Chapter 1: Introduction

Biodiversity is the result of 4.5 billion years of evolution and represents the biological variety of life on Earth at all its levels, from genetic and species diversity to ecosystem variability. It is a complex web of nature essential for all the living processes. In addition to its intrinsic value, it is the source of core benefits and services that humanity depends on including food, air, water, materials as well as health and wellbeing.

There is an urgent need to act decisively to address the two closely interlinked crises of climate change and nature loss. The Scottish Government declared a Climate Emergency in 2019. The 2020 Statement of Intent on Biodiversity outlined the Scottish Government’s commitment to the global drive to protect at least 30% of our land and sea for nature by 2030 (30 by 30 initiative). Scottish Ministers have also stated that the challenges facing biodiversity are as important as the challenge of combatting climate change.

The illustration below indicates the interaction between climate change and nature degradation. Climate change impacts include storms, flooding, drought, pests, disease, wildfires, and rising temperatures, while resilient ecosystems provide flood management, cooling, and carbon storage.

The mutually reinforcing link between biodiversity loss and climate change is well established. Healthy ecosystems stabilise the climate by absorbing and storing carbon dioxide but climate change is one of the key drivers of biodiversity loss. Changing temperatures, altered rainfall patterns and increased frequency of extreme weather events are devastating habitats on a local and global scale causing unprecedented shifts in species distribution and abundance.

Degradation of habitats such as forests and peatlands reduces their carbon storage capacity and potentially makes them carbon emitters, in turn exacerbating climate change and creating a negative feedback loop.

“80% of ecological processes that form the foundation for life on Earth are impacted by climate change”

International Union for Conservation of Nature (IUCN)

However, nature is also one of the best tools we have to build climate resilience. Nature-based approaches such as restoring indigenous forests and wetlands and improving land management are among the most effective strategies for mitigating the effects of climate change. Scottish National Adaptation Plan 2024-2029 (SNAP3) recognises this and includes measures to improve blue-green infrastructure, protect and enhance our natural carbon stores, and build Nature Networks.

The adoption of National Planning Framework 4 (NPF4) in 2023 marked a significant shift in Scotland's planning system, placing the twin climate and nature crises at the heart of planning policy and decision-making. NPF4 establishes climate change mitigation, adaptation and nature recovery as overarching priorities that should inform the preparation of plans and the determination of development proposals. In 2024, the Government published the Scottish Biodiversity Strategy to 2045 and announced ambitious targets for the nation to be nature-positive by 2030, and to have restored and regenerated biodiversity by 2045. Alongside the Scottish Biodiversity Strategy, Scotland’s strategic framework for biodiversity also includes six-year rolling Delivery Plans and the recently published Natural Environment (Scotland) Bill, which provides a framework for statutory nature restoration targets. The strategy is also supported by the Biodiversity Investment Plan and Natural Capital Market Framework which set out recommendations to foster value-led public, private and philanthropic investment into nature restoration. Furthermore, all public bodies in Scotland have a duty – the Biodiversity Duty (see Section 5) – to further the conservation of biodiversity when carrying out their responsibilities.

As the agency responsible for Scotland’s physical connectivity, as well as the management of one of the Scottish Government’s most valuable assets and a significant landholding (i.e. the strategic transport network), Transport Scotland must ensure its own policies and programmes align with these challenging targets. SNAP3 commits Transport Scotland to developing a biodiversity strategy by 2026. Establishing biodiversity as a strategic priority in the planning and management of Transport Scotland’s operations and activities is critical for safeguarding Scotland’s natural environment. It will also help highlight and unlock opportunities for transport sector improvements which contribute meaningfully to national climate change mitigation and adaptation objectives and targets.

Scotland’s natural capital assets make an important contribution to our prosperity. Pollinators support crop production, while woodlands, wetlands, and grasslands regulate the climate, improve air and water quality, and reduce the costs associated with drought, flood, and erosion. Recent analysis by Scottish Government summarised that natural capital annually supports over £40 billion in total economic output across a range of industries and area. The direct economic benefit of nature-based tourism in Scotland alone is estimated to be £1.4 billion each year. This is associated with 39,000 full-time equivalent jobs and annual visitor spending of more than £4 billion, accounting for nearly 40% of tourism expenditure.

