Marine and Terrestrial Species Indicators: Experimental Statistic

Published: 2021

An Experimental Official Statistics Publication for Scotland

This release 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.

Evidence

The indicator, which is presented as three lines, is based on trends in the occupancy and abundance of native species (in total trends of 3,049 species are used in this report) 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 (2,466 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. Two of the indicators measure change in terrestrial biodiversity, one for species’ abundance and one for species’ occupancy – note that the species compositions for these are very different. (Where trend data for a species is available for both abundance and occupancy, the abundance data is used.) Trends in marine biodiversity are represented by a separate indicator based on seabird abundance; trends for other marine species’ groups are shown as additional contextual information, but do not contribute towards the headline marine indicator. At present suitable occupancy data for marine species have not been analysed.

Any biodiversity indicator can only represent a sample of the species present. Overall, we have incorporated data for 3,049 species out of around 60,000 present in Scotland and its waters, and we have been able to include some the groups for which Scotland is internationally important such as seabirds, lichens and bryophytes.  This is the most comprehensive indicator for Scotland’s species to date. There are significant biases in the representation of Scotland’s biodiversity, they are, however, the best possible based upon current knowledge and as data from other species groups are made available these will be incorporated in the future.

The indicators

When presenting the indicators a common end date of 2016 was selected.

Marine species’ abundance

Terrestrial species’ abundance

Terrestrial species’ occupancy

Assessment of trends in the three Scottish biodiversity indicators

Marine abundance indicator

• The indicator has shown a statistically significant decrease of 36% from 1994 to 2016.
• The indicator rose by 2.7% over the comparison year 2015 to 2016; this change is assessed as stable. Although more recent marine data are shown below, we have reported on that year to allow easy comparison with the other indicators.

Terrestrial abundance indicator

• The indicator has shown a statistically significant decline of 31% since the start year of 1994.
• The indicator fell by 4.8% over the last year (2015 to 2016); this change is assessed as stable.

Terrestrial occupancy indicator

• The indicator has shown a statistically significant increase of 24% since the start year of 1994.
• The indicator rose by 0.8% over the last year (2015 to 2016); this change is assessed as stable.

Additional information on indicator trends

The three indicators summarise trends from a large number of species across many taxa collectively found in most if not all of Scotland’s habitats and regions, and responding both positively and negatively to a range of drivers. We can gain understanding into the patterns and causes of change in Scottish biodiversity by disaggregating the headline indicators into trends for subsets of species. Here we have split the two terrestrial indicators by their main taxonomic groups to provide some additional insight. In addition, we provide trends in abundance for additional marine taxonomic groups; although seabird abundance alone is regarded as a better measure of marine biodiversity overall, these trends provide further context.

Marine abundance indicators for additional species’ groups

Disaggregated terrestrial abundance indicator

 

Disaggregated terrestrial occupancy indicator

Drivers of change in Scottish biodiversity

The changes in average abundance and occupancy of terrestrial species and in abundance of marine species have been caused by a wide range of drivers of change acting on species’ populations and the environments they live in; identifying drivers for each of the species is beyond the scope of this report. Below we have considered the likely impact of the five key drivers of change identified by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) Global Assessment on Biodiversity and Ecosystem Services (IPBES 2019), as well as the positive impacts of conservation action taken to address biodiversity loss. A range of more comprehensive assessments of the impact of such drivers on Scottish biodiversity exist, such as The State of Nature Scotland 2019 report (Walton et al. 2019).

Of course, it should be remembered that changes happened to the Scottish landscape and its biodiversity before the start of the indicators given here. Although much evidence exists to the nature of these changes and their impacts on biodiversity, robust quantitative measures of change are lacking. However, Walton et al. (2019) reported a decline of 14% in an indicator of terrestrial species’ occupancy between 1970 and 1994 (the starting point for the indicator reported here), thus the increase in the terrestrial occupancy indicator presented here may represent a recovery from a decline since 1970.

Land- and sea-use change

Positive impacts: recent agri-environment schemes have arrested declines for some farmland species; generalist woodland species have benefitted from afforestation.

Negative impacts: agricultural intensification has driven declines in many species; afforestation has resulted in the loss of other habitats; urbanisation has resulted in habitat loss and fragmentation.

Scotland’s land area has been heavily modified through centuries of management for human purposes such as the production of food, timber, minerals and other commodities, and providing homes and other infrastructure. In recent decades, whilst there have been some changes in broad categories of land use, such as increased urbanisation and afforestation, probably the biggest impacts on biodiversity have arisen from changes in the management within existing land-uses. These have included agricultural intensification which has affected the 70% of Scotland that is farmed, with for example the increased use of chemical inputs, changed sowing seasons, the loss of non-cropped habitats and changes in livestock densities (as well as deer). At a UK-scale such intensification has been demonstrated to have severe impacts upon many species (e.g. Burns et al. 2016), although trends in Scottish biodiversity, such as farmland birds, suggest that the impact may not have been as great as in the wider UK, at least in more recent years (NatureScot 2021).

