Introduction

In the past few decades, the Indian growth story has become one to reckon with. Counted among the fastest growing developing nations of the world, the country’s efforts to achieve economic and social development are accelerating at a fast pace. The surge in economic development in the past few decades after the economic liberalisation in the early 90s has been partly fuelled by an aggressive push for infrastructure growth. Transportation linear infrastructure (LI), particularly roads and railway lines, figures among the top focal areas of the government to achieve this growth. Having overtaken Japan as the world’s third largest automotive market with more than a 500% jump in vehicle registrations in the past two decades (Balasubramanian, 2023), the road transport sector in India is showing no sign of slowing down. Additionally, railway line electrification and rail network expansion are being primed as lifelines to connect people, as well as help transport goods and key resources such as minerals and metals important for industry. Ambitious government projects such as those that seek to enhance accessibility and the Bharatmala project that seeks to optimize the efficiency of the movement of goods and people across the country (PIB, 2024), seem to be at the centre of the Indian growth story.

However, while these plans are working to transport India to greater economic self-sufficiency, the natural wealth of the country is at the receiving end. The country is rife with examples of busy roads and railway lines cutting through prime wildlife habitats and corridors, aggravating the impacts of human-presence on wildlife in these regions and hampering connectivity between habitats. While consolidated estimates of wildlife mortality on roads and railway lines in India are lacking, it is well established that such mortalities are highest in well-connected landscapes and Protected Areas (PAs) (Grilo et al., 2011). Further, such mortality in wildlife corridors can lead to the loss of ‘fit’ individuals of a population (Bujoczek et al., 2011) – a huge loss for the genetic pool of already endangered species. The problem is compounded by the prevalent LI planning framework that puts conservation concerns on the backburner while deciding LI design and alignments. Here, we delve into the present scenario of LI development in India as it pertains to its repercussions for long-term wildlife and landscape conservation. We also analyse the growth of transportation ecology and the science of mitigation planning worldwide and in India, concluding with future directions in the areas of research, developer initiatives and policy. Thus, our analysis aims to be a starting point for a concerted effort to achieve LI growth that is wildlife-friendly and sustainable.

Biodiversity conservation in India: beyond protected areas

Counted among the world’s most biodiverse regions, India boasts a rich array of natural landscapes, ecosystems and habitats. Despite being the most populous country in the world today, India is home to the highest number of large terrestrial carnivores (average body weight > 15 kg) in the world (Johnsingh, 1986), in addition to highly diverse aquatic, herpetofaunal, avian, mammalian, floral and invertebrate assemblages. The wildlife in the country majorly survives within a vast network of PAs, harbouring rich tropical, marine, montane and grassland ecosystems in almost pristine conditions. For example, the one-horned rhinoceros Rhinoceros unicornis is known to thrive in protected spaces like the Kaziranga, Dudhwa and Manas National Parks. On the other hand, some wildlife species have historically thrived in landscapes with low level of human activity or have adapted to life in multi-use landscapes. For example, about 30% of tigers Panthera tigris and 80% of Asian elephants Elephas maximus, live outside PAs (Jhala et al., 2021; Kshettry et al., 2020) Leopards Panthera pardus are famously known to be adaptable to living near human settlements (Surve et al., 2022), and the great Indian bustard Ardeotis nigriceps and the Indian wolf Canis lupus pallipes have co-evolved to live alongside low intensity agriculture and pastoralism (Dutta and Jhala, 2021; Jhala and Giles, 1991).

Spaces outside PAs have another pivotal role – to keep PAs and habitat networks connected. Currently about 5.43% of the country’s geographical area is legally protected under 1022 PAs https://wiienvis.nic.in/Database/Protected_Area_
854.aspx) with different levels of protection. However, only a few of these PAs are large enough to sustain wildlife populations and ecosystems for the long term by themselves. For example, a majority (32) of the 55 tiger reserves in the country have inviolate core areas smaller than the minimum 800 km² required to sustain a tiger population of 80-100 individuals that is viable in the long-term (https://ntca.gov.in/tiger-reserves/#-tiger-reserves-2). Similarly, elephant reserves are declared for Asian elephant conservation in India (https://wiienvis.nic.in/Database/eri_8226.aspx); however, these account for a mere 40% of the total elephant range and are not sufficient to contain the Indian elephant population.

