“Biodiversity Net Gain” is a problematic concept used as mitigation by developers
Biodiversity Net Gain (BNG) is a mandatory planning requirement for development sites, whereby wildlife habitats must be left in a better state than before development, either on the site to be developed or elsewhere. There are specific methods for working out the BNG for a site. BNG must be at least 10% enhancement of the current biodiversity value.
For full information on this concept: www.gov.uk/government/collections/biodiversity-net-gain
BNG is also frequently claimed on account of new tree planting, new hedgerows or more wildlife-friendly management of hedgerows (such as letting them grow taller and bushier, and planting up gaps), and sowing new grass margins adjacent to hedges and watercourses.
However, BNG needs scrutiny, as essentially an area of unspoilt countryside is lost, and any options for more ambitious and genuine biodiversity gains are impossible for the coming 40 years. There are also many problems with the practicalities of implementing BNG on the ground.
“Biodiversity Net Gain” if a Solar Farm is developed?
In the case of this proposal, for Old Hayes Solar Farm, there is already significant nature conservation value. The arable land is not of particular value, soils are likely to be in poor condition, compacted, and the light alluvial soils prone to erosion during floods. Thus in this context permanent grassland could indeed be a BNG. However, the site is an active and functioning floodplain, and the grassland should ideally be floodplain grassland, of value for wetland species including rare and declining wetland birds – not an option under the current proposal.
The various features of nature conservation value have been outlined – the River Cole, a Priority Habitat, tributary streams, hedges, trees and field margins.
Net Gain would be to manage the whole site as wet grassland or other wetland habitat, with minor changes to hedgerow management and enhancement of watercourse profiles. Such management is providing major wildlife benefits upstream on National Trust land and is proposed for other Trust land near this site. The proposal for a solar farm in this location conflicts with ambition for the River Cole corridor and its wider catchment.
“Biodiversity Net Gain” for the floodplain?
With a solar farm, the main gain could be a positive effect on water quality in the river and tributaries if the site was changed from arable to permanent grassland, reducing run-off of silt and nutrient-rich water. The grassland would probably be managed without fertilisers, reducing nutrient loading.
However, there are many unknowns related to the construction and management of a solar farm. It may well be difficult to establish a well-rooted grassland capable of intercepting pollutants and storing carbon.
Many grasslands in solar farms are poor in plant diversity and managed with herbicides for weed control.
https://solarenergyuk.org/resource/solar-habitat-2025/
Poor vegetation growth in the shade beneath the panels reduces soil carbon accumulation and soil quality (Carvallho et al, 2025. https://eprints.whiterose.ac.uk/221644/ ). And there are many likely changes to the hydrology of the River Cole, streams and the floodplain soils and function.
“Biodiversity Net Gain” for hedgerows & birds?
Solar Farm proposers frequently cite new and improved (taller, bushier) hedgerows as an element of claimed Biodiversity Net Gain. This Old Hayes site already has hedgerows, some of which are tall and bushy and already of value for birdlife. Indeed, modifying hedgerow management is an easy win for biodiversity that can be carried out by any land manager at little cost.
The RSPB has recently published a statement “Solar farms managed for nature can boost bird numbers and biodiversity”, February 2025.
This has been repeated by Solar Energy UK in their 2025 publication promoted to MPs.
https://solarenergyuk.org/resource/solar-habitat-2025
The RSPB's statement refers to solar farms in arable-dominated landscapes and is based on a study (reference below) comparing intensively managed arable land in the Cambridgeshire Fens with nearby ‘mixed-habitat’ solar farms and ‘simple’ solar farms. Read more...
The mixed-habitat solar farms had hedgerows, hedgerow trees and wild flower margins. The ‘simple’ solar farms and arable sites had none of these features. Unsurprisingly, the mixed-habitat solar farms benefitted both woodland and farmland birds, in both variety and numbers of species. These included blackcap, chiffchaff, dunnock, jay, lesser whitethroat and willow warbler amongst the ‘woodland’ species, and farmland birds corn bunting, goldfinch, greenfinch, linnet, reed bunting and yellowhammer, plus cuckoo. Neither the arable nor simple solar showed this positive effect, with the notable exception of skylark and lapwing being most abundant in the arable.
The study emphasised the critical importance of infrequent cutting or grazing management of the grassland around the panels; regular cutting or grazing rendered the solar farm no better than intensive arable for birds.
This finding on solar farm management is supported by bird monitoring at Westmill Solar Farm, near Coleshill, where the areas round and under the panels are sown with a meadow mix. The security fencing also has a positive impact, with lack of disturbance meaning that productivity for ground-nesting birds is much higher than in the surrounding landscape.
The RSPB study notes that there is very little existing research on the impacts of solar farms on biodiversity, and indeed the few studies that exist have produced mixed results – some positive, some negative. References are given to bat activity being negatively affected, bumblebees and pollinating insects being positively affected if the solar farm is wholly managed as wildflower meadow.
The conclusion is “In nature-depleted landscapes, like arable farmland, solar farms managed for mixed habitat can increase bird abundance and biodiversity”. It is important to repeat that the Old Hayes proposed site is not a nature-depleted landscape, already with mixed habitat.
Reference: Copping, JP, Waite, CE, Balmford, A, Bradbury, RB, Field, RH, Morris, I and Finch, T.. 2025. Solar farm management influences breeding bird responses in an arable-dominated landscape. Bird Study, published on line February.2025
https://www.tandfonline.com/doi/full/10.1080/00063657.2025.2450392
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