Marine and Terrestrial Species Indicator for Scotland

This indicator shows changes in marine and terrestrial biodiversity in Scotland. The indicator helps measure progress towards national biodiversity commitments, which include the Scottish Biodiversity Strategy and the national outcome to 'value, enjoy, protect and enhance our environment'. It also helps measure progress towards commitments under the Convention on Biological Diversity and the UN Sustainable Development Goals.

Background

The indicator, which is presented as three lines, is based on trends in the occupancy and abundance of native species (3,559 species in total) in Scotland, from all regions and habitats, and including Scotland's seas within the Exclusive Economic Zone (EEZ: up to 200 nautical miles from the Scottish coast). The trends in abundance (for 583 species) come from a range of established monitoring schemes, and trends in occupancy (2,976 species) from analyses of biological records held by the Biological Records Centre. Abundance trends reflect changes in the number of individuals of a species, whilst occupancy trends are based on the number of sites where a species is present, reflecting the size of the range within which it is found, and can be less sensitive to change than measures of abundance. Trends in abundance and occupancy may vary in different ways and at different rates within the same species, and so the two measures are reported separately. This indicator provides two lines measuring change in terrestrial biodiversity, one for species' abundance and one for species' occupancy – note that the species compositions for these are very different. Trends in marine biodiversity are presented in a separate indicator of species' abundance – at present suitable occupancy data for marine species have not been analysed.

The indicators are the average of the constituent species' trends, set to a value of 100 in the start year (the baseline). Changes subsequent to this reflect the average change in species abundance or occupancy; if on average species' trends doubled, the indicator would rise to 200, if they halved it would fall to a value of 50. A smoothing process is used to reduce the impact of between-year fluctuations, such as might be caused by variation in weather, and so make underlying trends easier to detect.

All the indicators presented measure change since 1994 (except the marine indicator where sufficient data are only available from 1998), and run to the most recent year for which data are available (this varies between the three indicators, due to differing lags in reporting between the main data sources). Although some longer time-series are available, this 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. 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.

Technical Details

There are considerable biases in the availability of species trends for incorporation in the indicator. For example, vertebrates are over-represented in the terrestrial abundance indicator in comparison to invertebrates, and plants are absent entirely. Thus, these indicators are not perfect representations of change in all of Scotland's biodiversity and have considerable biases; they are, however, the best possible based upon current knowledge.

The source data and statistical methods for creating these indicators are discussed in detail in the technical report describing the development of this indicator (Eaton et al. 2021).

In brief, each indicator is the geometric mean of multiple species' trends (in either abundance or occupancy) for as many species of terrestrial and marine species for which Scottish-specific trends were available. The indicators are created using a new hierarchical modelling method for calculating multi-species indicators within a state-space formulation (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.

Data on individual species came from a variety of sources, principally:

• Abundance-based data generated by well-established national monitoring schemes. For a few species of birds and moths, trends were available from more than one source. In such cases we have identified a hierarchy for identifying the most suitable trend for incorporation. Marine fish data came from long-term sampling programmes for deepwater and demersal fish.
• Occupancy-based metrics using more ad hoc presence-only biological records. Following several years of development and refinement of methods to produce such trends at the UK scale (e.g. Outhwaite et al. 2018), trends for the UK's four constituent countries have been produced by the UK Centre for Ecology and Hydrology (UKCEH; previously CEH). Whilst the production of such trends at a smaller (national-level) spatial scale means a reduction in sample size and a consequent reduction in the number of species for which sufficiently robust trends can be derived, this work presents a step change in the ability to report upon Scotland's biodiversity. For Scotland 2,976 species have sufficient data to be included in an indicator.

Data for the indicators comes from a wide range of recording and monitoring schemes, as follows:

Marine abundance indicator:
Seabird Monitoring Programme; ICES International Bottom Trawl Survey; Marine Scotland Science Deepwater Trawl Survey.

Terrestrial abundance indicator:
BTO/JNCC/RSPB Breeding Bird Survey; Rare Breeding Birds Panel; BTO/JNCC/RSPB Wetland Bird Survey; Statutory Conservation Agency and RSPB Annual Breeding Bird Scheme; National Bat Monitoring Programme; UK Butterfly Monitoring Scheme; Rothamsted Insect Survey.

Terrestrial occupancy indicator:
occupancy trends were derived from data held from the Biological Records Centre at the UK Centre for Ecology and Hydrology, submitted from a wide range of recording schemes and societies, namely:
Aquatic Heteroptera Recording Scheme; Bees, Wasps and Ants Recording Society; British Arachnological Society - Spider Recording Scheme; British Bryological Society; British Lichen Society; British Myriapod and Isopod Group - Millipede Recording Scheme & Centipede Recording Scheme; Conchological Society of Great Britain and Ireland; Cranefly Recording Scheme; Empididae, Hybotidae & Dolichopodidae Recording Scheme; Fungus Gnat Recording Scheme.