The Transport Scotland Biodiversity Strategy is applicable to all areas of the agency's responsibilities, which include trunk roads, bridges, ports, airports, harbours, canals, rail networks, and ferry transportation. The quality and connectivity of ecosystems associated with the agency’s assets require urgent attention to ensure they are fit to mitigate and adapt to climate change and deliver positive outcomes for biodiversity. Transport Scotland is committed to introducing the required changes to help achieve the national outcomes and targets in this ambitious timescale.

What is Biodiversity and Why is it Important?

Biodiversity is the variability of living organisms fundamental to the processes that sustain life on Earth, including humanity. It encompasses genetic diversity within species, between species and across ecosystems and habitats in a particular place. Biodiversity holds enormous value, enriching our lives in countless ways.

We depend on biodiversity. Scotland’s natural environment, viewed in terms of natural capital assets, is valued at £230 billion and it underpins our prosperity. While a single native plant or animal species may not appear to have an obvious benefit to humankind, it may be vital to the survival of others, which we rely on for food, medicine, building materials, recreation, or climate change resilience. Pollinators are critical to crop production, invertebrates and microorganisms help to maintain soil health, while forests, wetlands and grasslands store carbon, filter pollution, improve air and water quality and reduce the risk of drought, flood and erosion. Biodiversity also positively influences our physical and mental health.

A high level of diversity in animals, plants and microorganisms at genetic, species and ecosystem levels not only allows us to thrive but it is the source of resilience, providing us with greater capacity to respond to and recover from a wide variety of disturbances caused by climate change, pests and diseases and natural disasters.

Biodiversity is a complex web of interdependencies. Loss of just one species can have a disproportionately large effect on its natural environment resulting in cascades or extinctions or the uncontrolled rise. of another species’ population to the detriment of others.

Once the balance in an ecosystem is lost, restoring it is a long and difficult process, and may not always be possible.

Various large-scale landscape restoration projects in progress in Scotland, such as Cairngorms Connect or The Great Trossachs Forest, work to a 200-year vision. There are examples of successful species reintroductions, such as white-tailed sea eagles and beavers, to areas of Scotland. However, given the challenges of habitat, species, and ecosystem restoration, concerted efforts are needed to safeguard and enhance existing biodiversity assets before further losses occur.

To tackle the direct and indirect impacts of biodiversity decline, prevention is better than cure - ecological and economic frameworks show that preventing habitat destruction and environmental degradation is exponentially more cost-effective and practical than trying to reverse or 'cure' biodiversity loss after the fact.

Transport Scotland’s Need for a Biodiversity Strategy

The Scottish Government has committed to halting biodiversity loss by 2030 and reversing the decline by 2045. The first of these targets is less than five years away and with each passing year meeting this deadline becomes more challenging. Although the Natural Environment (Scotland) Act has recently received royal assent, the associated statutory nature recovery targets have still to be established and agreed. Transformative action is needed now to prevent irreversible biodiversity loss, and the Delivery Plan forming part of this Strategy is aimed at driving this forward, whilst remaining adaptable and capable of being updated when the relevant legislation becomes official.

The edges of Scotland’s transport networks support a rich variety of flora and fauna and provide valuable habitats for a range of wildlife, including protected species of national and international importance. The quality and interconnectivity of ecosystems alongside and adjacent to the agency’s transport estate require urgent attention to ensure they deliver positive outcomes for biodiversity.

Although legislation requires that Environmental Impact Assessment is undertaken for any new developments likely to result in significant impacts on the environment, it does not prevent impacts on biodiversity from occurring. Transport Scotland already looks for opportunities to extend new development boundaries where appropriate to provide suitable areas for ecological mitigation and biodiversity enhancements. However, availability of land alongside transport networks is often constrained, and the compulsory purchase route to secure land for such a purpose is both challenging and emotive. It is critical that ecological mitigation measures are carefully planned, designed, and implemented. Works must be delivered at the right time and within the anticipated window of opportunity, whilst being appropriately managed in the long term, including suitable monitoring and full documentation to ensure their success.

