The Biodiversity Net Gain Metric

As of November 2021, biodiversity net gain (BNG) has become an essential part of the majority of development projects moving forward that will transform the way we think about planning projects, with a focus on preserving and enhancing the natural environment. But even with an ongoing two-year transition period in place, developers have had to adapt quickly to the requirement, as an uncertain number of local planning authorities within local councils already began to roll out BNG as if it was already universally enforced by law.

A part of the Environment Act 2021, the concept of biodiversity net gain centres around an intention to increase the biodiversity value on a site by a minimum of 10%. For it to work successfully, an ecologist conducting a biodiversity net gain assessment will need to measure ecological value using a calculation that accurately assesses the current state of biodiversity on the development site. The recorded calculation can then be used to gauge how much the level of biodiversity needs to be improved upon to meet or exceed the 10% increase.

In order for ecologists to create a biodiversity net gain plan that has effective measures to achieve the 10% increase, a predetermined biodiversity metric has to be utilised correctly. Over the course of this article, we use our insight to explain what the biodiversity metric is, why it is crucial during any BNG assessment, and how we approach using the metric to measure biodiversity value and satisfy the BNG planning policy.

ecologist measuring ecological value on a site
An ecologist making notes of ecological features on a site as part of a biodiversity net gain assessment.

What is the Biodiversity Net Gain Metric?

The biodiversity net gain metric is a unit of measurement used to quantify the ecological value of a site. Primarily determined by the value of an area based on natural features that could act as habitats to local wildlife, the biodiversity metric is a fundamental part of any biodiversity net gain assessment and plays a pivotal role in evaluating the ecological value of the site.

Criteria used within a calculation made using the BNG metric include both land and intertidal areas, particularly factoring in hedgerows, rivers and steams inside the boundary lines of the plot of land. In terms of the professionals tasked with using the metric, ecologists often need to utilise it to calculate biodiversity value as part of a biodiversity assessment. However, the biodiversity metric may also be used by other stakeholders in the planning process.

For instance, this may include developers during a biodiversity assessment, landowners who need to offset biodiversity units or biodiversity credits from one site to another, local communities that want to gauge the impact of planning projects on local biodiversity, and local planning authorities that need to confirm that the biodiversity values in front of them are correct before granting or denying planning applications.

How does the Biodiversity Metric Contribute to BNG?

Potential changes to biodiversity value on a site – both positive and negative – can be quantified through the use of the biodiversity metric. Features of utilising the metric include analysing the biodiversity unit value of a specific plot of land, gauging likely impacts on biodiversity, identifying gains and losses to biodiversity, and mapping out necessary on-site and off-site enhancements.

Not only does the BNG metric give an accurate representation of the current state of biodiversity on a site, but it is also used to predict the resulting condition of biodiversity once the development has been completed. To do this, an ecologist will speak to the developer in charge of the project and view all plans they have for the development to understand what biodiversity on the site will look like afterwards.

Through looking at both the pre-development measurement and the predicted post-development measurement, an ecologist can clearly view the difference between the two figures before developing methods of eliminating the deficit and building upon it by at least 10%.

Biodiversity Metrics

Created with input from the Environment Agency, the Forestry Commission, Natural England / Natural Resources Wales and numerous local planning authorities, the biodiversity metric was developed by the Department for Environment, Food and Rural Affairs (DEFRA). Additionally, DEFRA also developed a small sites metric that has a similar structure but for use on small development sites.

Over time, the biodiversity net gain metric has been altered following natural trial and error during the implementation of the planning policy and suggestions from environmental organisations, responsible bodies and local authorities, as well as the insight taken from professional ecologists and developers following biodiversity net gain consultations.

Below, we have outlined the core updates to biodiversity metrics spanning from the first version up until the most recent:

Biodiversity Metric 1.0

Released in 2012, the first biodiversity metric was piloted before changes were made based on early tests of biodiversity net gain and suggested changes put forward by industry experts. Among the updated version of the first biodiversity metric were capabilities to cover a broader range of habitat types, more consideration in regards to ecological connectivity, and integration with a spreadsheet tool that supports application of the metric.

Biodiversity Metric 2.0

By 2019, opportunities for feedback from a wide range of stakeholders presented numerous changes to biodiversity metric 1.0 that would make measuring biodiversity more effective. Along with the significant change that enabled both area and linear habitats to now be measured, biodiversity metric 2.0 also introduced measurements of intertidal habitats and the ability to look up canal data on river metrics.

It also eliminated several faults in the first BNG metric such as data outside of the chosen plot of land factoring into the measurement, incorrect options from dropdown tabs, missing options in hedgerow length lookup, retained off-site habitats being left out of the measurement, and rows missing in the enhancement tabs.

Biodiversity Metric 3.0

Following further tests and government consultations, DEFRA released biodiversity metric 3.0 in 2021. Key changes from DEFRA biodiversity net gain metric 2.0 to 3.0 include adding a contextual understanding over created or enhancing habitats in advance or over phased developments, changing the target of improved biodiversity from 32+ years to 30+ years, removing the accelerated succession and connectivity tools, updating the physical appearance, and various changes to flora and fauna data.

Even though many alterations were made between biodiversity metric 2.0 and 3.0, a future update that will include an effective approach for incorporating marine net gain for English waters is expected.

an ecologist utilising the biodiversity net gain metric to gauge biodiversity value
With the biodiversity metric, an ecologist can determine ecological value from features on the site.

Using the Biodiversity Metric

When an ecologist needs to measure the biodiversity value of a development site as part of a BNG assessment, the same factors will be used. Components used to measure biodiversity value consist of the types of habitat parcels on the site, the condition of each habitat parcel, the size of each habitat parcel in kilometres or hectares, and the locations of each habitat parcel.

All of the data from the biodiversity net gain assessment can then be submitted into the government website’s biodiversity metric 3.0 calculation tool to determine an accurate score based on ecological features present on the site.

Measuring Net Gain Based on the Biodiversity Metric

If you are in the process of staging a land development, it is highly likely that you will be required to achieve BNG and increase the biodiversity value on the site by at least 10% post-development compared to pre-development.

Ever since it was conceptualised, Arbtech has been keeping tabs and maintaining an understanding of biodiversity net gain as it was gradually rolled out and incorporated into UK legislation. Whether you need a BNG plan to satisfy your local planning authority or another form of ecology survey to develop an awareness of ecological features on your site, speak to us and we can use our expertise to point you in the right direction.