Support for wildlife-friendly farming through agri-environment schemes and High Nature Value farming has enabled the recovery of some target species such as Corn Bunting (Perkins et al. 2011) and biodiversity more widely (Wilkinson et al. 2012).

Large-scale afforestation over past decades has supported increased populations and range expansions in generalist woodland species, such as some moths, particularly those that use conifer host-plants such as the Pine Carpet (Randle et al. 2019, SNH 2019). However, specialist woodland species across a wide range of taxa have declined in the absence of suitable management. In addition, afforestation has led to the loss of upland peat bog habitat home to rare habitat specialist species, as well as having a range of other impacts on biodiversity (e.g. see Burton et al. 2018).

Climate change

Positive impacts: northward and altitudinal range expansions in many species leading to increases in abundance and occupancy.

Negative impacts: range retractions in northern and montane species; complex influences on ecosystems and foodwebs e.g. in Scotland’s seas.

Climate change is a significant and growing threat to biodiversity globally (IPBES 2019), and its impact is being increasingly detected in Scotland and its seas. The effects are many, and varied, with a mixed impact in biodiversity; presently, at least, many species are benefiting. For example, species are shifting ranges northwards, through Scotland, with consequent increases in abundance and range for birds such as Nuthatches (Gillings et al. 2015) and butterflies including Ringlets (Fox et al. 2015). A trend towards milder winters may have resulted in improved survival for bird species such as Goldfinches. For many species, the influence of climate change may be serving to soften the impact of land-use changes and other adverse environmental changes. However, species with their southern range limits in Scotland, and those associated with cold montane habitats where, for example, duration of snow-lie is decreasing, are likely to undergo range contraction and possibly even extinction from Scotland (ASC 2016). Further threats arise from increased frequency and severity of storms and other extreme weather events, and the impact of sea-level rise on vulnerable coastal habitats.

In marine ecosystems climate change is having rapid and widespread effects through warming waters, salinity changes, acidification and seasonal stratification (MCCIP 2020). The impact of climate change is being felt through marine food webs, with changes in phytoplankton, feeding through to zooplankton (in which rapid northward shifts in range have been detected) to fish and top predators including large fish, seabirds and marine mammals. The substantial increases in fish abundance shown by the marine indicator may in part reflect increases in small-bodied non-commercial species such as the Boarfish in response to warming seas (Monterro-Serra 2015). Cephalopods too have expanded their ranges in the North Sea as a response to warming conditions (van der Kooij et al. 2016). Declines in seabirds such as Kittiwakes have been linked to climate change impacts upon food webs including key prey species such as Sandeels (Carrol et al. 2017).

Pollution

Positive impacts: levels of principal air and water pollutants have declined in Scotland in recent decades; recoveries in some bryophytes and invertebrates may have occurred in response.

Negative impacts: long-term effects of atmospheric pollutants on sensitive habitats remain evident and in addition, novel air and water pollutants are a concern.

There has been marked progress over the lifetime of the indicators presented here in reducing the principal sources of air and water pollution in Scotland; for example, levels of all the principal air pollutants have declined since 1990, substantially in most cases (Scottish Government 2020). However, many pollutants have had long-term impacts so the influence on sensitive habitats remains substantial; the impacts of nutrient enrichment and acidification remain, although some recoveries have been detected such as in sensitive bryophytes (Pakeman et al. 2019). The strong recovery in the occupancy of freshwater invertebrates between 2000 and 2010 suggests a beneficial impact of environmental regulations and consequent management changes in reversing biodiversity declines (Outhwaite et al. 2020). Nonetheless, concerns have been raised over the impact of a range of new pollutants, most notably plastic pollution in marine environments, but also light and noise pollution, and new chemical contaminants including novel agrichemical products. For a large part the impacts of these newer pollutants on biodiversity have not yet been well quantified.

Natural resource use and exploitation

Positive impacts: recent changes in fisheries management, with an increasing number of commercial fish stocks being fished sustainably, have led to some signs of recovery albeit from very low baselines.

Negative impacts: fishing with bottom-towed gear continues to cause damage to sensitive seabed habitats.

