8. A headline biodiversity indicator for Scotland: recommendations
145. We recommend an indicator based on trends in species' status, measured at the scale of Scotland or Scottish marine waters (defined by the EEZ). These trends should be measured in either abundance or occupancy, across as many species as possible to provide taxonomic breadth and thus represent the scope of Scottish biodiversity as best as possible.
146. The indicator will begin in 1994 and run to the most recent year for which data are available. The start year has been identified as the best balance between providing as long a time-series as possible, but keeping the taxonomic groups contributing to the index broadly consistent throughout (a substantial tranche of bird trends become available from 1994 onwards). Based on current data, we would recommend the current final year should be 2016. The indicator will have annual index values, and be capable of annual updates, with nearly all of the constituent species' trends being updated annually.
147. The indicator will be based on trends in abundance from a range of established monitoring schemes and trends in occupancy from analyses of biological records held by the Biological Records Centre. Set rules, either imposed by those organisations that operate these monitoring schemes, or created for the purposes of this indicator, will filter species trends for suitability for inclusion, ensuring individual species trends are robust. With the exception of marine fish trends, which were produced specifically for this indicator, these species' trends are created by existing work programmes, meaning that future updates of the proposed indicator will be efficient and low-cost. All single species trends are derived using well-established and published methods.
148. Whilst the combination of abundance and occupancy trends in the same metric, as proposed, is not currently used for other government biodiversity indicators in the UK, it is not without precedent. The same approach, also without the use of weighting to correct perceived biases, was used in the State of Nature 2016 report (Hayhow et al. 2016, Burns et al. 2018) and in a Dutch 'Living Planet Index' (Van Strein et al. 2016).
149. However, we should caution that this approach does combine trends measured in two different 'currencies', of abundance and occupancy (distribution), which may vary in different ways and at different rates within the same species. There is evidence that changes in occupancy may differ in scale to those in abundance (Van Strien et al. 2019), or even show trends in a different direction (Dennis et al. 2019), and so combining the two currencies in a single metric is far from ideal. Our recommendation to do so is on the basis that we feel the much greater taxonomic representation this gives the indicator warrants this approach; if the requirement of the NPF indicators was not for a single indicator line, we would not recommend the combining of the two currencies. Note the 2019 State of Nature report (Hayhow et al. 2019, Walton et al. 2019) did not combine abundance and occupancy data in a single measure, but was able to present measures of change in each separately.
150. Note that using the two currencies together means that the indicator can only be described in abstract terms; a change cannot be described in terms of either abundance, or distribution.
151. The draft indicator is based upon species' trends from the following sources: UK Breeding Bird Survey, Wetland Bird Survey, Statutory Conservation Agency and RSPB Annual Breeding Bird Scheme, Rare Breeding Birds Panel, Seabird Monitoring Programme, UK Butterfly Monitoring Scheme, Rothamsted Insect Survey, National Bat Monitoring Programme, International Bottom Trawl Surveys, and a wide range of biological recording schemes collated within the Biological Records Centre. In total, trends for 2,073 species have been combined in the draft indicator presented here. A breakdown of species trends by source is given in Appendix 1. We recommend that as new species trends become available they are adopted within the indicator; this might include single or small numbers of species through the development of existing schemes (e.g. when increasing survey coverage on the Breeding Bird Survey increases to enable the production of Scotland-specific trends for additional species), or larger numbers in the event of new monitoring programmes maturing to the point at which species trends are available (e.g. the National Plant Monitoring Scheme, which started in 2015).
152. There are considerable biases in the availability of species trends for incorporation in the indicator. For example, the draft indicator contains trends for far more terrestrial and freshwater species than marine species, and vertebrates are over-represented in comparison to invertebrates and plants. However, we have failed to identify an objective approach to weighting the indicator to address these biases, so propose the indicator should be the unweighted average or all available species' trends. Most notably this means that taxonomic groups measured using trends in distribution have a greater impact on the indicator than those for which we have abundance trends, and terrestrial and freshwater species have a far greater influence than marine species.
153. We recommend that the indicator is created using a new hierarchical modelling method for calculating multi-species indicators within a state-space formulation developed by CEH (Freeman et al. 2020) which offers some advantages over the more traditional geometric mean method; it is robust, precise, adaptable to different data types and can cope with the issues often presented by biological monitoring data, such as varying start dates of datasets and missing values. As Figure 5 shows, the two methods provide similar results.
154. The project team, and stakeholders involved in consultations as part of this project, hold substantial reservations about the value of the proposed indicator for assessing change in Scottish biodiversity. A number of the decisions made, particularly regarding whether to combine trends in abundance and occupancy, and whether to weight to address biases in data availability, had no obvious "correct" answer and other choices to those made may have been equally valid. We therefore retain substantial reservations about the value of an indicator summarising biodiversity trends at such a high level, particularly across terrestrial, freshwater and marine realms combined. Even if we were able to do this perfectly, the value of such a high-level measure is doubtful as it will hide considerable, and important, changes in biodiversity.
155. The draft indicator presented here is derived from much the same data sources as used for metrics in the recent State of Nature Scotland 2019 report (Walton et al. 2019), but does not fully match the metrics presented in that SNH-endorsed report. Walton et al. did not seek to produce a single metric, as is required by the format of the NPF indicators; therefore abundance and occupancy trends were not combined, nor were data for terrestrial and marine biodiversity. The general pattern of the separate State of Nature indicators for abundance and occupancy can be seen in the draft composite indicator (Figure 6), but with some variance caused by differences in the use of data. Chiefly, the State of Nature occupancy indicator started in 1970 (not 1994) and underwent a pre-1994 decline omitted from the draft combined indicator; it incorporated vascular plant trends for this longer period (omitted from the joint indicator due to the lack of annual data; see paragraph 107); and the composite indicator includes many (largely increasing) trends for marine fish that were not included in the State of Nature metrics.
156. Differences being the now widely circulated and used metrics in the State of Nature Scotland 2019 report and the draft composite indicator do have the potential to cause confusion unless carefully communicated. However, a similar broad pattern of biodiversity loss is shown by both measures
157. As stated previously, the draft indicator is derived from existing data sources that are updated annually by funded monitoring programmes. To a large extent these programmes also run analyses to produce updated indices on an annual basis, or routinely make data available for those analyses to be conducted (biological records submitted to the BRC are used by the CEH to generate occupancy trends annually, under a JNCC-funded work programme). A relatively small amount of work would be required on an annual basis to update the combined indicator: to collate species trends, derive trends for the small proportion of species for which these are not readily available (e.g. to analyses ICES data for demersal fish using the approach developed by this project) and to calculate the indicator using the code provided.
158. The indicator proposed is, we feel, the best option currently available to represent change in terrestrial and marine biodiversity in Scotland although, as emphasised above, is very imperfect. We have in this report identified a range of steps that might be taken to improve upon this indicator. Some are far-reaching changes to the structure of biodiversity recording, such as those recommended by the SBIF review (Wilson et al. 2018) to lead to a much improved system for biodiversity data collection, collation, curation and use in Scotland: if implemented this would lead to many improvements in data availability.
159. This indicator increased by 9% from 1994 to 2006, but declined thereafter to a level 13% below the 1994 level in 2016.