Roads, verges and nature

Roads form vast and pervasive networks across the earth, with diverse and often profound negative environmental impacts. However, these roads are bordered by a parallel network of habitats in the form of road verges. In this thesis, I propose that roads present a nearly ubiquitous environmental pressure, but that their verges offer an equally widespread opportunity to benefit nature, the environment and, as a result, people. I begin by using spatial mapping and modelling to estimate the extent of road pollution, in various forms, across Great Britain. This reveals that half of land is less than 216 m from a road, and that roads have a zone of influence that extends across > 70% of the land area. Whilst high levels of road pollution are estimated to be relatively localised, low levels are pervasive. Second, I synthesise the literature to determine the global potential of road verges for mitigating such pollution, for supporting nature and for providing further environmental and social benefits via ecosystem services. I estimate that road verges may currently cover 270,000 km2 globally, and that they provide a wide range of benefits, including biodiversity provision, regulating services (e.g. air and water filtration) and cultural services (e.g. health and aesthetic benefits by providing access to nature). However, verges also displace other habitats and provide ecosystem disservices (e.g. plant allergens and damage to infrastructure). Third, I use satellite and ground-level imagery to estimate the extent of road verges across Great Britain and to explore their potential to benefit nature, the environment and people. I estimate that there are 2579 km2 (2149-3010 km2) of road verges in Great Britain, equivalent to 1.2% of land area, of which 707 km2 (27.5%) is short, frequently-mown grassland, 1062 km2 (40.9%) is regular grassland, 480 km2 (18.7%) is woodland, and 272 km2 (10.7%) is scrub. Only 27% of frequently-mown grassland verges contain trees, indicating potential for planting trees and shrubs to provide environmental and social benefits. Fourth, I take insect pollinators as a case study and, using a combination of roadside surveys, field experiments, a literature review, and meta-analyses, determine the suitability of road verges for pollinator conservation, and how they could be enhanced for this purpose. The findings demonstrate that road verges can be hotspots of flowers and pollinators in human-dominated landscapes, but that traffic and road pollution can cause mortality and other negative impacts on pollinators. Surveys in Cornwall, UK, show that pollinator densities are lower closer to the road edge – particularly within the first 2 m (55% lower than at 7-9 m) – where pollution is greatest. Field experiments reveal that these trends can be explained by pollinators being deterred by turbulence, and avoiding concentrations of metals that are more frequently found in flowers within 2 m of roads. Overall, evidence suggests that the benefits of road verges for pollinators outweigh the negative impacts of road pollution and pollinator-vehicle collisions. The research shows clearly that road verges already provide many environmental and social benefits, but that these could be enhanced considerably through strategic design and management. Several reoccurring management recommendations are reducing mowing frequencies to no more than twice per year, leaving some areas uncut on rotation, and planting trees in urban verges and in low quality verge habitats. I finish by reflecting on the project and its outcomes, and on the remaining barriers to managing road verges for nature, the environment and people. This reveals the most important directions for future research, namely, the importance of overcoming social barriers to change that inhibit uptake of environmental management recommendations.Natural Environment Research Council (NERC)Cornwall Area of Outstanding Natural Beauty (AONB) Uni