Development of a combined marine and terrestrial biodiversity indicator: research

Executive summary

This report details work undertaken to produce a single high level indicator to measure trends in biodiversity in Scotland. The Scottish Government commissioned the work, keen to include an appropriate metric within the National Performance Framework (NPF). In the process of developing a single measure which combined data on both terrestrial and marine species' abundance and terrestrial species' occupancy (distribution), the authors raised concerns about whether such an approach could produce a meaningful indicator.

Following completion of the report, but prior to its publication, the Scottish Government has taken account of the concerns raised. With the exception of data on seabirds, it was noted that the data available for marine species abundance are largely restricted to seabed species fished commercially and that these species are subject more to fishing effort than environmental factors. As such, recent increases in fish stocks from a low base provide a misleading trend, and one that differs from the assessment of marine species across a wide range of ecosystem components provided in Scotland's Marine Assessment 2020 (SMA2020). Data from the SMA2020 was not suited to the NPF process due to the fact that its assessments do not report on an annual cycle. The Scottish Government has decided that for this indicator the NPF website should show trends in seabird species' abundance, terrestrial species abundance and terrestrial species' occupancy separately.

Indicator performance will be assessed as follows:

• if one or more measures of the three measures show deterioration then indicator performance is assessed as "Performance worsening";
• if one or more measures show improvement and any remaining measures show no clear change then indicator performance is assessed as "Performance improving";
• otherwise indicator performance is assessed as "Performance maintaining".

Showing the three measures will be more meaningful than one merged metric, and trends will be easier to describe. The Scottish Government wished to make readers of the report aware of this positive decision, but the report itself remains as presented to the Scottish Government. Some of the sections of the report where the concerns referred to above are raised by the researchers are now, therefore, less relevant than they were, since they have been addressed. The detail in the report about how data on different species can be gathered and amalgamated to reflect their abundance and distribution remains entirely relevant and will be adopted.

This report describes the work conducted by the RSPB, Centre for Ecology & Hydrology, James Hutton Institute, and University of Sheffield, under contract to the Scottish Government (reference SPB/001/18), in order to identify the most appropriate high-level indicator to measure and report trends in both terrestrial and marine biodiversity in Scotland. This indicator will enable trends in biodiversity to be considered as one of the 81 National Indicators in the National Performance Framework (one of eight used to measure progress towards the National Outcome for the Environment).

The combined biodiversity indicator proposed by this report will be an important measure of progress towards national commitments towards biodiversity. These include the commitments in the Scottish Biodiversity Strategy, and to the national outcome to 'value, enjoy, protect and enhance our environment'. Further than this, it can be regarded as a measure of progress towards targets in the European Union Biodiversity Strategy (such as target 1, to protect species and habitats), to the global Aichi targets used to assess progress towards commitments under the Convention on Biological Diversity and the UN Sustainable Development Goals.

The project entailed a number of components: i) a review of published literature relevant to the aims of this project; ii) a review of data on biodiversity in Scotland that might be used in a high-level indicator; iii) extensive consultation with a range of stakeholders to select the most suitable data and indicator format; iv) the collation of datasets required for indicator construction; and v) the creation of a draft indicator as presented in this report.

Most nations either publish or are developing biological indicators that assess the condition of specific aspects of the natural environment. The approaches adopted are diverse, but most fall into one of three definitions: the average trend in a measure of species' status (e.g. abundance); species status assessments often based around the International Union for Conservation of Nature (IUCN) Red List process; extent and condition of protected sites, habitat and ecosystems. The use of indicators showing the average trend in a measure of species' status is widespread, understood and accepted within the UK including within the indicators intended to measure progress against the Scottish Biodiversity Strategy. These existing indicators cover a range of measures encompassing some of Scotland's most valued biodiversity, and collectively give a valuable overview of trends in nature. The State of Nature reports at a UK (Hayhow et al. 2019) and Scotland scale (Walton et al. 2019) published government-endorsed high-level metrics of the status of biodiversity that are reasonably robust and credible measures of change. However, these reports did not publish a single headline indicator; the format of the NPF indicators require a single annually updated line. We also review other approaches to creating indicators using data on biodiversity, and proxies for biodiversity, but conclude that there is no compelling case for the use of these approaches given the availability of robust species' trend data for Scotland.

Our review of biodiversity data for Scotland found a considerable volume of species' data suitable for inclusion in a combined terrestrial and marine indicator. Robust abundance trends are available for 380 species of bird, mammal, butterflies and moths, and trends in occupancy (distribution) are available for an additional 1,578 species across a much broader taxonomic range including bryophytes, lichens and invertebrates. Constraints on the availability of these data are discussed: there are many gaps in data availability; for example, at present vascular plants are not included. Further still, we have far less data for marine species, and much of that which has been collected is not readily available for analysis.

An extensive consultation process was undertaken with Scottish Government and a wide range of other stakeholders in biodiversity policy, research and conservation. We report on this, describing how using stakeholder input helped us make key decisions on data to be used in a combined indicator, the treatment of this data, and the construction of the indicator itself.

We recommend an indicator based on the average of trends in species' status, measured at the scale of Scotland or Scottish marine waters. 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. In total, trends for 2,073 species have been combined in the draft indicator presented here. We recommend that as new species trends become available, they are adopted within the indicator.

The recommended indicator begins in 1994 and runs 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. Based on current data (January 2020), we would recommend the initial, final year should be 2016. The indicator has annual index values, and is capable of annual updates, with nearly all of the constituent species' trends being updated annually. The recommended indicator is 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, filter species trends for suitability for inclusion, ensuring individual species trends are robust. With the exception of marine fish trends, 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.

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. 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, or even show trends in a different direction 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 given the requirement of the NPF indicators to be single trend lines. Without this constraint 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. 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.

There are considerable biases in the availability of species trends for incorporation in the indicator. For example, the draft indicator contains trends for many 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 of 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.

We recommend the indicator be 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.

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 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 utility of such a high-level measure is doubtful as it will hide considerable, and important, differences in biodiversity trends between taxa, realms and ecosystems.

Differences between 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.

The single line in the proposed headline indicator, incorporating trends in an extremely wide range of species across disparate taxa, collectively found in most if not all of Scotland's habitats and regions, and responding both positively and negatively to a disparate range of drivers, is intended to reflect the most broadscale changes in the country's biodiversity. However, amalgamating such a wide range of data means that the single line can mask massive variation in trends between species, and such variation may reflect wider patterns of change. As such, the headline indicator alone may be a misleading measure of biodiversity health; we strongly recommend the publication of disaggregated indicators to aid interpretation (and avoid misinterpretation) of the headline indicator and potential ways of disaggregating the headline indicator are demonstrated in this report.

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 Biological Records Centre are used by CEH to generate occupancy trends annually, under a Joint Nature Conservancy Council-funded work programme). A relatively small amount of work would be required on an annual basis to update the combined indicator.

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 Scottish Biodiversity Information Forum (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.

Finally, we make a number of recommendations; regarding (i) the communication of the changes shown by the indicator, particularly as regards the use of disaggregated indicators to enable better understanding of the underlying causes of change in the headline metric, (ii) further analytical developments, and (iii) priorities for the collation and analysis of existing biodiversity data to enable future improvements of the indicator.