Biodiversity considerations have historically been perceived as less important than other aspects of the transport network management such as road safety. This translated to less funding and resourcing as well as limited awareness of biodiversity issues. The development of a Transport Scotland sectoral Biodiversity Strategy is needed to broaden the focus of the transport supply chain culture, to raise awareness of biodiversity importance, and to enable strategic investment into biodiversity, alongside the means necessary to understand and fully consider the natural capital value of the national transport estate. The Strategy will allow Transport Scotland to understand and deliver changes to its processes and practices to ensure it does all it can to support the challenging national biodiversity goals. As a government agency with extensive landholdings, Transport Scotland has both an obligation and an opportunity to contribute to these targets through the promotion of sustainable land management and integrating biodiversity considerations across its activities.

The Strategy will assist in indicating where the agency can best contribute to the goals and targets of the Scottish Biodiversity Strategy. It will help promote the restoration of biodiversity and seek to identify where the delivery of the responsibilities and actions of the agency, and its supply chain, will be adjusted to maintain and, where possible, enhance biodiversity.

It is recognised that there is a need to develop a biodiversity metric to support delivery of NPF4 and other planning policy. NPF4 Policy 3b states that national and major developments, or those subject to EIA, will only be supported where it can be shown that they will conserve, restore and enhance biodiversity, including nature networks, so they are in a demonstrably better state than without intervention. The policy requires that such proposals demonstrate significant biodiversity enhancement, in addition to any proposed mitigation and this, in turn, requires comprehensive and consistently applied measurement. The absence to date of a Scottish biodiversity metric has resulted in a range of methods being adopted to assess biodiversity impacts related to developments in Scotland, including broad use of the English statutory biodiversity metric (often referred to as the ‘DEFRA metric’ or the ‘English metric’) which calculates the biodiversity of a site and currently requires developers to demonstrate a 10% minimum Biodiversity Net Gain (BNG). It is universally recognised that, whilst this metric has many positive attributes, its application is not wholly appropriate to the policy landscape in Scotland, or the specific requirements of our native habitats.

Work commissioned by the Scottish Government is progressing on the development of a Scottish biodiversity metric, specifically to support delivery of NPF4 Policy 3b and with a focus on adapting the existing English metric to produce a tool that is suitable for use within Scotland’s policy and environmental context. NatureScot expects the Scottish biodiversity metric to be fully available in 2027, with a consultation planned this year on a working draft metric tool and accompanying guidance.

The main perceived benefits of metrics are that, where used appropriately, they can provide a consistent and transparent approach to calculating changes in biodiversity at a project level. Metrics provide clear assessment criteria and relatively easy to interpret outputs, supporting robust, evidence-based decision making. However, it is recognised that metrics have limitations as they apply a relatively simple and broad-brush approach and do not consider species or ecosystem function. Transport Scotland’s natural capital approach (see below) will capture the potential value of ecosystem services provided by land within and immediately surrounding the Trunk Road Network.

1, 2. Vehicle collisions on rural roads are among the conservation threats to barn owls and Scottish wildcats in Scotland.

3. Populations of orange-tip butterflies are found widely across southern and central Scotland. Recent surveys suggest that their geographic range has increased in Scotland over the past 25-30 years and are now found as far north as Moray, possibly due to climate change.
4. Red squirrels are vulnerable to woodland habitat loss and severance resulting from transportation schemes.

Transport Scotland is already embedding the requirement to consider biodiversity enhancements, in line with NPF4 Policy 3b, on new road schemes.

Our Biodiversity Responsibilities

The aims and objectives that underpin the Transport Scotland Biodiversity Strategy are applicable to and can be applied across the full scope of the agency’s assets. However, the strategy is most relevant to areas where actions and operations directly impact the condition of the landscape within and adjacent to Scottish Ministers’ ownership or control. The most obvious and significant such area is the Scottish Trunk Road Network.