Scottish marine waters hold important stocks of fish and shellfish which are commercially exploited, and such fishing has had radical impacts on fish populations and thus marine ecosystems more widely over centuries, with a lengthy history of overexploitation. In recent decades, however, fisheries management has improved, although there is some way to go; in 2017, 54% of key Scottish fish stocks were assessed as being fished sustainably (Scottish Government 2018). There are welcome signs of recovery in fish stocks in both the North and Celtic Seas, as shown by existing indicators showing increases in species such as Cod and Haddock (Walton et al. 2019). In addition, fishing pressure in the deep seas off the west of Scotland has been decreasing since the early 2000s with the introduction of management measures (Mindel et al. 2018). Increases in fish abundance shown in the marine abundance indicator reported here may to some extent reflect increases of smaller less commercially valuable species increasing in response to perturbation of fish communities by over-fishing, although this is not the case in the deep sea where size-based indicators have shown a positive recent trend (Mindel et al. 2018). The increasing trends in sharks and their relatives is also particularly encouraging, as this group is among the most highly threatened of all vertebrate groups (Dulvy et al. 2014). Bottom-towed fishing gear do, however, continue to cause extensive damage to vulnerable seabed habitats and populations of the species that occupy them; a recent expansion of the Marine Protected Area network to 22% of Scottish waters will hopefully make a substantial step towards protecting seafloor integrity.

Invasive species

Positive impacts: some projects have removed or reduced populations of invasive non-native species (INNS).

Negative impacts: numbers of INNS established continue to grow, with northward range shifts, and intensifying impacts on native wildlife.

Invasive non-native species (INNS) are recognised as one of the greatest threats to biodiversity globally, and numbers of species established in Great Britain are continuing to grow (JNCC 2020); to date 180 species have had a documented negative impact on biodiversity (Roy et al. 2017). Specific examples of negative impacts, such as on breeding seabirds from introduced mammalian predators (e.g. Towns et al. 2011), are well-documented; mechanisms range from predation on native species, competition, hybridisation, habitat damage and disease and pathogen transfer. At present impacts are limited mostly to certain groups of native species, and particular habitats such as islands, freshwater bodies and native woodlands, but the problems caused by INNS seem very likely to increase. Localised projects have provided significant benefits for native biodiversity by removing INNS, such as non-native Black Rats on the Shiant Islands (Main et al. 2019) enabling populations of seabirds such as Storm Petrels and Manx Shearwaters to recover.

Conservation action

Over the twenty-plus years spanned by these biodiversity indicators, a wide range of conservation actions have helped biodiversity in Scotland. These include legislation and policies aimed at delivery broad environmental improvements, such as those which have driven down levels of damaging air pollution and delivered improvements in fisheries management, to action to designate and improve the management of Scotland’s most important sites and landscapes for wildlife, and to species-focused work, targeted at endangered species such as the Corncrake, Twinflower, Marsh Fritillary and Scottish Wildcat. In some cases, we can identify species recoveries, in both abundance and range (i.e. as measured by changes in occupancy), which can be linked to conservation action. However, for the most part we lack knowledge on counterfactuals – what would have happened to species’ populations in the absence of conservation action – to adequately quantify the impact of such action. That said, we have evidence of local and population-specific benefits from conservation, such as the recovery of the White-tailed Eagle and Red Kite, and the restoration of large areas of degraded peat bog in the Flow Country, to give an understanding how conservation action has helped Scottish biodiversity over the period of these indicators.

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.

Technical Background and Source Data

All the indicators presented measure change since 1994 (except trends for marine species’ groups other than seabirds, for which 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.

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 vascular 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. Species’ trends are entered into the indicators without weighting (each species has an equal impact upon the average indicator value).

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 the 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 2,466 species have sufficient data to be included in an indicator.

Change in the indicators is reported over two periods; the long-term change since the start year of the indicator, and single-year change between the penultimate and final year of the sequence. Long-term changes were assessed by whether the values for the start and end year were statistically significantly different. To measure whether changes over the final year were meaningful, we have applied percentage thresholds informed by variation in the indicator previous, and the measurement error around the indicator.

• The final-year change in the marine (seabird) indicator will be regarded as increasing if the final value is 5% greater than that for the penultimate year; decreasing if the final value is 5% lower; otherwise stable.
• The final-year change in the terrestrial abundance indicator will be regarded as increasing if the final value is 6% greater than that for the penultimate year; decreasing if the final value is 6% lower; otherwise stable.
• The final-year change in the terrestrial occupancy indicator will be regarded as increasing if the final value is 1% greater than that for the penultimate year; decreasing if the final value is 1% lower; otherwise stable.

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;
• Gelechiid Recording Scheme;
• Grasshopper Recording Scheme;
• Ground Beetle Recording Scheme;
• Lacewings and Allies Recording Scheme;
• National Moth Recording Scheme;
• Riverfly Recording Schemes: Ephemeroptera;
• