6th December 2021

Biodiversity Net Gain – the magic 10%

What is Biodiversity Net Gain?

With the new Environment Act coming into effect and COP26 having recently taken place, buzzwords relating to the climate ecological emergency are being banded around left right and centre. One of these is biodiversity net gain (BNG) – a term used to describe how the value of habitats (and in theory, wildlife) on a site may change over time. In terms of planning and development, it is an approach to demonstrate how a project will comply with both wildlife legislation and planning policy to bring about gains for wildlife. In turn, it encourages developers to put more focus on creating green spaces and infrastructure within their development. Or, to introduce another new term for you, Natural Capital.

If you do some reading around, you will find articles arguing the pros or cons of BNG, from the perspective of ecologists or conservationists. It is by no means a perfect process, but from our perspective, if there is a requirement to achieve biodiversity net gain, this can mean habitats that may not have otherwise been retained or created, are included in the overall design for the site. Whilst current wildlife legislation can protect specific species, a requirement for BNG can ensure habitats that are good for wildlife in general are also protected. This is important for a number of reasons. If, due to legislation, we only protect habitat specific to a small number of protected species, this will cause habitats to become fragmented, and put common species at risk of decline.

Inclusion of these habitats are not just beneficial for the species that use them – it’s worth considering the wider ecosystem services that could be achieved as a result of the habitats chosen for the site. For example, if allotments have been included within a residential development, inclusion of habitats that support pollinators will ensure a good yield for tenants using allotments to grow vegetables, with many food plants relying on pollination to produce fruit. There are also benefits such as carbon sequestration, and flood prevention. The benefits of spending time in natural and green spaces are also widely known to improve mental health[1], and numerous studies finding that during the recent Covid-19 pandemic there were sharp increases in the number of people utilising their local parks and spending time outside for leisure or exercise[2].

How does it work?

Net gain is calculated by inputting habitats, pre- and post- development, into a metric, created by Defra (Department for Environment, Food & Rural Affairs). The metric (Defra Metric 3.0, as of July 2021) provides a baseline biodiversity score for the site, and then the percentage change between pre- and post-development is given – which is where the “net gain” comes in. The metric will have fixed weighting to a habitat’s “distinctiveness”, but we can add in additional elements that may vary between site, such as the habitats “condition.” For example, a pond would have a fixed distinctiveness score of “high.” However, if during the site surveys it was found to have little emergent vegetation, evidence of high nutrients or possibly pollution, then we could specify that it was of “poor” condition. These classifications are associated with the score and get multiplied along with other factors, to provide a biodiversity score for the habitat. Other things that are considered include how long a habitat might take to reach maturity, and difficulty of creation.

What does a quote for BNG calculations include?

Usually if a net gain calculation is required, we will include a biodiversity impact assessment at the end of a report, or we can produce a standalone document. This will include a written summary of the change in ecological value and any assumptions made when completing the calculations and interpreting the landscape plans provided. We can also provide recommendations for increasing the net gain score.

To conduct the actual calculation and complete the metric we require a detailed landscape plan, which demonstrates the different areas of habitat on site, with the illustration showing clear boundaries and sections that we can measure. For the pre-development score, we will base these habitats and areas off a habitat map created during the preliminary ecological appraisal of the site. The post-development score is based on measuring the sections of habitat illustrated on the agreed landscape plans. Often a landscape plan can have different terminology to describe areas of habitat to the codes/names used on the Biodiversity Metric. For example, an area described as wildflower meadow on a landscape plan could be listed as any of around eight habitat types on the Defra metric, which may all have different impacts and weightings on the biodiversity score for the site. To get clarity on this it may be necessary to have consultations with the landscape architect involved with the project to determine what type of species mix they are proposing for the habitat or what type of management they are likely to carry out. All this information will help us determine the most suitable habitat to put and the most realistic condition the habitat will reach.

In full, a quote will need to allow time for consultations, mapping habitats on GIS software, inputting the data into the Defra metric, and completing the analysis and reporting elements. The time allowed for all of this may vary depending on the scale of the project.

What happens if my project doesn’t reach 10%?

We can understand the frustration felt when it can feel impossible to reach that golden figure of 10% net gain, which is now so often cited. This can often happen when developments occur on greenfield sites, as even if the baseline habitats are not of significant importance, they are still significantly higher scoring than hard standing and buildings, which will score zero. This means any remaining habitat must be significantly enhanced to compensate for the biodiversity lost. A common pitfall is not having enough space to create new habitat, even though in relative terms, the developer is not planning to build over a huge amount. However, if this habitat is of high value and has a high multiplication factor for reaching maturity, it can take a much larger area to replace the biodiversity units lost.

As ecologists, we understand some of the pitfalls of the metric, and will always try to be pragmatic when it comes to making a written assessment on the biodiversity impact on the site. Sometimes it gets to a stage where the only way to reach 10% is to remove a perfectly good habitat and create a higher scoring one in its place, and realistically this may (arguably) cause more disruption to the site and displace species that the habitat currently supports. If we think certain habitat creation practices may be inappropriate, we will reflect this in our report, and similarly, emphasise the benefits of the proposed habitats if we feel they would be of particular benefit to the ecology on site. Alternatively, we will work alongside you to identify opportunities for offsetting biodiversity with a landowner/provider.

Here are some tips and pointers to keep your project on track and hopefully reduce delays due to low BNG scores:

Involve and consult with an ecologist from an early stage. Even if they have not completed the metric yet, they may be able to advise on whether the existing habitats on site are likely to score highly, and therefore how much (roughly) might need to be retained/enhanced. We would always recommend retaining as much high-quality habitat as possible (e.g., woodland).
Similarly, if your project involves covering a substantial portion of a green site with hard standing, they may be able to advise that it will be difficult to reach net gain, allowing you to investigate alternatives and adjustments at an early stage.
Generally, a site is much more likely to achieve a net gain in biodiversity if it is a brownfield site, or partially developed already.
Enhancing habitat that is being retained is a fantastic way to achieve gain which may use less labour and resources than creating an entirely new habitat. For example, if a pocket of woodland is identified on site, some general maintenance and understorey planting may take the condition of the habitat from poor or moderate, to good, or change the woodland type to a higher scoring habitat (e.g., from a fairly species poor plantation woodland, to a diverse deciduous mixed woodland).
With the new Environment Act coming into action, if 10% cannot be achieved on-site it will need to be achieved using off-site enhancement. If we anticipate at an early stage, that it may be difficult to achieve 10% gain on-site, we can inform the client early on to allow time for finding a suitable area for off-site enhancement.