introducing novel competitors that hinder adaptive responses (Alexander et al. 2015), or even entombing animals or hindering passage (especially of aquatic species) via poorly maintained culverts (Lovich et al. 2011). However, such risks can be reduced through proper siting, design, and maintenance, and are outweighed by the benefits. Although many of these benefits are interrelated and mutually reinforcing, we group them by several themes below (see Appendix S1: Panel S1 for a description of how relevant literature was identified).
Individual fitness and mortality
In the face of ongoing population declines and extirpations attributable in part to climate change (Ceballos et al. 2017), reducing wildlife mortality is increasingly important. Diversion fencing is particularly critical for reducing mortality (Rytwinski et al. 2016), and crossings may also mitigate other harmful aspects of roadways that indirectly contribute to mortality or maladaptive behavior. For example, the low albedo of roads coupled with minimal shade causes localized warming, which may cause some species (eg desert tortoises [Gopherus spp]) to exceed their physiological thermal limits (Peaden et al. 2017). This risk, exacerbated by climate change, may be mitigated with culverts and natural vegetation cover. The emissions, light, and noise from roads and traffic may compel individuals to change their behavior such that they trade-off foraging for sustained vigilance or flight (Gagnon et al. 2007). As resource patterns shift under climate change, mitigating the negative impacts of roads and traffic is especially important for individuals to maintain adaptive behaviors (Aikens et al. 2020).
Population dynamics and gene flow
By reducing isolation among populations, wildlife crossings may enable adaptive population dynamics to unfold under changing conditions. Wildlife crossings can support gene flow and counteract genetic isolation, which may enable climate-adaptive genotypes to propagate within and between populations (Sawaya et al. 2019). For species with temperature-dependent sex determination, warming temperatures may contribute to sex-biasing (Roberts et al. 2023), which can be compounded by the disproportionate dispersal or road mortality of one sex (Moore et al. 2023); crossings can prevent or forestall such biasing. Finally, mitigating the impacts of roads may be particularly important for meta-populations, especially at the “trailing edge” of ranges and where diminishing habitat quality may shift subpopulations into sinks (Aiello et al. 2023).
Species interactions
The effects of climate change on wildlife interactions are complex and uncertain (Bastille-Rousseau et al. 2018), but predators and species with large ranges and resource requirements will likely be impacted more than prey and species with small ranges (Parsons et al. 2022). How wildlife crossings affect predator–prey dynamics also varies by context and taxa, with examples of crossings either benefiting predators (by concentrating prey) or benefiting prey (by facilitating evasion/escape), as well as examples of predators and prey using crossings both independently and in response to one another (Caldwell and Klip 2020; Mata et al. 2020). However, crossing structures clearly affect the movements of both predators and prey and thus influence the broader predation landscape (Sabal et al. 2021). Although our understanding of the presence, direction, and magnitude of the effects of wildlife crossings on predator–prey dynamics remains incomplete, allowing ample space for these interactions to unfold and evolve will be important for protecting key ecological processes under changing conditions.
Habitat and resource access
Wildlife crossings can protect access to high-quality habitat and resources such as forage—especially as the abundance and timing of those resources shift or become unreliable (eg due to drought). Many ungulates, for instance, respond to clear environmental cues to initiate migration and track available forage; as those cues change, there is a wide spectrum of migratory plasticity across and within species (Xu et al. 2021). These varied responses highlight the importance of reducing barriers (eg with wildlife crossings) along both present-day migration routes and where animals are projected to move in the future (see case study below). Aquatic species that maintain high fidelity to breeding grounds (eg natal rivers of salmonids) may be particularly vulnerable to changing conditions that result in warmer waters and reduced or disconnected flows—risks that can be reduced through the use of appropriately sized culverts and natural cover along waterways (Wilhere et al. 2017). Species that are more flexible in their movement patterns may increase risk-taking behavior (eg crossing roads) as resources become more limited or are otherwise altered by disturbances such as wildfire (Blakey et al. 2022), which again underscores the importance of ensuring safe passage over and under roadways.
Range shifts
As they have done during past periods of global change, species are already moving to track suitable climatic conditions (Chen et al. 2011). However, contemporary changes are outpacing the dispersal abilities of many species (Schloss et al. 2012; Dobrowski et al. 2021), while ongoing habitat loss and climate impacts (eg sea-level rise, reduced snowpack) may compromise or literally erode existing corridors and connections (McKelvey et al. 2011; Leonard et al. 2017). Reconnecting landscapes and restoring climate connectivity (via wildlife crossings, for instance) is thereby crucial to facilitating successful range shifts (McGuire et al. 2016; Parks et al. 2020). The importance of connectivity in enabling range shifts highlights the need to explicitly consider projections of where species may move in planning connectivity-enhancing investments (Littlefield et al. 2019), which we explore further in the case study discussed below.
