Time-to-target condition

Only ‘Difficulty of creation/ enhancement’ counts towards HBU  while ‘Time-to-target condition’ is referred to for planning, monitoring and review purposes only. ‘Time-to-target condition’ refers to a timescale, ranging up till ‘30’ years and beyond, required for a proposed habitat to be either newly created or enhanced from the baseline. The computed duration is a function of habitat distinctiveness and condition and not considered further in the computation of HBU, for reasons outlined below:

Minimise bias

The presence of more than one multiplier contributing to the calculation of a HBU score in England’s Biodiversity Metric 3.0 has been demonstrated to work against Biodiversity Net Gain (BNG) attainment for projects with proposed habitats of high distinctiveness by way of resulting in a lower HBU score 1. This is especially true when two multipliers are less than zero, the product of which is a lower numerical value than either individual multiplier due to an increase in decimal places. Since high priority habitats are difficult to create, naturally a particular risk multiplier of habitat creation would be less than zero. This bias in fact encourages the creation of habitats of lower diversity due to their ease of attainment. 

Adaptive localisation

The relegation of ‘Time-to-target condition’ to a reference role recognizes the value of preserving it for the purposes of planning, monitoring and review only, as mentioned above, while at the same time reflects the operational rationale driving the on-going development of the SGBA: to minimise measurement subjectivity, increase parsimony for better understanding, enhance practicality on the ground, and tailoring to a unique local context (e.g., habitat creation via land reclamation as extensively practised in Singapore [discussed later]). Another instance of adaptive localisation is the exclusion of one weight present in the Statutory Biodiversity Metric, namely, ‘Strategic Significance’, from the SGBA. This was due to the absence of explicit local authority policy planning governing the development of geographical areas. This would effectively remove another layer of subjectivity in assessing HBU as  ‘Strategic Significance’ scores are assigned on a narrow 3-point scale according to the presence and type of local planning authorization or gazetting. 

Metric evolution

The adaptive and selective use of weights discussed above indicates the context in which the SGBA has been developed and the criteria upon which it is formulated. This evolution in weight selection and usage is not unique to the SGBA. As a tool develops, it is usual to modify or drop certain measurement parameters or weights. For instance, a parameter called an ‘Ecological Connectivity multiplier’ which featured in earlier versions of the England’s Biodiversity Metric was dropped by its third iteration 2. Therefore, the broadening accessibility and implementation of an ecological metric tool across multiple areas (e.g., planning and development) represents a trade-off between tool sophistication and usability which works to the advantage of non-expert practitioners in the field 3. The SGBA represents a pioneering tool in a local and possibly regional context, open to on-going adaptation and improvement in the absence of globally standardised instrumentation. 

Minimise redundancy

While potentially successful offsetting attempts might be achieved through the use of risk-related multipliers (or weights), one study indicated that they were neither always necessary in all conditions to offset biodiversity unit losses nor free of shortcomings 3. Utilising the Biodiversity Metric (version 1.0), sufficient biodiversity net gain was judged attainable without multipliers such as habitat creation difficulty and time to target conditions for proposed habitat changes to Scottish villages and golf course sites which already had some vegetation. For instance, existing intensive grassland and plantation coniferous woodland were ‘traded upwards’ to grassland with wildflowers to broadleaved native species woodland, respectively. However, the multipliers were only required for an open cast coal mine slated for restoration which presumably had little vegetation, mistakenly classified as ‘arable’ instead of treated as ‘unrestored’, presumably biassing upwards (i.e., increasing) the demands for compensation. These two findings were attributed to the sufficient flexibility of the Biodiversity Metric to account for compensatory mechanisms (i.e., "trading-up") to enhance a particular habitat with additional vegetation types. Furthermore, it was argued that the presence of additional weights (e.g., distinctiveness and condition scores, multipliers) would obscure the evaluation of biodiversity units for both practitioners and audience to whom the assessments are targeted. 

Minimise risk

On a similar note, another study reported an additional shortcoming of restoration difficulty and temporal risk multipliers, namely, that projections into the future entailed risks (e.g., policy reversals, implementation and restoration failure, etc. 4). 

Thus, the fewer risk-related multipliers there were,  the more transparent the accounting process, the higher the practical usability by non-experts and the lesser the associated risks. 

1: Glenister (2022)

2: Martinez-Cillero et al. (2023)

3: McVittie & Faccioli (2020)

4: zu Ermgassen et al. (2021)