The Network is divided into four regional units and managed on behalf of Transport Scotland by Operating Companies through the Network Management Contract (NMC). Some parts of the Network are managed by private consortia under long-term Public-Private Partnership (PPP) concessions to deliver new infrastructure via Design Build Finance and Operate (DBFO) contracts.

As part of their roles both sets of contractors, the Operating Companies (OCs) and the DBFO concessionaires, are responsible for the annual preparation and delivery of area-focussed landscape strategies, landscape development plans, and landscape management reports, as well as annual sustainability reports. The contractors take on duties such as regular grass cutting, verge maintenance, and the control of invasive species. They must also ensure that the works they carry out support the promotion of biodiversity and conservation interests, which may require the careful use of pesticides and sensitive management of wildlife issues.

At the same time, other essential road safety and management responsibilities and obligations can have unintended negative consequences for biodiversity - these include preventing vegetation encroachment and maintaining the integrity of sightlines and CCTV camera visibility spays; delivering winter maintenance activities like salting and snow clearing; and the management of drains and ditches, including gully cleaning. It is important to harness a careful and balanced approach to these activities to maintain the safe operation of the Network whilst supporting nature connectivity and the agency’s legal obligations to further the conservation of biodiversity when exercising its functions.

Transport Scotland operates a data management and performance monitoring platform known as Asset Management Performance System (AMPS), which records the assets that make up the Trunk Road Network estate which the OCs are required to manage and update. This includes a landscape inventory that records all aspects of the Trunk Road Network soft estate (the vegetated areas alongside roads). Transport Scotland’s Trunk Road Information Manual (TRIM) sets out the OCs’ contractual obligations for asset data inspection and maintenance required to achieve the agency’s corporate management objectives.

These incorporate a suite of requirements related to the soft estate, including grass and verge maintenance, the management of trees, shrubs, and scrub vegetation, and the surveying, inspection, prioritisation, and control of non-native invasive and injurious species.

The promotion of active travel and expanding the National Walking, Wheeling and Cycling Network, offers opportunities for biodiversity enhancement and contribution to green networks, while also preventing large numbers of car journeys and thereby helping to avoid pollution as well as greenhouse gas emissions.

Natural Capital

The natural resources all around us, including air, water, minerals and all living things, provide flows of benefits or ecosystem services – direct and indirect contributions that nature provides for human wellbeing and quality of life, whether individually or to wider society. Recognising and incorporating ecosystem services in development models and management processes is referred to as a natural capital approach.

The importance of measuring natural capital is widely acknowledged as fundamental if Scotland is to become Nature Positive by 2030 and to restore and regenerate biodiversity by 2045. This approach considers nature as a capital asset to encourage policymakers and businesses to recognise the immense, often unvalued positive effect that nature provides to society. This can foster more sustainable decision-making and improved environmental protection, whilst supporting a more comprehensive understanding of wealth and value by recognising the inherent contribution of natural assets.

Transport Scotland has prepared a natural capital baseline assessment (and set of accounts) of the entire Trunk Road Network. A natural capital approach helps capture the true value of Transport Scotland’s estate and demonstrate the potential value of ecosystem services provided by land within and immediately surrounding the Trunk Road Network.

The natural capital baseline assessment also provides a useful resource to inform the development of new projects as well as existing estate management opportunities, and a means to measure progress on the delivery of biodiversity objectives. The baseline is presented on two scales – Scope 1, within the maintenance boundary, which is land owned or managed by Transport Scotland; and Scope 2, within a 1km buffer (reducing to 100m in urban areas), representing land that may be influenced by, or have an influence on the management of the Trunk Road Network.

Biodiversity is not treated as an individual ecosystem service but is instead considered as a core component of natural systems that underpin the delivery of all other ecosystem services.

