Beyond the BNG Metric: evolving the approach to Nature Recovery on the GB Railway

Being deeply invested in the environmental health of our railway corridors , I've spent much time reflecting on the wisdom of relying solely on the Defra Biodiversity Metric to measure the success of the railway manager's land management approach, coupled as it is to a Government transport directive to ‘help stop the decline in native species and enhance biodiversity for all to enjoy and benefit ’.

The wonderful Lawton Review's clarion call for a "more, bigger, better, and joined" approach to nature conservation has deeply influenced my thinking since its publication in 2010, and coloured my own experiences analysing years of habitat data collected from railway projects and observing the responses of invertebrates, small mammals, birds and other taxa within these environments.

In recent years, the Kent Pilot project, a multi-year initiative to test various Habitat Management Techniques on the lineside, yielded results that reinforced my concern. The project involved 70 trial sites with varying habitat types and conditions, and botanical and invertebrate surveys were conducted to monitor changes in biodiversity following the implementation of the selected HMTs (Figure 1). While we observed some encouraging increases in plant species richness, the early phase outcomes for invertebrates, particularly butterflies, were mixed.?

Figure 1. Habitat trials on the Kent Route. While we observed some encouraging increases in plant species richness, the early outcomes for invertebrates, particularly butterflies, were not as consistent. With grateful thanks to COOMBES Group Craig Mills

This further strengthened my resolve to explore and advocate for enhancements and supplementation of the biodiversity metric for railway land management purposes, considering ecological function metrics and metrics for taxa .

So, while the Defra metric, with its focus on habitat quality and distinctiveness, provides a valuable framework for assessing potential Biodiversity Net Gain (BNG), it has seemed to me that it falls short in capturing the nuanced reality of on-the-ground biodiversity. This disconnect is not confined to my, largely qualitative, experience and opinion; there is growing evidence now, and recent research caught my eye.

Work by Duffus et al. (2024) was one such piece, highlighting the limitations of the BNG metric in accurately reflecting biodiversity. Their study found no significant relationship between the metric's combined area-condition score and the richness, abundance, or composition of invertebrate communities (Figure 2). This finding is particularly thought-provoking given the essential role invertebrates play in ecosystem functioning and overall biodiversity. As the author’s state: "If use of these metrics result in trades of large, low-scoring sites for smaller, high-scoring sites... wider biodiversity benefits may not be achieved", suggesting that BNG metric methodology may lead to the undervaluing of certain habitats and fail to adequately address the needs of diverse species.

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The findings of Marshall et al. (2024) also caught my attention. This team demonstrated that the metric's emphasis on plants can lead to a skewed understanding of the overall ecological impact of interventions. The metric failed to demonstrate any significant relationship with bird or butterfly biodiversity, highlighting the risk of overlooking the specific needs and sensitivities of these taxa (Figure 3). Thus, the metric is likely not adequately capturing or promoting gains for these taxa, potentially leading to a skewed understanding of BNG.

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The apparent limitations of the current BNG metric then raise questions about its efficacy in delivering meaningful biodiversity gains and contributing to broader ecological networks. The Lawton approach to nature conservation resonates deeply in this context. The railway, with its extensive network of landholdings, has a unique opportunity to contribute to this vision (Figure 4.). Railway land traverses diverse landscapes and connects fragmented habitats, offering valuable potential to support ecological restoration and enhancement.

To fulfil this potential, I suggest the current directive for the railway land manager 'to achieve biodiversity net gain' for 'its existing lineside estate' using the BNG metric be amended and evolved. We need to move beyond the confines of a single, plant-centric metric and work towards a layered strategy that leverages the unique advantages of railway land. This would involve:

1. Developing comprehensive Land Management Plans: plans that go beyond simply incorporating BNG principles and consider the specific needs of a wider range of taxa. Requiring detailed work to home in on key species and their habitat requirements, as well as broader management actions to create and enhance suitable habitats. For example, we could focus on creating and managing habitat mosaics that cater to the needs of various species, including invertebrates, birds, and small mammals, contributing to a more balanced and resilient landscape.

This need not be overcomplicated - (relatively) simply manage vegetation on a rotational basis, at a regional level. If this rotation and management is structured properly, then the creation of a predictable mosaic of habitats, in different stages of development, would increase biodiversity value as a whole. Why then be wedded to site-specific biodiversity calculations for such an approach?

Additionally, LMPs should incorporate nature-based solutions to address performance and environmental challenges, such as flood mitigation, temperature regulation and carbon sequestration, maximising the ecological, financial and societal benefits of railway land management.

2. Expanding assessment metrics: to capture a more holistic view of biodiversity, we need to move beyond habitat-based assessments and incorporate ecological function metrics and taxa-specific metrics . This could involve monitoring key ecological processes, such as pollination and land permeability, and tracking the populations of indicator species to assess the overall health of the system.

As Marshall et al. suggest, refining the distinctiveness and condition scoring systems, incorporating a change index to track habitat trends, and adjusting net gain trading rules to incentivise the creation of high-quality habitats are essential steps in this direction.

As to the ‘how’ - well, as I have advocated elsewhere, Ecology 5.0 is the way to go.

3. Securing funding and resources: implementing these comprehensive LMPs and expanding assessment metrics will require significant investment, a step-scale change. This needs to be articulated to and recognised by Department for Transport (DfT), United Kingdom and Office of Rail and Road (ORR) . Collaboratively, we need to explore and secure diverse funding streams, including government grants, private sector partnerships, and biodiversity offsetting, to support long-term habitat management and ensure the sustainability of efforts. This involves working with the grain of Nature Recovery Networks , conservation organisations, and engaging with local communities to leverage expertise, resources and support.

The railway's inherent protected status, with fenced-off land and reduced disturbances, makes the landholding IMHO an ideal candidate for securing long-term funding through biodiversity offsetting and habitat banking. By adapting the principles to the railway environment and leaning into biodiversity covenant agreements, we can ensure the long-term funding, protection and proactive management of the rail estate for nature and nature-based solutions.

4. Prioritising connectivity: enhancing land permeability to facilitate wildlife movement and gene flow is crucial for creating a more connected and resilient ecological network. This involves creating wildlife corridors, underpasses, and overpasses to allow animals to safely cross railway lines and access a wider range of habitats. This will not only benefit individual species but also contribute to the overall health and resilience of the ecosystem.

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The message I think is becoming clearer: refining the management requirements for a railway that "helps stop the decline in native species and enhance biodiversity for all to enjoy and benefit " necessitates a broader, bolder, approach. Time to embrace a more holistic strategy that encompasses the full spectrum of biodiversity, moving beyond BNG, as currently conceived, and tackling a more nuanced approach that considers the intricate dynamics of species interactions, ecosystem functions, and landscape connectivity. By doing so, we can ensure that our efforts more robustly contribute to nature recovery and create a railway network that optimally supports a rich and diverse natural environment.

References:

Marshall, C. A. M., Wade, K., Kendall, I. S., Porcher, H., Poffley, J., Bladon, A. J., Dicks, L. V., & Treweek, J. (2024). England's statutory biodiversity metric enhances plant, but not bird nor butterfly, biodiversity. Journal of Applied Ecology, 61, 1918–1931.

Duffus, N., Atkins, T., Ermgassen, S., Bull, Greyner, R., Bull, J., Castell, D., Stone, B., Tooher, N., Milner-Gulland, E.J., & Lewis, O.. (2024 Metrics based on habitat area and condition are poor proxies for invertebrate biodiversity. Pre-print - doi: https://doi.org/10.1101/2024.10.02.616290

Views in this article represent the author’s personal opinions only.