Migratory and dispersal pathways, also called corridors, are important for facilitating wildlife movement across PAs, but often lie outside the PA network and are already under immense anthropogenic pressures. The few studies on species-specific corridors mapped for certain landscapes or across the country, including those for tigers and elephants (Habib et al., 2021a; Qureshi et al., 2014; WTI, 2017), have revealed rapid shrinkage of these corridors. Despite this shrinkage, certain non-forest and moderately-modified landscapes facilitate wildlife movement between habitat patches, regularly used by species such as tigers, leopards and wolves (Habib et al., 2021b; Hussain et al., 2022), and as such form connecting links between populations. Such land uses also account for 40.67% of the elephant range in India (Padalia et al., 2019). Rapid development that does not take into account the role of corridors and such land use matrices in maintaining ecological connectivity will have repercussions for the long term viability of increasingly isolated wildlife populations, and may threaten species persistence in the face of climate change. Thus, it is not only the legally protected wilderness areas, but also these unique spaces outside PAs and the mosaics of natural-human modified spaces that require conservation action to maintain their ecosystem function.

Corridors at crossroads

The ongoing threats to India’s biodiversity are compounded by the intrusion of LI networks in sensitive wildlife habitats as well as designated ecological corridors. Fast-moving vehicular and train traffic on a road or railway track passing through a wildlife habitat or corridor is like a death trap for unsuspecting animals. Depending on the behavioural and cognitive abilities of wildlife species (Jacobson et al., 2016), animals may continue to collide with vehicles and trains over time, or learn to avoid these areas at certain times of the day or entirely. Unfortunately, both responses can lead to population declines (Fahrig and Rytwinski, 2009). Such declines can be caused by the loss of individuals in a population by collision-related mortality. While high animal mortality is expected inside protected habitats because of high local animal abundances (Grilo et al., 2011), the mortality of the occasional ‘fit’ disperser on a road or railway line passing through a corridor can be extremely detrimental to the local gene pool (Bujoczek et al., 2011). Secondly, LI can cause the separation of once contiguous regional wildlife populations because of the avoidance of habitats adjacent to road and railway line clearings. This can happen through multiple pathways, such as by avoidance of the physical road surface, avoidance of traffic emissions such as noise, and avoidance of cars and vehicular traffic (D’Amico et al., 2015). Roads and railway lines also decrease the quality of the habitat through which they pass, the effects percolating a few kilometres inside the habitat (Forman, 2000). Light, sound and air pollutants can leach into natural systems, cause disturbance to wildlife, interfere with inter- and intra-species communication, and add to the barrier already created by the physical road and railway track (Silva Lucas et al., 2017; van der Ree et al., 2015). The situation is compounded by the creation of smaller habitat fragments by road networks, more than half of which are < 1 km² in size (Ibisch et al., 2016) that are unsuitable for sustaining wildlife populations. Further, secondary development such as human settlements and establishments following the construction of a road or railway track can create more disturbances and reinforce the barrier to animal movement.

Nature, and the complex web of ecological interactions within it, relies on a delicate balance. Imbalances in these processes introduced through human activities can have adverse fallouts. For example, the unnatural removal of top predators from an ecosystem can lead to the proliferation of herbivores that would lead to unregulated grazing and trophic cascades (Kuijper et al., 2016). Excessive unnatural removal of herbivores or prey species can lead predators to switch to domestic prey for survival, which would, in turn, increase conflict with human communities living near wildlife habitats. Tipping the scale towards natural imbalance would also deprive us of ecosystem services originating from wilderness areas, such as the provision of water, carbon sequestration, maintenance of mineral and hydrological cycles, and provision of valuable resources to forest-dependent communities. There are several other repercussions of human intrusion in natural spaces that we do not yet fully comprehend, but anticipate that it would be to the detriment of natural and human well-being in the long run.

