TRICKY METRICS

There has been a move by Planning Authorities and others in recent times to try to put numbers on biodiversity and habitats in an attempt to quantify gains and losses in a way that sets numerical targets for ecology in change of land-use. This has been termed Biodiversity Metrics and is doubtless well-intentioned, but like its predecessor “Biodiversity Offset”, it does not work. Worse, it misrepresents and weakens the science. The situation is further confused by the wording of the National Planning Policy Framework and other policy documents requiring “measurable” biodiversity gain[1]. I will try to explain in the context of an increasingly frequent request: to use the metrics in order to show that a specified level of Biodiversity Net Gain (BNG) in a development project can be attained.

Biodiversity Net Gain (BNG) Metric Calculations – the caveats.

Below are listed our eight principal caveats which, please note, are mostly taken from the published metrics methods documentation. The emphasis is mine.

1. The metric calculation only accounts for direct impacts on habitats within the footprint of a development or project. It is only a simple assessment tool and only considers direct impacts on biodiversity through impacts on habitats. Indirect impacts, which it is important to consider, are not included in the metric.

2. Biodiversity unit calculations are not absolute values but provide a proxy for the relative biodiversity worth of a site pre- and post-intervention. The metric is not a substitute for expert ecological advice. The metric should never be used to override or undermine the mitigation hierarchy (see below) or any existing planning policy or legislation.

3. The metric does not include species explicitly but uses habitat types as a proxy for the so-called “biodiversity value” of the species communities of those different habitats. (Metric outputs do not change existing levels of species protection and do not replace the processes linked to species protection regimes.)

4. Using habitats as a proxy for biodiversity is a simplification and biodiversity metric unit calculations/scores are not scientifically precise or absolute values, only a proxy for the relative biodiversity worth of a habitat or site.

5. The metric and any numerical outputs are not absolute values but must be interpreted using ecological expertise and common sense. If they are used at all, it should only be as one (potentially misleading, so great care needed) element of the evidence that informs plans and decisions. The metric is not in or of itself a solution to biodiversity decisions. The metric does not give instructions, for example on the species to use in habitat enhancement/ compensation.

6. Biodiversity metrics have a focus on typical habitats and widespread species; protected and locally important species’ needs are not considered; protected sites and irreplaceable habitats are not adequately measured by the metric. Notable habitats and features require appraisal separately by a skilled ecological scientist, as we at Betts do as part of a survey and report.

7. Biodiversity metric instructions state that “metric design aims to encourage enhancement, not transformation, of the natural environment. Proper consideration should be given to the habitats being lost in favour of higher-scoring habitats, and whether the retention of less distinctive but well-established habitats may sometimes be a better option for local biodiversity. Habitat created to compensate for loss of natural or semi-natural habitat should be of the same broad habitat type unless there is a good ecological reason to do otherwise”. However, the metrics are not required to do what is stated here and may well confuse the situation.

8. Biodiversity metrics are not designed to override expert opinion. “Management interventions should be guided by appropriate expert ecological advice and not just the biodiversity unit outputs of the metric. Ecological principles still need to be applied to ensure that what is being proposed is realistic and deliverable based on local conditions such as geology, hydrology, nutrient levels, etc. and the complexity of future management requirements”.

Betts Ecology In-House Procedure

• We make clients aware of the caveats above.
• We add site- or job-specific details that are relevant and explain why our stand-alone survey-and-report approach for the project concerned is superior to the metrics approach (and saves time and money).
• Because there is so much subjectivity in making the metrics assessment decisions, we promote an expert view and Ecological Constraints and Opportunities method with listed requirements in the Avoid, Minimise, Remediate, Compensate hierarchy to which are added the enhancements (as a specification of measures on the ground rather than a numerical score) necessary to demonstrate BNG. Biodiversity enhancements (or losses) are described qualitatively and quantitatively where this is meaningful (habitat type/area, evidentially-projected species changes, alterations in ecosystem services, etc.) but not as a metric for the reasons given above.

*  *  *

Betts Ecology do not recommend the use of Biodiversity Metrics for calculating BNG because of their limitations, ability to be confusing and their tendency towards lack of application to the circumstances of projects and the sites’ ecology. It is especially important to be wary of using any outcome of the metrics in isolation, segregated from appropriate site survey and fieldwork. Clients will obtain a much more accurate, useful, reliable and relevant explanation and description of BNG for their projects from the site surveys and report recommendations we make. Our experienced ecological scientists warn that BNG metrics can be seriously misleading.

[1] Paragraph 179 of the NPPF reads, “ To protect and enhance biodiversity and geodiversity, plans should:

… identify and pursue opportunities for securing measurable net gains for biodiversity.”