Ecosystem functioning and resilience
Wildlife movement is declining globally (Tucker et al. 2018), which can undermine the processes that not only shape communities and trophic structures but also underpin ecosystem functioning (Bauer and Hoye 2014). Therefore, the benefits of supporting animal movement extend to ecosystem resilience. For example, mounting evidence shows that seed dispersal by animals is helping plants track suitable climatic conditions (Nuñez et al. 2023), thereby enabling ecological communities to shift and reshuffle. Although the net carbon storage/sequestration potential of wildlife remains unresolved (Cromsigt et al. 2018; Malhi et al. 2022), animals’ contributions to flows of matter and energy within ecosystems and across landscapes are undisputed and underscore the importance of enabling their free movement. In aquatic systems, well-designed culverts and underpasses contribute to hydrologic connectivity that maintains aquatic food webs and can enable the inland migration of some coastal ecosystems like salt marshes (Perry et al. 2022). These resilience-enhancing benefits are likely to persist even if an initial target species no longer uses a particular crossing structure or passage. Thus, infrastructure that is properly located, designed, and maintained can represent a strategic investment that supports myriad ecological processes.
Capturing present-day and future movements with wildlife crossings: a case study
Realizing the substantial climate-adaptive benefits of wildlife crossings requires incorporating climate considerations directly into infrastructure planning. Here we present a case study that (1) highlights the critical need to account for climate-driven changes in movement and habitat use and (2) provides guidance on how to incorporate such changes into the siting of new crossing structures. Our case study focuses on elk functional connectivity—that is, the degree to which the landscape facilitates or impedes elk movement (Tischendorf and Fahrig 2000)—during migration in southwestern Colorado and northwestern New Mexico, both now and in the future. Many elk populations migrate between low-elevation winter ranges and high-elevation summer ranges to avoid deep snow and track seasonal changes in forage availability. However, as noted above, climatic changes, roads, and intensifying human land use can threaten seasonal migrations by elk and other ungulates and affect the amount and location of suitable habitat (Rivrud et al. 2019; Aikens et al. 2020).
In 2019, the Colorado Department of Transportation (CDOT) and Colorado Parks and Wildlife (CPW) released the Western Slope Wildlife Prioritization Study (WSWPS) (Kintsch et al. 2019), which identifies highway segments where new crossing structures would be most effective at reducing wildlife–vehicle collisions and improving connectivity based primarily on data from CDOT and CPW. However, the WSWPS did not explicitly consider how climate change will impact migratory connectivity and thus which crossings are most likely to support that connectivity into the future. To address this gap, we used global positioning system (GPS) locations from 43 elk collared by Southern Ute Tribe biologists between 2013 and 2021 (Appendix S1: Figure S1) to model migratory connectivity under present and future (2050) conditions and to evaluate the potential crossing locations identified in the WSWPS in light of those changing conditions (Appendix S1: Panel S2). We used resource selection functions (RSFs; Muff et al. 2020; Fieberg et al. 2021) to quantify elk habitat selection and suitability on winter and summer ranges, comparing either winter or summer range used locations (represented by 8-hr elk GPS points) to random locations from across an animal's entire range (ie winter range, summer range, and areas in between). We then quantified landscape resistance to movement while migrating using integrated step selection analysis (iSSA; Avgar et al. 2016), comparing used movement steps (based on 2-hr elk locations) to available steps along each elk's migration route. We drew on climate variables, projected changes in road traffic, and a plausible scenario of future changes in land cover/use (ie development, forest cover; Appendix S1: Panel S2) to map elk winter and summer range habitat. We then used a circuit theory-based approach (McRae et al. 2008) to model functional connectivity between these seasonal ranges in the present and future. Finally, we quantified the relative connectivity value for each WSWPS potential crossing location to determine which crossings would experience the most migratory movement by elk under present and future conditions. We note that the elk under study here use only a portion of the area considered in the WSWPS and are one of several species that would potentially benefit from new crossing structures in the region. Extending this analysis to consider other relevant species, such as mule deer (Odocoileus hemionus) and carnivores, and considering additional scenarios for future land cover/use, would be valuable avenues for further research.
We used k-fold cross validation to confirm that top RSF and iSSA models had strong predictive power (all k-fold values between 0.66 and 0.99; Appendix S1: Panel S2). The strongest drivers of elk habitat selection on winter range were climatic, with elk strongly avoiding areas of high snowfall and selecting for relatively high minimum temperatures (Figure 2a; Appendix S1: Figure S3, a and b). Climate and landscape modification were both important factors affecting elk habitat selection on summer range, highlighting the potential compounding stressors of ongoing climate change and development (Figure 2b). In summer, elk selected for areas of moderate-to-high precipitation (Appendix S1: Figure S3c) while avoiding oil and gas fields and areas close to development (Appendix S1: Figure S3d).