The development of transportation LI sectors in sensitive landscapes is one such activity that is in direct conflict with wildlife and nature conservation through its myriad direct, indirect and cumulative effects. It would be unfortunate if the path to India’s development was paved with severe repercussions for India’s wildlife and ecosystems. Thus, maintaining the current Indian growth trajectory with India’s place in the world as one of the most biodiverse regions is a humongous yet responsible task that requires small steps in ingenuity and innovation, backed by rigorous science.

Balancing infrastructure growth and conservation: inspirations and pathways

The mitigation hierarchy is a four-step framework that aims to manage biodiversity and ecosystems while balancing conservation with developmental needs (CEQ, 2000). The four steps are: Avoidance, Minimisation, Restoration, and Offsetting. With regard to the primary and foremost action in the hierarchy i.e., ‘Avoidance’, the Indian government has taken active steps to regulate any new infrastructure development within legally protected areas and designated wildlife and ecological corridors (MoRTH, 2019). However, most of this infrastructure development is expected to occur in rural areas where many development schemes of the government are focused. These are also the regions where humans and wildlife live in close proximity, in very elastic and open interfaces. Human activities and land uses like agriculture and settlements are often used equally if not more by some wildlife species that reside in fragmented but useful forest patches, largely cohabiting with each other in shared spaces (Jhala & Giles, 1991; Mahajan et al., 2021). It is in these areas that the construction of infrastructure such as roads and railway lines pose a threat to the delicate balance of coexistence, and threaten the role of these rural landscapes as habitats and permeable corridors (Hussain et al., 2022).

Experiences from other parts of the world have shown that building bridge-like structures (underpasses, overpasses, culverts, via-ducts, canopy bridges) under and over the ‘grade’ of the road or railway line (Smith et al., 2015), and fencing to deter wild animals from accessing the road or railway line (Donaldson & Elliott, 2021) can help reduce the impacts of roads and railway lines. These measures can help save human lives as well since collisions with large animals such as elephants, deer like sambar and spotted deer, and wild pigs can be fatal for motorists and passengers. Further, technological solutions available today, including animal detection and early warning systems that alarm and alert wild animals near the road or railway line, and motorists, respectively, can be useful in averting possibly fatal collisions. Mitigation measures for other species groups, such as herpetofauna and birds, are specific to their ecological needs. The applicability of each of these measures can vary with the road or railway line project, landscape and target animal communities. Given the long life of transportation infrastructure and the large ecological footprint they have, the choice of mitigation measures requires careful research and understanding of the ecosystem, animal community and species most likely affected.

The field of road, railway and general LI ecology that forms the basis of wildlife mitigation measures gained traction prominently in the 1990s, with recognition of impacts and contemplation of solutions to this fast-growing problem (Forman & Deblinger, 1998; Forman et al., 1997; Forman & Alexander, 1998). Around the same time, Canada became a world leader in mitigating the impacts of roads on wildlife by constructing 38 wildlife underpasses, 6 overpasses and exclusion fencing when the Trans-Canada Highway (TCH) passing through the Banff National Park was being widened between 1981 and 1996. Over 20 years old today, the wildlife crossing structures along the TCH are the highest in number along a single highway in the world, and the most diverse in terms of crossing structure types (Parks Canada Agency, 2022). The crossing structures are also the subject of the world’s longest running wildlife mitigation monitoring programme, making the passages the most studied crossing structures in the world. Long-term monitoring of the crossing structures have revealed that these measures reduced wildlife-vehicle collisions by up to 80% (Clevenger et al., 2001), and that the crossings allowed sufficient gene flow to prevent genetic isolation (Sawaya et al., 2014). The research also shed light on the structural preferences of different species and species guilds (Clevenger and Waltho, 2005), that informed mitigation measures in other parts of the world.

These findings have also helped convince decision-makers that crossing structures work for wildlife, and have inspired the construction of several crossing structures around the world. For example, Malaysia took the lead on green highways in Southeast Asia by incorporating wildlife passages into highway construction and upgradation projects. Crossing structures and viaducts on highways in Malaysia today range from 80 – 900 m in width, and have even been constructed on an existing highway where no upgradation was planned (Wan Nordin et al., 2020).

