Oxford Long-Term Ecology Lab

Long-Term Ecology, Biodiversity Conservation, and Environmental Stewardship Technologies

Over the past decade there has been a radical shift in the way governments, corporations, NGOs and landowners are starting to think and talk about the natural environment in the UK. This has been evident in an increased understanding not just of the critical role nature plays in providing landscapes for biodiversity (including rare and iconic species, communities, and habitats) but also of the ecosystem service flows from this nature which provide important societal benefits. The most obvious one, and the one with a clear and well-established market, is carbon offsetting and the role of trees, peats, wetlands, and soils in providing carbon storage and CO2 sequestration. Other important benefits, however, include nature’s role in water-flow regulation (reducing flood risk), clean-water, soil erosion protection, pollination, and recreational spaces important for physical and mental well-being.

The Government’s 25-year Environment Plan, published in 2018, the imminent Environment Bill, the 2020 report of the Government’s Natural Capital committee and Dasgupta’s recent 2021 review on the Economics of Biodiversity all highlight the importance of nature in underpinning these valuable societal benefits – potentially resulting in a win/win situation; conserved, protected and enhanced biodiversity funded by the public and private purse because of the multiple societal benefits that it provides. These reviews/policies and the Environment Bill should be welcomed; they represent a definite step in the right direction for biodiversity in the UK.

However, as with all new policies and directions of travel, it is the devil in the detail that may in the end turn something that is potentially very positive for nature in the UK into something that is highly detrimental. Unfortunately, this is the direction we appear to be heading in with the policies and practices surrounding Net Biodiversity Gain – which is one of the policies linked through to the Environment Bill which mandates that new development that impinges on landscapes must achieve either no net loss of biodiversity or 10% net gain elsewhere if biodiversity is damaged or reduced on-site.

On the face of it, the Net Biodiversity Gain policy therefore seems a pragmatic approach to what happens when other urgent societal needs, most often due to new infrastructure developments (e.g. house building, new rail routes, road building) result in the potential destruction of natural habitats and landscapes that are located “in the way”. For all new developments, if destruction of habitats cannot be avoided, developers will be required to demonstrate that their proposals deliver a net biodiversity gain – either in proximity of the current development, or in another location that has high potential to achieve even more biodiversity gain (habitat banks). Only once this has been demonstrated will they receive planning permission from the local planning authority.

But herein lies the problem. The tool that has been developed to enable planners and local planning authorities to work out the current biodiversity of the site and net biodiversity gain, is simply not fit for purpose. Rather it sadly looks as if it will promote further loss and fragmentation of some of UK’s natural environment and even more critically, the important ecosystem service flows that emerge from the species, communities, and habitats on these fragments of land. There are many issues with this biodiversity metric but three that have been recently highlighted and warrant real concern are as follows:


  1. A biodiversity metric that is best suited to cows?

The first issue is to do with the metrics used to measure biodiversity in the field. The existing biodiversity on the site should first be assessed using Biodiversity Metric 3.0 which is an Excel-based biodiversity calculator devised by NE/Defra. To calculate a measure of the biodiversity, the tool requires a field-based assessment to collect quantitative data on the area of the different habitat types, then a qualitative assessment of the different habitat types (selected from a pre-existing list), their condition and distinctiveness. These different measures are then analysed along with some guesstimate multipliers (e.g. how long it takes for habitat to reach current condition) to provide a biodiversity score. The majority of the biodiversity metric assessment, is therefore based on the qualitative skills of the ecological consultants who are employed by the developer.

Even if you employ the best ecologists in the world there will always be huge variation between individuals when using such a qualitative approach – not only in judging different habitat types but even more determining what classifies as ‘degraded’. A case in point is in the use of this biodiversity metric for scoring grasslands. Field experience from those trying to use this measurement on the Knepp rewilded estate in West Sussex, for example, have found that when using this metric to score the degraded state of grassland you get marked down for the presence of ragwort and bramble scrub; its presence is viewed as indicative of a high level of degradation.