Our models predicted that availability of high-suitability winter range (defined as pixels with RSF values in the top 20% relative to all present-day pixels; Appendix S1: Panel S2) will remain relatively stable through 2050 (Figure 3d), and will even expand slightly as snowpacks decline. By contrast, high-suitability summer range will contract by half (49%) as summer precipitation decreases and as human development continues to expand (Figure 3e). Our top migratory iSSA, which was used to quantify landscape resistance to movement under present and future conditions, indicated that migrating elk avoided crossing highways as traffic volumes increased (Figure 2c) and avoided areas closest to development (Appendix S1: Figure S3e), suggesting that resistance along elk migratory routes will also shift between now and 2050 as development and highway traffic increase.
The combination of climate-driven changes in summer range availability and altered landscape permeability along migration routes led to substantial differences in predicted migratory connectivity between the present and 2050 (Figure 4a; Appendix S1: Figure S4) and thus changes in where elk may cross highways. This in turn affected the relative connectivity value of crossing locations proposed by the WSWPS. The top ten crossing locations are all located in the center of the study area both now and in the future, but the highest-value locations shift eastward under future conditions as predicted movement routes between seasonal ranges change (see Figure 4, b and c). This shift in the relative importance of crossing locations highlights the critical need to consider future climatic and landscape conditions when siting new wildlife crossings and further suggests a method for prioritizing infrastructure investments that will have both immediate and long-term benefits. For example, our models identify four proposed crossing locations that are predicted to be of greatest value to reduce wildlife–vehicle collisions and enhance elk migratory connectivity both now and in the future, thereby providing potential targets for initial climate-informed investments.
Crucially, consideration of future climate-driven movements must not eclipse the very real needs for reducing wildlife–vehicle collisions and enhancing connectivity today—hence our approach of identifying sites where present and future benefits coincide. Securing funding for crossing infrastructure is most likely to succeed when there is alignment with existing transportation infrastructure plans and when benefits are likely to be immediately realized (Elton and Drescher 2019), even as we strive for more integrated, long-term, and landscape-scale approaches to planning (see below; Newell et al. 2022). In addition, there are inherent uncertainties to any models of future dynamics (eg migratory movement) that rely on projections and anticipated rates of change in climatic conditions, vegetation cover, and human land use (Sohl et al. 2016). Accordingly, planners may elect to use models of future movement as an additional criterion for refining a set of prospective crossing locations that are otherwise based on present-day needs (eg wildlife–vehicle collision hotspots), feasibility, and other considerations (eg adjacency or proximity to protected lands).
Advancing climate-informed wildlife crossings: key considerations
Jointly, our case study and the literature reviewed above illustrate the potential climate-adaptive benefits of wildlife crossings and highlight the need to incorporate climate-change information into their siting and development. Successfully translating this science-based premise into practice requires several key considerations, as discussed in the following sections.
Planning for the long term at the landscape scale
Embedding climate adaptation principles in criteria for funding mechanisms (eg Panel 1) incentivizes comprehensive crossing systems (eg Figure 1, a and b) that not only address wildlife movement across entire landscapes and through time but also include structures, fencing, and adjacent land protections. Establishing clear methodologies for incorporating wildlife movement into short- and long-term transportation planning and resiliency programs (eg funding for culvert replacements to accommodate increased flooding) may provide cost-effective, win–win solutions for both wildlife and drivers while enhancing infrastructure resilience. More broadly, further fragmentation of intact habitat and corridors (eg by new road construction) should be avoided whenever possible, as reduced landscape permeability will compromise species ability to adapt to changing conditions (McGuire et al. 2016; Parks et al. 2020).
Promoting equitable engagement, cross-jurisdictional coordination, and interdisciplinary expertise
Engaging Tribal nations and historically marginalized communities is essential for building equitable and enduring adaptation solutions that reflect local knowledge and priorities (Brondízio et al. 2021). Beyond federal recommendations (eg 2022 White House Guidance on Indigenous Knowledge) and Tribal resources (eg Tribal Adaptation Menu Team 2019), greater coordination across jurisdictions and agencies like state departments of transportation and wildlife could support the exchange of data and resources, integration across agency plans (eg state wildlife action plans), and ensure mutual accountability and buy-in.
Stimulating innovation and actionable science
Government and private funds should reward strategic innovation in less carbon-intensive materials and construction and in novel designs likely to be durable and resilient under changing conditions (Lister et al. 2015). Emphasizing societal cost-savings from climate-informed wildlife crossings—and, conversely, the costs of inaction—may accelerate their uptake. Finally, sustained support for accessible, actionable science and boundary-spanning organizations (eg US Geological Survey Climate Adaptation Science Centers) will be important for ensuring incorporation of the best available science in wildlife crossing decision making.