The beginning of the road for LI ecology in India

In India, while the recognition of the impacts of the increasing network of LI was still growing in the past decade, we were still in the infancy of understanding the direct impacts of LI on wildlife species found in the subcontinent (Rajvanshi et al., 2001), and policy aspects of LI in ecologically-sensitive areas (Raman, 2011). The general understanding was that wide roads with fast-moving and heavy traffic would deter wildlife movement and cause wildlife mortality. Faced with a rampage of projects that threatened severe wildlife connectivity in the past few decades, we turned to the vast knowledge of road and railway ecology in the developed world. Images of the great animal crossing structures on the Trans-Canada Highway on the Banff National Park and eco-ducts on highways in Peninsular Malaysia inspired strategies to mitigate LI impacts in our country.

One of the landmark projects that paved the way for mandatory wildlife mitigation measures on LI in India was the improvement of the National Highway 44 (NH 44, earlier called NH 7). NH 44 is the longest highway in India, connecting 11 Indian states and several important urban and agricultural centres. On its way north from Nagpur (Maharashtra) towards Seoni (Madhya Pradesh) in central India, the highway passes through the Pench Tiger Reserves in Maharashtra and Madhya Pradesh states (together called Pench hereafter), and surrounding forests. Pench is a fabled tiger haven, and currently holds one of the most important source populations of tigers in the Central Indian Landscape (Qureshi et al., 2014). Dispersing tigers from the reserve make their way across the majorly agrarian landscape with forest tracts towards other tiger habitats, and help revive wildlife and forests there. Such forest tracts originating from the eastern Pench form part of the corridors connecting Pench to other tiger habitats, most importantly Kanha Tiger Reserve in Madhya Pradesh and Navegaon Nagzira Tiger Reserve in Maharashtra. The erstwhile 2-lane NH 44 with rapidly increasing traffic, was also aligned along the eastern boundary of Pench, threatening the links between Pench and other tiger habitats in the landscape.

In view of the grave danger to the permeability of the corridors originating at Pench because of the expansion of NH 44 and the possible rise in animal mortality because of a further increase in traffic, the project gained extensive public and legal attention during the mid-2010s. Multiple civil society organisations and conservation NGOs came together to demand mitigation measures for the highway through litigation. Initially, the Central Empowered Committee (CEC) of the Supreme Court of India recommended using an alternate route, that would impair a different tiger corridor (Habib et al., 2015). However, in a fait accompli situation on the existing NH 7 resulting from the prevalent piecemeal approach to LI development, following this approach would funnel traffic from already upgraded sections into a narrower space (alignment inside the forest) creating a greater barrier to animal movement (Habib et al., 2016). Consequently, the National Highway Authority of India (NHAI) which was incharge of the upgradation project, was required to construct mitigation measures on the highway section passing through the tiger reserves and corridors. Finally, the crossing structures on NH 44 were ready to be used by the wildlife of Pench after multiple field visits, site assessments, technical reports, and deliberations involving an army of conservationists, decision-makers and highway officials. The 60 km stretch of the 4-laned highway now has 13 dedicated wildlife crossing structures between Chorbahuli village in Maharashtra and Mohgaon village in Madhya Pradesh, including the 1400 m long underpass on NH 44 passing through the Pench Tiger Reserve in Madhya Pradesh, the longest operational animal underpass in the world today.

The construction of the crossing structures has not only been a conservation milestone for India, but has also contributed to our knowledge of what works best for the Indian wildlife in terms of mitigation strategies on LI. The set of 9 crossing structures on the Maharashtra section of NH 44 are now the subject of a long-term monitoring programme that seeks to answer some basic questions regarding crossing structure design and use by wildlife of the Indian subcontinent (Saxena and Habib, 2022). Regular monitoring and camera traps set up under the crossing structures since early 2019 have revealed that the structures are currently being used by at least 23 wild mammals. While the learning curve varied for different species, a 240% increase in use of the structures during the first three years was observed (Habib et al., 2020). The monitoring has also helped understand why group-living prey species prefer using underpasses at sites with vegetation cover nearby, and how larger structures make it possible for different tiger individuals to use the same structure without confronting one another. The study has also indicated how constructing larger structures can help buffer the effects of humans using the crossing structures as well, a situation that is typical of and omnipresent in India.