The ecosystem services included in the natural capital assessment are:

• food production
• wood production (timber)
• carbon sequestration and storage
• flood regulation
• air pollutant removal
• water quality regulation
• recreation
• noise reduction and
• pollination

Whilst many of these services will be more prevalent beyond the trunk road boundary, there is potential for Transport Scotland to have an influence on their provision through construction and operation related to the Trunk Road Network. The development of a natural capital baseline to inform decision making is of strategic importance in terms of understanding and influencing Transport Scotland’s sum contribution to managing the natural environment and maximising opportunities for the agency to support the delivery of national biodiversity targets.

Our Estate

Most of the Transport Scotland estate comprises linear areas around the Trunk Road Network and Scotland’s rail network. Other areas, such as parcels of land around ferry terminals, tend to be small and disconnected from other parts. Scotland’s railway network is almost entirely owned and managed by Network Rail on behalf of the UK government. Thus, the Trunk Road Network estate is the prime beneficiary and driver of the Transport Scotland Biodiversity Strategy. Whilst most obviously associated with the roadside verges, the Trunk Road Network estate also incorporates remnants of habitat listed on the Ancient Woodland Inventory and Sites of Special Scientific Interest (SSSIs). The estate also contains wetland areas which contribute to a blue-green infrastructure network.

Transport Scotland manages 3,745km (2,327 miles) of trunk roads and motorways (the Trunk Road Network) which account for 7% of all roads in Scotland and carry 40% of all traffic and 70% of HGV traffic. The agency is responsible for over 11,000 hectares of land associated with the Trunk Road Network. Transport Scotland oversees the trunk road OCs’ programmes of inspection, maintenance and improvement work and their delivery of routine and cyclic

The trunk roads operations and maintenance schemes, alongside new infrastructure projects, are all required to meet statutory, legislative and contractual requirements in relation to biodiversity, along with a range of standards set out in the Design Manual for Roads and Bridges (DMRB), including LD 118 Biodiversity design and LD 117 Landscape design (including Scotland National Application Annex).

Scotland’s railway network (managed by Network Rail) extends 2,819km (1,751 miles) across the country. Transport Scotland is responsible for Scotland’s rail policy and delivery, as well as providing input on matters reserved to the United Kingdom Government.

In addition, there is land associated with other parts of the transport network including various ports and harbours which come under Caledonian Maritime Assets Limited (CMAL) and are owned by Scottish Ministers. Transport Scotland acts as the Scottish Government's sponsor to provide management oversight and corporate governance via the Ferries and Ports Directorate.

Scottish Ministers own Highlands and Islands Airports Limited (HIAL) which operates 11 airports mainly across the west and north of Scotland. Separately, Scottish Ministers also own Glasgow Prestwick Airport. However, Transport Scotland’s role in shaping biodiversity strategy here is likely to be limited given the constraints on habitat and wildlife management at airports and the fact that aviation policy is generally reserved to UK Government.

Scottish Canals is a stand-alone public body, funded (via Transport Scotland) by Scottish Government grant aid, working to protect and improve Scotland’s canal network. The body manages an array of assets associated with the canal network, including structures, buildings, waterways, and towpaths. Historically it has had its own Environment Strategy which includes Biodiversity Management.

The Decline in Biodiversity

The Scottish Biodiversity Strategy cites the five direct drivers of global biodiversity loss which are:

• changing use of land and sea resources, especially for agriculture, forestry, fish farming, and coastal infrastructure;
• direct exploitation of organisms via harvesting, logging, hunting, and fishing;
• climate change;
• pollution; and
• invasive non-native species (INNS) and growing prevalence of pathogens.

Understanding the current state of biodiversity and the extent and rate of its decline in Scotland is the starting point for developing a strategy aimed at halting and reversing losses. Indicators of the abundance of marine seabird populations and selected terrestrial species in Scotland show significant declines. Research published in the State of Nature Scotland 2023 report highlights that:

• since systematic monitoring of 407 species in began in 1994 the abundance of those species has reduced on average by 15% and in the last decade alone 43% have declined;
• since 1970 the distribution of half of flowering plants has decreased;
• one in nine Scottish species are threatened with extinction including mountain bumble bees, red squirrels, Scottish wildcats, capercaillies, swifts, puffins, curlews, kestrels, tree sparrows, brown hares, hedgehogs and alpine blue-sow-thistle to name just some terrestrial species; and
• Scotland’s seabirds have declined by half between 1986 and 2019.

