A database of Defra statutory biodiversity metric unit values for terrestrial habitat samples across England, with plant, butterfly and bird species data

Policies requiring biodiversity no net loss or net gain as an outcome of environmental planning have become more prominent worldwide, catalysing interest in biodiversity offsetting as a mechanism to compensate for development impacts on nature. Offsets rely on credible and evidence-based methods to quantify biodiversity losses and gains. Following the introduction of the United Kingdom’s Environment Act in November 2021, all new developments requiring planning permission in England are expected to demonstrate a 10% biodiversity net gain from 2024, calculated using the statutory biodiversity metric framework (Defra, 2023). The metric is used to calculate both baseline and proposed post-development biodiversity units, and is set to play an increasingly prominent role in nature conservation nationwide. The metric has so far received limited scientific scrutiny.

This dataset comprises a database of statutory biodiversity metric unit values for terrestrial habitat samples across England. For each habitat sample, we present biodiversity units alongside five long-established single-attribute proxies for biodiversity (species richness, individual abundance, number of threatened species, mean species range or population, mean species range or population change). Data were compiled for species from three taxa (vascular plants, butterflies, birds), from sites across England. The dataset includes 24 sites within grassland, wetland, woodland and forest, sparsely vegetated land, cropland, heathland and shrub, i.e. all terrestrial broad habitats except urban and individual trees. Species data were reused from long-term ecological change monitoring datasets (mostly in the public domain), whilst biodiversity units were calculated following field visits. Fieldwork was carried out in April-October 2022 to calculate biodiversity units for the samples. Sites were initially assessed using metric version 3.1, which was current at the time of survey, and were subsequently updated to the statutory metric for analysis using field notes and species data. Species data were derived from 24 long-term ecological change monitoring sites across the Environmental Change Network (ECN), Long Term Monitoring Network (LTMN) and Ecological Continuity Trust (ECT), collected between 2010 and 2020.

Methods
Study sites

We studied 24 sites across the Environmental Change Network (ECN), Long Term Monitoring Network (LTMN) and Ecological Continuity Trust (ECT). Biodiversity units were calculated following field visits by the authors, whilst species data (response variables) were derived from long-term ecological change monitoring datasets collected by the sites and mostly held in the public domain (Table S1).

We used all seven ECN sites in England. We selected a complementary 13 LTMN sites to give good geographic and habitat representation across England. We included four datasets from sites supported by the ECT where 2 x 2m vascular plant quadrat data were available for reuse. The 24 sites included samples from all terrestrial broad habitats (sensu Defra 2023) in England, except urban and individual trees: grassland (8), wetland (6), woodland and forest (5), sparsely vegetated land (2), cropland (2), heathland and shrub (1). Non-terrestrial broad habitats (rivers and lakes, marine inlets and transitional waters) were excluded. Our samples ranged in biodiversity unit scores from 2 to 24, the full range of the metric. Not all 24 sites had long-term datasets from all taxa: 23 had vascular plant data, 8 had bird data, and 13 had butterfly data. We chose these three taxa as they are the most comprehensively surveyed taxa in England’s long-term biological datasets. Together they represent a taxonomically broad, although by no means representative, sample of English nature.

Biodiversity unit calculation

Baseline biodiversity units were attributed to each vegetation quadrat using the statutory biodiversity metric (Defra, 2023) (Equation 1). Sites were visited by the authors between April and October 2022, i.e. within the optimal survey period indicated in the metric guidance. Sites were assessed initially using metric version 3.1 (Panks et al., 2022), which was current at the time of survey, and were subsequently updated to the statutory metric for analysis using field notes and species data.. Following the biodiversity metric guidance, we calculated biodiversity units at the habitat parcel scale, such that polygons with consistent habitat type and condition are the unit of assessment. We assigned habitat type and condition score to all quadrats falling within the parcel. Where the current site conditions (2022) and quadrat data (2010 to 2020) differed from each other in habitat or condition, e.g. the % bracken cover, we deferred to the quadrat data in order to match our response and explanatory variables more fairly. Across all samples, area was set to 1 ha arbitrarily, and strategic significance set to 1 (no strategic significance), to allow comparison between sites. To assign biodiversity units to the bird and butterfly transects, we averaged the biodiversity units of plant quadrats within the transect routes plus a buffer of 500 m (birds) or 100 m (butterflies). Quadrats were positioned to represent the habitats present at each site proportionally, and transect routes were also positioned to represent the habitats present across each site. Although units have been calculated as precisely as possible for all taxa, we recognize that biodiversity units are calculated more precisely for the plant dataset than the bird and butterfly dataset: the size of transect buffer is subjective, and some transects run adjacent to offsite habitat that could not be accessed. Further detail about biodiversity unit calculation can be found in the Supporting Information.