Conclusions
Even as the science advances, uncertainties remain. Precisely where and when will wildlife species move under changing conditions? Where will humans move, with implications for transportation infrastructure and traffic? Does a single large crossing investment (eg Figure 1f) yield greater net benefits than smaller, distributed investments (Helldin 2022)? How can crossing investments simultaneously accommodate the needs of numerous wildlife species—including aquatic taxa—that may respond individualistically to design elements (Clevenger and Waltho 2000; Denneboom et al. 2021)? The persistence of these unresolved questions does not mean action must be forestalled. Rather, ample scientific evidence shows that wildlife crossing infrastructure can provide myriad climate-adaptive benefits to wildlife and ecosystems while improving human safety. Explicitly considering climate-change information can ensure that maximal benefits are realized while fortifying crossing investments so that they can support wildlife movement today and into the future. Such climate-informed wildlife crossings likely represent one of the most cost-effective, tangible solutions for jointly protecting wildlife, ensuring driver safety, and enhancing the resilience of infrastructure and ecosystems in our interconnected and rapidly changing world.
Panel 1. Federal funding and policies with relevance for climate-informed wildlife crossings, embedded in a global context
The 2021 US Infrastructure Investment and Jobs Act (IIJA), also known as the Bipartisan Infrastructure Law (Pub L 117-58), invests over a half-trillion dollars in US infrastructure. The Act includes $350 million in dedicated Wildlife Crossings Pilot Program funding—managed by the US Federal Highway Administration—for projects to reduce wildlife–vehicle collisions while improving habitat connectivity (23 USC § 171) and expands or maintains eligibility for wildlife-friendly infrastructure through multiple programs. Notably, also under the Act, the Promoting Resilient Operations for Transformative, Efficient, and Cost-saving Transportation (PROTECT) Discretionary Grants Program seeks to improve infrastructure resilience to climate change, extreme weather, and sea-level rise by paying for “natural infrastructure” and “protective features” (eg larger culverts, longer bridges) that offer co-benefits to aquatic and terrestrial species through improved habitat connectivity (23 USC § 176).
Beyond the IIJA, the US federal government has elevated the importance of habitat connectivity, corridors, and crossings in other policies and initiatives, including the 2020 action plan of the US House Select Committee on the Climate Crisis, which recognizes wildlife crossings as an important climate adaptation tool. More broadly, habitat connectivity is a key pillar in the Biden Administration's “America the Beautiful” conservation agenda. In 2020, the Council on Environmental Quality convened an interagency habitat connectivity working group and produced guidance directing federal agencies to coordinate addressing the impacts of transportation on connectivity. In addition, the Department of the Interior has invested in numerous wildlife crossing projects through the National Fish and Wildlife Foundation Big Game Seasonal Habitat and Migration Corridors Fund, which serves to implement Secretarial Order 3362 “Improving Habitat Quality in Western Big-Game Winter Range and Migration Corridors”.
Collectively, these landmark funding opportunities and policies empower federal, Tribal, state, and local decision makers to invest in innovative solutions that protect motorists and wildlife, reconnect habitats, and save taxpayer dollars by improving infrastructure resilience. This momentum in the US is echoed across the globe in international initiatives for advancing wildlife–highway mitigation and safeguarding connectivity for wildlife. For example, the 2022 Kunming-Montreal Global Biodiversity Framework elevates the importance and focus on connectivity relative to the 2010 Aichi Biodiversity Targets and is accompanied by a monitoring framework that includes key indicators for connectivity. Guidance set forth by international bodies includes the International Union for Conservation of Nature (IUCN) report detailing strategies for supporting wildlife during development of linear transport infrastructure (ie roads, railways, canals; Ament et al. 2023), the IUCN's guidance on conservation connectivity more broadly (Hilty et al. 2020), and guidelines from the Bonn Convention for addressing linear infrastructure impacts on large migratory mammals (Wingard et al. 2014). Large-scale, coordinated efforts and alliances include the Global Tiger Initiative, which has advanced green infrastructure guidelines for countries within the range of the tiger (Panthera tigris) (Quintero et al. 2010); Tanzania's comprehensive action plan for wildlife corridors (Penrod et al. 2022); and the Jaguar Corridor Initiative, which spans the range of the jaguar (Panthera onca) across Central and South America and expressly addresses climate change (Zeller et al. 2013).
Open Research
Data Availability Statement
Data used in our analyses are available on figshare (https://doi.org/10.6084/m9.figshare.26090950.v1) and code files are available on Zenodo (https://doi.org/10.5281/zenodo.13240130).