A roadmap for the present and future

As we await more interesting and novel insights from the crossing structure monitoring exercise at Pench, mitigation measures on several road and railway line projects across the country are under construction or nearing completion. Notable among these are the 1797 crossing structures comprising underpasses and overpasses on the 700 km long Mumbai-Nagpur Samruddhi Mahamarg (Mumbai-Nagpur Expressway), including the world’s widest wildlife overpass measuring 60 m. Much earlier, crossing structures for elephants were constructed under the NH 54E passing through the Lumding Elephant Reserve, Assam (Singh et al., 2010). The Delhi-Dehradun expressway connecting the capital cities of Delhi and Uttarakhand states would soon be ready with multiple large elevated sections totalling 21 km (Pandav and Habib, 2020), that would help clear the historic bottleneck between the Rajaji Tiger Reserve and the north-western limit of the Terai Arc Landscape that extends till Haryana. It is indeed a matter of pride for India to have begun the journey of mitigating the impacts of LI quite late as compared to the rest of the world, and yet have some of the world’s largest wildlife-friendly structures on LI today.

These crossing structures offer us a unique opportunity to gather more information on the design and implementation of mitigation strategies through monitoring. This is because these are much larger than the mitigation measures present in other parts of the world (Denneboom et al., 2021). The varied geographic spread of these projects, and variability in structure types, landscapes and target faunal communities also make these projects apt for a country-wide monitoring exercise.

Moving forward, it is essential to invest in rigorous research in an adaptive management framework to keep up the momentum of LI development that proactively integrates biodiversity conservation concerns into their plans. In this direction, ecologists must build on the wealth of global road and railway line ecology research, and supplement it with the diversity of unique problems and solutions applicable to wildlife in the Indian subcontinent. Further, solutions must move towards ecosystem and multi-species-focussed strategies from those focussing on single species, considering the varied ecological needs of different ecosystems and species groups. Novel solutions require relying on traditional wildlife management techniques combined with innovative and technological approaches. We need to enhance our current understanding of the natural world with how species ecology and behaviour are changing in the ‘Anthropocene’ in response to infrastructure development. Fostering transboundary cooperation for natural landscapes traversing nation-states is also vital for the immensely biodiverse Indian subcontinent. Close coordination and consultation with LI development agencies to understand engineering perspectives while designing viable mitigation structures would help put us in good stead, and achieve realistic biodiversity conservation targets.

In this direction, we outline a few guiding research elements and areas of research required to take LI ecology research in India forward. The outputs of the research should have direct practical influence on LI and mitigation planning decisions. These include basic questions that need to be answered for such ecological research to be applicable (Roedenbeck et al., 2007; van der Ree et al., 2015), as well as questions that we think are specific to and typical of developing economies, like India. These questions may require a synthesis of different studies and preferably field-based experimental (as opposed to observational) studies.

1. What are the wildlife population-level consequences of LI-related impacts?
2. What is the relative importance of LI-related impacts on wildlife as compared to that of other human activities?
3. Under which scenarios and within which landscapes/regions should LI development be avoided completely, and what are the compelling arguments for the same?
4. How do species behaviour, ecology, energetics and interspecific interactions change in the vicinity of human infrastructure, and what consequences can it have for LI impacts (e.g., increase/decrease in roadkill rates), human- wildlife interactions, and mitigation measure effectiveness?
5. How effective are elements of existing infrastructure and mitigation structures, and how can they be modified or retrofit to be more effective?
6. With respect to mitigation measures, which impacts demand priority as a consequence of the weight of their influence on wildlife?
7. In terms of mitigation measure design and planning, can combinations and designs of mitigation measures reduce more than one impact, and for more than one species or taxa (focus on community-level instead of focal species approach)?
8. How can interactions between species and physical infrastructure be explained through a trans-disciplinary lens (e.g., ecology, engineering, design), and how can these findings influence mitigation measure design?
9. In terms of monitoring, what statistics (crossing frequency, roadkill rate) of crossing structure/mitigation measure certify its effectiveness in context of the impact it was envisioned to mitigate?
10. How can existing LI segments passing through ecologically- sensitive habitats be identified, prioritised and mitigated, including options such as rerouting existing infrastructure, if objectively necessary ?
11. What are the economics of LI development through sensitive areas in terms of socio-economic benefit, environmental and ecological cost, cost of mitigative actions, and long-term costs/benefits of incorporating (or not) ecological concerns into infrastructure planning?