Ragwort and scattered bramble scrub might indeed be a problem for farmers if the area is being using for intensive grazing of cows and sheep; ragwort is known to be poisonous to livestock in large quantities – but this is not a problem for insects. Quite the opposite in fact. Ragwort and scrub provide important habitat for many insects. At Knepp, for example, it has been demonstrated that at least 117 insect species use ragwort as a source of nectar and pollen including 30 species of solitary bees, 18 species of solitary wasps and 50 insect parasites1 . The presence of ragwort thus provides habitats for a biodiverse assemblage of insects that are providing critical ecosystem services (pollination and pest control). So what biodiversity is this habitat assessment as part of the biodiversity metric tool hoping to protect? That for a single species of cow or multiple species of insects? To me, one of these is does not support the definition of biodiversity…!


  1. A game of “guess the ecosystem services”

Once the biodiversity metric scores have been obtained, the second step is to assess the ecosystem services that flow for these different habitat types using the Defra /NE Environmental Benefits from Nature tool (EBN). This is branded as a “voluntary decision support tool” where the consultant is given a list of ecosystem services that the habitat (recognised using the Biodiversity metric tool) “may” support.

Herein lies at least two insurmountable problems. First, without quantitative modelling and measurements, it is impossible to assess the true nature and extent of ecosystem services flows from a patch of land. Second and even worse, most ecosystem services (e.g. soil erosion prevention, waterflow regulation, pollination) do not flow from habitat units. They are instead as a result of the combination of species, communities, soil types, and habitat structure and location (e.g. where they are situated in the drainage catchment/basin). The nature that supports critical ecosystem services is on the whole, spatially constrained. Take for example a block of trees and scrubland that has a low  score in the biodiversity metric. In fact, if this block of trees is located upstream it might be incredibly important for water-flow regulation and thus reducing flood risk downstream, along with soil erosion protection, habitats for pollinators and recreation, yet could well be deemed suitable for offsetting elsewhere due to its state and habitat type. If you replace this block of trees and scrub with a biodiversity elsewhere, you have effectively lost all the ecosystem services and societal benefits it provided. I would go as far as to say this biodiversity/habitat approach simply does not work for ecosystem service metrics.

As recommended recently by the Environmental Audit committee, the requirement for Net Biodiversity Gain should be replaced with a quantitative tools and approaches to measure and ensure Net Environmental Gain as soon as possible.


  1. Habitat banking in “banks” yet to be established… is this a wise fiscal approach?

But is all the above merely noise?  Do “gut-feelings” in fact produce the correct decisions to protect and indeed enhance biodiversity and the ecosystem services it provides in the UK? Unfortunately, a recent paper that has analysed all the Biodiversity Net Gain assessments that have accompanied planning applications submitted between January 2020 to February 2021 suggests quite the opposite2. This assessment looked at the applications that were submitted to the six councils that have adopted the BNG-equivalent policies in advance of its national rollout later this year, a total of 55 BNG assessments. From this work the authors found that the dataset revealed a 34% reduction in green space despite promising a 25% increase in biodiversity area. This accounting loss is excused by the promise that these habits will be traded for habitats of higher distinctiveness and condition in the future. So that biodiversity losses of today will be banked in banks yet to be established. How on earth can this be acceptable to anyone?

The result of all the above will be increasing fragmentation of UK landscapes, loss of corridors for biodiversity to move across the landscape, and loss of the critical ecosystem services that we rely upon nature to provide. We simply must do better than this – and quickly. The blame cannot be placed on the farmers, developers, or even the ecologists. We are currently providing them with tools that are the equivalent of giving a farmer a trowel to dig a whole field. What we urgently need are quantitative approaches, metrics, and tools that are transparent, repeatable, and easy-to-use. Without the right approaches put in place quickly, net biodiversity gain in the UK will end up being net biodiversity loss and the only gain will be infrastructure developments; quite the opposite of what is intended by the Environment Bill.


Kathy Willis,

Professor of Biodiversity, University of Oxford

1 Tree, I. 2018. Wilding: the return of nature to a British Farm. Picador, London

2 zu Ermgassen, S.O.E. et al., 2021. Exploring the ecological outcomes of mandatory biodiversity net gain using evidence from early-adopter jurisdictions in England. Conservation letters; e12820

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