Scotland’s environment is also influenced by indirect factors like consumption habits, demographics and urban growth, economics and technology as well as political, institutional and governance factors. They include the societal values, attitudes and behaviours that contribute to substantially exceeding our share of planetary resources.

While many of these fall outside the scope of this sectoral biodiversity strategy, Transport Scotland is committed to taking responsibility for raising awareness and influencing attitudes in areas within the scope of the strategy.

The Impact of Transport Infrastructure Upon Biodiversity

Construction of new transport infrastructure usually results in loss or fragmentation of habitats which can lead to their general long-term decline as they cease to be ecologically functional units. New construction also impacts wildlife, whether through habitat loss and noise and light pollution, introduction of dust, silt and other pollutants (e.g. fuel or exhaust fumes) or the danger posed by construction traffic and, following completion, the fast-moving vehicles on the new road. Unless mitigated, new infrastructure creates barriers to the movement of animals, thus fragmenting their populations and territories.

Development on greenfield land needs to be carefully considered, given its biodiversity potential, and take account of the mitigation hierarchy, which prioritises avoiding impacts in the first place.

Any recognised need for a new transport corridor must be balanced with robust environmental assessment of the project, appropriate siting informed by the natural capital value of the receiving landscapes and the need to provide biodiversity enhancement.

Construction of new transport infrastructure usually involves felling trees, which constitutes a loss of biodiversity and contributes to habitat fragmentation. Improper disposal of material can also add to the spread of INNS and pathogens. The removal of vegetation also exposes soil to erosion from wind and water. Associated loss of soil (both particulate and nutrients) can lead to an unsuitable media for plant growth with the eroded material contributing to the pollution of freshwater habitats.

New transport infrastructure construction typically involves extracting, stripping, storing and moving large volumes of materials including minerals and soil. Construction traffic results in soil compaction, reducing the porosity and increasing the density of soil. This makes it more difficult for plant roots to grow through the soil and uptake nutrients and water. It also affects the diverse microorganisms that live in the soil (microbiome), which play a crucial role in maintaining soil health in a natural context and overall ecosystem functioning. Creation of impervious surfaces, such as roads, seals the soil, diminishing its capacity to support animals and plants and absorb and clean water. Pollution from construction activities results in soil contamination.

In natural environments, topsoil contains the highest concentrations of organic matter, nutrients and microorganisms, making it the most productive soil layer. One hectare of it can contain up to 5 tonnes of living organisms.

However, it takes more than 500 years to form two centimetres of topsoil which means it is a priceless, non-renewable resource. When improperly stored soil becomes degraded which in turn impacts on carbon sequestration and ecosystem resilience. New construction frequently results in surplus topsoil, which necessitates planning and finding uses for it. It can also adversely affect the local peat stores and the water environment, which impacts local ecosystems.

New transport schemes usually involve planting and seeding, which aim to replace or offset the vegetation and habitats lost due to construction but require appropriate ground preparation, regular maintenance and plenty of time to successfully establish and become an effective part of the ecosystem.

Biodiversity impacts can also be associated with business planning and decisions made related to supply chains and materials, and it is important that Transport Scotland continues to implement sustainable sourcing policies whilst supporting circular economy principles that reduce waste, promote recycling, and carefully consider the use of natural resources.

Existing transport infrastructure impacts biodiversity mainly through:

• forming a physical barrier within and between ecosystems and natural habitats
• traffic-related wildlife mortality and disturbance of adjacent habitats – affecting animal populations
• spread of INNS through seed and vegetative propagules, typically in the form of fragments of stem or rhizome, along transport corridors
• effects of pollutants such as heavy metals, nitrogen, particulates, ozone or substances used on roads in winter on the surrounding ecosystems and
• challenges associated with maintenance and management of the roadside vegetation and the culture within organisations responsible for it

The establishment of existing infrastructure has resulted in the fragmentation of areas of habitat, such as woodland, heathland or bog. This spatial disconnection has led to the isolation of species populations and the disruption of ecological connectivity resulting in a reduction in the functional integrity of these ecosystems, and the eventual decline and loss of biodiversity. Red squirrels, for example, require a minimum woodland area of 200 hectares for a viable population although an area of 2000-5000 hectares is considered ideal.

