A biodiversity metric is simply a tool used by ecologists to measure changes in biodiversity. Crucially, the Defra metric is only concerned with habitats and does not take protected (or other) species into consideration.

The Defra Metric works by assigning every habitat on a site a ‘unit value’, A proxy, weighted to its relative importance for biodiversity, rather than its true value. 

The unit value encapsulates the habitats biodiversity value, by considering a number of factors, such as the type of habitat, its condition and connectivity to other habitats. In this way, a very small area of a particularly valuable habitat, such as ancient woodland, can be equated to a much larger area of a less valuable habitat such as an arable field – both can be included under the label of a ‘habitat unit’. 

Similar provisions apply to hedgerows and other linear features which might occur on site – small lengths of high quality, connected linear features may well equate to much longer stretches of poor quality, unconnected linear features.

Using a Metric enables meaningful comparison between the existing value of a site and what will be delivered through development or management.

• Baseline biodiversity units = Distinctiveness x Condition x Significance x Connectivity x Area in                                             hectares (or length in km)

A biodiversity score is calculated based on credits assigned to each of these components and then multiplied by the area or length (if it is a linear corridor) of the site. The predicted gain is then calculated in the same way, adding-in new biodiversity elements and new components relating to risks associated to the development, regarding:

•     Spatial risk: distance of offset from site

•     Temporal risk: time for habitats to reach target condition

•     Delivery risk: difficulty of habitat creation

These elements are applied to calculate the onsite (and offsite) post-development score:

• Post-development biodiversity units = Distinctiveness x Condition x Significance x Connectivity x Area in hectares (or length in km) / Spatial x Temporal x Delivery risks

The Biodiversity Net Gain is then a fairly simple calculation:

• Biodiversity units created = Post development biodiversity units – (minus) Baseline biodiversity units

Watercourse Unit values are far more difficult to explain or indeed understand. The surveying methodology has been designed by a corporate business and in truth whilst ‘standardising’ survey values, it is clunky and does not mesh well with the Defra metric. It has to be shoehorned into the Defra Metric and few consider it to represent real BNG values. Nonetheless, we are saddled with it until at least 2029. 

https://www.gov.uk/government/publications/statutory-biodiversity-metric-tools-and-guides

Defra highlights that the proposed Biodiversity Net Gain (BNG) metric is just one part of the biodiversity puzzle. 

Existing ecological legislation protecting key species, habitats and designated sites remain valid. 

This means BNG is not a licence to trash a site– 

If the ecological Mitigation Hierarchy is not followed, the Local Authority can refuse planning permission. This means that important habitats should be protected from harm (avoidance), some habitats (mitigated) and others compensated for, if lost. Whilst this does mesh with BNG in some regards, it is not the same thing.

Removal of hedgerows and scrub will have impacts to breeding birds, bats and potentially dormice (other species too), yet in BNG only the loss of the hedgerow / scrub is considered, not the impacts it will have on the species.

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