The 2021 Environment Act and BNG
The 2021 Environment Act will make it mandatory for many developments in England to demonstrate they will result in a 10% increase in Biodiversity Net Gain (BNG). BNG is currently measured in units described by the government’s Biodiversity Metric, a simple indicator reflecting the area, ecological condition and distinctiveness (a proxy for conservation value) of each patch of habitat within the development boundary. The Act, however, requires developers to provide evidence that their BNG obligations have been met, and the requirements for management and monitoring extend for 30 years. With this long delivery period, there is a critical need to define standards for monitoring design, data collection, storage and analysis that will reliably demonstrate support for net gain outcomes, and for those standards to be sustainable and cost-efficient over decadal timescales.
As monitoring a wide range of species has traditionally been expensive, we have relied on a limited range of criteria to define habitat quality and value in the hope they represent most biodiversity values. The Environment Act specifically requires the calculation of units impacted and delivered to be made using the Metric, and the base monitoring requirements are limited to a summary of habitat type, extent and condition, i.e. a repeat of the Metric habitat assessment every few years. This minimal and strict focus on habitats is, however, at odds with the broader objectives of the policy to restore biodiversity in the wider sense, for example in line with the Good Practice Principles (Baker 2016), which define BNG as “development that leaves biodiversity in a better state than before, and an approach where developers work with local governments, wildlife groups, landowners and other stakeholders in order to support their priorities for nature conservation”. We would, therefore, recommend that good practice monitoring of BNG schemes should accept multiples lines of biodiversity evidence, including more detailed species assessments alongside habitat mapping.
Changes in habitat type, condition and area, as summarised in the Metric, provide one measure of ecosystem change, but it is widely recognised that this is only one blunt proxy for biodiversity, and is potentially biased and insensitive to wider changes. Many threats can impact the persistence of species without observable changes in habitat scores (e.g. impact of introduced diseases on red squirrels; Everest et al. 2021), and habitat creation does not imply the recovery of all plant or animal species (Fuentes-Montemayor et al. 2022, Hughes et al. 2023).
Putting to one side the suitability of habitat classes to represent the breadth of biodiversity, the ambiguity in how status or even some habitat classes are assigned poses clear problems for demonstrating measurable improvement for BNG. The legitimacy of offset systems rests on the assertion that (1) biodiversity units accurately portray the ecological value of different sites, meaning that compensation is proportionate to impact and results in a positive outcome and (2) the measured gains are the result of habitat creation/ restoration actions, not chance. While the scale by which the first point is measured has been the subject of the Metric’s development, far less attention has been paid to how surveyors will effectively demonstrate gains have occurred on the ground. Payments for offsets will be based on biodiversity outcomes, not because landowners simply performed all the ‘right’ actions (Pressey et al. 2021). Despite this, information on the monitoring methods that should be undertaken for BNG schemes is scarcely mentioned in the legislation, policy or guidance. Defra’s (2022) consultation documents have suggested that monitoring should provide a “summary of habitat type, extent, and condition (with a comparison where applicable against the expected condition)” – however, this limited information is unlikely to allow the success of wider BNG principles to be properly assessed. Site owners, land managers, local authorities and statutory agencies rapidly need to establish good practice for monitoring protocols and evidence types that will both satisfy the legal obligation to deliver measurable improvement and properly demonstrate that biodiversity is being conserved and enhanced.
Multiple lines of evidence
The new legislative requirements will require repeated habitat mapping surveys that cannot be achieved cost efficiently by increasing the time spent by individuals in the field – especially with the ongoing capacity crisis in the ecology sector (CIEEM 2022). Instead, we need to consider what tools are available for condition assessment of hundreds of individual sites and allow audit of the effectiveness of the BNG policy. One such technology that could serve this demand is ecoacoustic monitoring, alongside other methods such as satellite remote sensing, trail cameras and environmental DNA.
Ecoacoustic monitoring is increasingly undertaken using automated recorders that minimise human involvement in fieldwork and enable consistent surveying across landscapes (Browning et al. 2017). Passive acoustic monitoring can record sound data over long time periods, allowing standardised reproducible protocols independent of observer biases (Stowell and Sueur 2020). The raw sound files can be permanently archived for comparison with data collected in the future, potentially using analysis methods that have not even been created yet. Developments in ecoacoustics have now progressed to such a point that good practices for survey and monitoring can be implemented (Metcalf et al. 2023).
Ecoacoustics can be used to survey a wide range of species or to gather data on the entire soundscape (including environmental sounds and humangenerated noise) within an area. The breadth of taxa includes birds, mammals, orthoptera, amphibians and aquatic invertebrates. Additionally, soundscapes can be recorded above ground, below ground and in marine and fresh water (Farina and Gage 2017). The data gathered for all these species and ecosystems can provide information on changes in species composition, and the frequency of detection across sampling sites (occupancy) can act as an indicator of species rarity (Abrahams and Geary 2020). For some taxa, acoustic data may also provide an indication of abundance (Pérez-Granados and Traba 2021). The ecoacoustic approach is therefore well suited for long-term assessments of BNG compensation sites, offering an increased level of biodiversity information beyond that provided by vegetation assessments alone.
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