We expect that focussed research with strong inferences regarding impacts and viable working solutions would help provide weightage to the conservation vs. development argument in favour of considering ecological concerns into infrastructure development and planning, which currently tips in favour of socio-economic considerations.

At the same time, we provide a blueprint for action by LI development agencies to streamline the process of integration of biodiversity conservation in development plans. These actions would not only ensure timely integration of conservation concerns into development plans, but would also reduce risks of litigation, monetary loss and delayed project implementation.

1. Initiate SEA or sectoral assessments including regional environmental impact assessments that consider the cumulative impacts of multiple infrastructure on a landscape (Saxena et al. , 2016),
2. Align multiple infrastructure in the same corridor through inter-agency cooperation, for example between road agencies that work at different levels (NHAI, PWD) but may have projects aligned nearby,
3. Include mitigation and biodiversity offset costs into project costs to avoid cost overruns later on,
4. Provide alignment alternatives to forest and conservation agencies at the planning stage, before the process of clearance and land acquisition have been initiated, and
5. Invest in research on effectiveness of mitigation measures and structural components of mitigation measures likefences, crossing structure construction material, roadlighting, automated animal detection and collision avoidance systems.

Further, collective action from multiple stakeholders can also help maintain the momentum of achieving sustainable transportation infrastructure. Some areas of action by these stakeholders can be concerted outreach efforts by non-governmental organisations (NGOs) and the media. NGOs working at national and local levels can help collate wildlife mortality data from roads and railway lines in collaboration with the relevant forest departments. Greater awareness and recognition of the impacts of LI on wildlife can help garner public support. This can also be achieved by widening the scope of environmental and wildlife science studies in educational institutes to include thematic areas addressing conservation challenges in the Anthropocene, including that of LI impacts.

More importantly, it is vital to make concerted efforts to realise policy imperatives to achieve sustainable LI development. It is important to suitably advance and adapt our environmental protection policies and frameworks to keep up with the challenge of the rapidly expanding transportation sector. Development of LI-specific guidelines within the framework of existing environmental assessment laws through intersectional research in environmental, social and wildlife policy is imperative, given the uniqueness of the problem. A step towards this direction could be the use of tools such as strategic environmental assessments of LI development plans at the sectoral and regional levels (Saxena et al., 2016).

Considering the vital link between an efficient transportation system and economic progress, the coming decade is expected to be crucial for India. The efforts we make in this decade will also dictate how we are able to sustain a thriving natural world in the face of accelerated human development. If we are to maintain the aspired quality of infrastructure, and overall human and environmental well-being, solving this crisis is imminent and would require inventive and often simple solutions. By investing in green infrastructure to reduce wildlife-vehicle collisions and maintain the continuity of natural processes like wildlife movement, the nation can ensure road and rail safety, and secure resilient natural landscapes. In other words, investing in green LI that connects habitats maintains and/or facilitates habitat connectivity can help reduce conservation costs by reducing the costs of translocating wild animals and enhancing wildlife in smaller habitats. Thus, a strong commitment to biodiversity inclusive growth that is sensitive to both economic development as well as nature conservation will truly help us achieve development that is sustainable in the long term and demonstrate the nation’s will towards responsible ecological stewardship.

Acknowledgements

The authors would like to acknowledge the support from Director, Dean and Registrar Wildlife Institute of India. Authors also acknowledge the support and guidance of former Director Dr. V. B. Mathur and Dr. Asha Rajvanshi Former Sc – G and EIA Nodal Officer, Wildlife Institute of India. Authors also acknowledge the support from other linear infrastructure agencies especially NHAI, MoRTH, MoR, CEA, Ministry of Power and other for support and relevant information.

CONFLICT OF INTEREST
The authors declare no conflict of interest.

DATA AVAILABILITY
No data was used in this research.

AUTHOR CONTRIBUTIONS
BH conceived the idea, AS wrote the first draft of the paper. Both BH and AS revised the initial draft and approved the final draft for submission.