Whilst it is increasingly recognised on recent and new transport schemes that there is a need for providing safe wildlife passage under or over the road to link habitats, such permeability is often lacking on historic transport routes.

Traffic-related wildlife mortality is another cause of biodiversity loss on existing transport infrastructure. Between January 2022 and May 2023 over 1,500 dead animals were recorded on Scotland’s main roads. More than half of these were deer whilst 20% were badgers and 17% were foxes. Other animal deaths recorded included otters, owls and pine martens as well as domesticated animals and livestock.

Marine wildlife, such as whales, seals and basking sharks, is also occasionally seriously injured or killed in collision with vessels (ship strikes). There is no readily available data on the mortality of birds resulting from collision with aircraft, however, due to the considerable danger posed by birds to aircraft and the people they carry, efforts are made to prevent bird strikes by discouraging them from areas around airfields and avoiding flight through areas where they are likely to be in higher concentrations. Ground nesting birds such as oystercatchers, which are highly vulnerable during their breeding seasons, are known to nest within areas of the Transport Scotland estate.

Transport routes also facilitate the spread of INNS with vehicles acting as major vectors of long-distance seed distribution through the slip stream around a vehicle as it moves and/or by physical attachment onto the vehicle’s frame. Due to the higher proportion of traffic supported, national roads can have a greater volume of non-native species compared with local roads.

Existing impervious road surfaces and their runoff affect groundwater and its movement, which can result in deterioration or loss of important habitats such as bogs. Data is not yet collated for the Scottish Trunk Road Network, however, a report published in 2024 noted that there are more than 18,000 known outfalls associated with the motorway and Trunk Road Network in England and likely more than a million local highway drains discharging directly to watercourses. Highway runoff samples show elevated levels of toxic and bioaccumulative priority pollutants that exceed published standards that have been set to protect aquatic organisms and wildlife from harm. Poorly designed road drainage systems such as upright kerbs and uncovered gully pots can act as ‘death traps’ for amphibians including frogs, toads and newts, which travel along kerbs to breed or migrate, leading to mass mortality or the isolation of populations.

Air pollution around motorways has been shown to affect the health of oak and beech trees, and vascular plant composition in blanket bog. Effects extended to 100m from the roads and were correlated with nitrogen dioxide (NO2) concentrations.

Whilst there is a clear requirement to manage the risk associated with icy road surfaces in winter, the use of road salt is well-known in the enabling of maritime (salt-tolerant) species to spread along British roadsides.

Sea plantain is found along inland road verges and continues to spread on the verges of salt-treated roads in Scotland, supplanting less salt tolerant locally indigenous species.

The maintenance and management of the extensive trunk road landscape is subject to various resource constraints and the need to ensure public safety, which can sometimes conflict with achieving positive biodiversity outcomes. For example, vegetation must be cleared to maintain visibility required for road safety. Similarly, where ash dieback affects roadside trees, they need to be felled if they pose a risk to the public, rather than left to naturally decompose. In addition, special care needs to be taken when disposing of diseased trees and invasives to ensure compliance with the relevant regulations and guidance and to avoid spread of disease and INNS.

Some of the existing trunk road maintenance funding arrangements, such as performance indicators and payment mechanisms, can encourage the automatic adoption of frequent grass cutting or tree pruning and felling which can be detrimental to the biodiversity of the roadside landscape. However, there is potential flexibility within maintenance contracts that can be explored for future operations, including biodiversity-focussed specialist input, both at the higher decision levels and on the ground. The importance of preserving and enhancing biodiversity needs to be at the heart of the organisations managing the existing transport network through appropriate communication, training, new appointments and new incentives.