Equation 1. Biodiversity unit calculation following the statutory biodiversity metric (Defra, 2023)

Size of habitat parcel × Distinctiveness × Condition × Strategic Significance = Biodiversity Units

Species response variable calculation

We reused species datasets for plants, birds and butterflies recorded by the sites to calculate our response variables (Table S1). Plant species presence data were recorded using 2 x 2m quadrats of all vascular plant species at approximately 50 sample locations per site (mean 48.1, sd 3.7), stratified to represent all habitat types on site. If the quadrat fell within woodland or scrub, trees and shrubs rooted within a 10 x 10 m plot centred on the quadrat were also counted and added to the quadrat species records, with any duplicate species records removed. We treated each quadrat as a sample point, and the most recent census year was analysed (ranging between 2011-2021). Bird data were collected annually using the Breeding Birds Survey method of the British Trust for Ornithology: two approximately parallel 1 km long transects were routed through representative habitat on each site. The five most recent census years were analysed (all fell between 2006-2019), treating each year as a sample point (Bateman et al., 2013). Butterfly data were collected annually using the Pollard Walk method of the UK Butterfly Monitoring Scheme: a fixed transect route taking 30 to 90 minutes to walk (c. 1-2 km) was established through representative habitat on each site. The five most recent census years were analysed (all fell between 2006-2019), treating each year as a sample point. Full detail of how these datasets were originally collected in the field can be found in Supporting Information.

For species richness estimates we omitted any records with vague taxon names not resolved to species level. Subspecies records were put back to the species level, as infraspecific taxa were recorded inconsistently across sites. Species synonyms were standardised across all sites prior to analysis. For bird abundance we used the maximum count of individuals recorded per site per year for each species as per the standard approach (Bateman et al., 2013). For butterfly abundance we used sum abundance over 26 weekly visits each year for each species at each site, using a GAM to interpolate missing weekly values (Dennis et al., 2013). Designated taxa were identified using the Great Britain Red List data held by JNCC (2022); species with any Red List designation other than Data Deficient or Least Concern were summed. Plant species range and range change index data followed PLANTATT (Hill et al., 2004). Range was measured as the number of 10x10 km cells across Great Britain that a species is found in. The change index measures the relative magnitude of range size change in standardised residuals, comparing 1930-1960 with 1987-1999. For birds, species mean population size across Great Britain followed Musgrove et al., 2013. We used the breeding season population size estimates to match field surveys. Bird long-term population percentage change (generally 1970-2014) followed Defra (2017). For butterflies, range and change data followed Fox et al., 2015. Range data was occupancy of UK 10 km squares 2010-2014. Change was percent abundance change 1976-2014. For all taxa, mean range and mean change were averaged from all the species present in the sample, not weighted by the species’ abundance in the sample.

·       Bateman, I. J., Harwood, A. R., Mace, G. M., Watson, R. T., Abson, D. J., Andrews, B., et al. (2013). Bringing ecosystem services into economic decision-making: Land use in the United Kingdom. Science (80-. ). 341, 45–50. doi: 10.1126/science.1234379.

·       British Trust for Ornithology (BTO), 2022. Breeding Bird methodology and survey design. Available online at  https://www.bto.org/our-science/projects/breeding-bird-survey/research-conservation/methodology-and-survey-design

·       Defra, 2023. Statutory biodiversity metric tools and guides. https://www.gov.uk/government/publications/statutory-biodiversity-metric-tools-and-guides. 

·       Dennis, E. B., Freeman, S. N., Brereton, T., and Roy, D. B. (2013). Indexing butterfly abundance whilst accounting for missing counts and variability in seasonal pattern. Methods Ecol. Evol. 4, 637–645. doi: 10.1111/2041-210X.12053.

·       Department for Environment, Food & Rural Affairs (Defra) 2017. Wild bird populations in the UK. All species, species trends, UK, 1970-2014. http://data.defra.gov.uk/Wildlife/all_species_index_species_trends_UK_1970_to_2014_rev.csv

·       Fox, R., Brereton, T. M., Asher, J., August, T. A., Botham, M. S., Bourn, N. A. D., … Roy, D. B. (2015). The State of the UK’s Butterflies 2015. Wareham, Dorset.

·       Hill, M. O., Preston, C. D., & Roy, D. B. (2004). Plantatt Atributes of British and Irish Plants: Status, Size, Life history, Geography and Habitats. Centre for Ecology and Hydrology, 80.

·       Joint Nature Conservation Committee (JNCC), 2022. Conservation designations for UK Taxa 2022. Available online https://hub.jncc.gov.uk/assets/478f7160-967b-4366-acdf-8941fd33850b

·       Musgrove, A., Aebischer, N., Eaton, M., Hearn, R., Newson, S., Noble, D., … Stroud, D. (2013). Population estimates of birds in Great Britain and the UK. British Birds, 106(4), 231–232

·       Natural England (2023a). The Biodiversity Metric 4.0 - User Guide. Retrieved from http://publications.naturalengland.org.uk/file/659370772502937

·       Natural England (2023b). The Biodiversity Metric 4.0 Technical Annex 2 - Technical Information. Retrieved from http://publications.naturalengland.org.uk/file/659370772502937

·       United Kingdom Butterfly Monitoring Scheme (UKBMS), 2022. Methods for recording butterfly transects. Available online at https://ukbms.org/methods