Biodiversity Net Gain and Metric 3.0

Rewilded field at the Knepp Estate in the Sussex Weald.

Biodiversity Net Gain and Metric 3.0

What is biodiversity net gain?

Biodiversity net gain has been a topic of interest amongst planners and ecologists for a few years. Its implementation as a legal requirement through the Environment Bill has been mooted since 2018, however is not due to come into force until late 2023, following a 2 year transition period after the bill is enacted this Autumn.

As a basic principle, biodiversity net gain (BNG) seeks to leave the natural environment in a better state than it was before. The incoming legal requirement will be that development must achieve a 10% increase in biodiversity, whether on-site, off-site, or through "biodiversity credits" (i.e. a financial contribution). 

Introducing 3.0

In recent years, many LPAs have sought to enforce BNG across new developments through a variety of individual policies, measured through a variety of metrics. The statutory version will use a standardised metric produced by Defra and Natural England, and recent discussion has centred around the release of "metric 3.0", which is set to be that finalised standard version. This is a methodology which assigns value to the habitat within a site, which functions as the baseline biodiversity value of a site. From this the appropriate level of net gain can be calculated.

The actual changes between metric 2.0 and metric 3.0 are relatively minor - wood pasture and parkland have been assigned greater value, and continuing the government's current obsession with "tree lined streets", so have urban trees.

What 3.0 has done is re-inflamed debate both about the concept of BNG, and the attempt to assign rigid numerical values to habitats. 

Habitats such as sand quarries are apparently undervalued in the new metric.

The single most dangerous thing for nature conservation?

Indeed, one insect specialist describes the metric's method of assigning value as potentially "the single most dangerous thing to be done by a statutory agency I've seen in all my 40 years working in nature conservation." The reason for the furore? Some ecologists are concerned that scrubby landscapes are not valued by the metric – and could be classed as "degraded" – despite their real ecological value. Other habitats which could be classed as degraded include sand quarries and field margins.

​If habitats are being undervalued through the metric, not only will not enough weight be placed on their loss through a proposed development, but inadequate compensation will be provided, overall perhaps leading to a loss in biodiversity.

Some ecologists are also concerned that the metric does not, for example, assign a higher value to a well-connected habitat which can provide a far more ecologically rich habitat than an isolated one. Overall, fans of rewilding-style conservation believe the metric only works with traditional conservation management in mind, and penalises efforts to "rewild" landscapes.

Urban scrubland in Teesside. Photo source

 Ecologists are also worried about the requirement that any compensatory habitats be maintained for 30 years – a short amount of time in ecology terms. The risk is that ancient habitats could be lost through development on the promise of a replacement habitat that may only be in place for the next 30 years.

That all said, Natural England's own website notes that the metric can in fact be used to assess rewilding projects by taking a conservative approach given the more uncertain outcomes involved, and that "bonus" biodiversity units may even be accrued by such schemes which could then be sold on if unexpectedly rich and distinctive habitats are created.

Natural England also makes it clear that the metric is not intended to replace ecological judgment and there is more scope for flexibility and to change a rating if it appears the metric has got it wrong than some of the more hysterical responses seem to suggest.

There was never going to be an easy or uncontroversial answer to the question of how to quantify the biodiversity value of a site, so any loss as a result of development can be measured and the appropriate compensation and net gain identified. There will always be a trade-off, too, between accuracy and ease of use. Metric 3.0 appears to be a bit of a blunt tool – but only time will tell whether it has the intended results. 

Brownfield sites can be wildlife havens

Viability ramifications

What is clear is that for many developers the new BNG requirements – which can be provided via compensatory tariffs expected to cost between £9,000 and £15,000 per "biodiversity unit" if on or off-site habitat creation is not possible – will be just another drag on their viability assessments, added to affordable housing requirements, CIL, and any other S106 requirements. There isn't usually enough in the pot for all these financial obligations – and the reality is they will be traded off against each other. Once BNG is a statutory requirement, it may erode the capacity for non-statutory contributions meaning hoped for gains in sustainability credentials or affordable housing do not pan out.

This could be a particular concern for brownfield sites which can be extremely ecologically rich containing a varied mix of habitats. Such sites would then require a commensurately high amount of BNG – which could be prohibitively expensive on top of the other abnormal expenses associated with developing brownfield sites. This might disincentivise developing brownfield sites – generally preferred over greenfield in broad sustainability terms – or even incentivise developers to actively downgrade the site, perhaps through destroying or degrading habitats, before putting it forward for development. 

​It will be interesting to see how developers come up with cost-effective ways of meeting the BNG requirements – perhaps by purchasing parcels in a high-scoring nearby habitat – and equally how landowners will make money off the requirements themselves. For example, if their land has little development potential they could look to increase its biodiversity and sell it as offsetting habitat. Other entrepreneurs could look to acquire parcels of land at a lower price than it would go for as a development opportunity, and sell it off to developers in need of "biodiversity units".

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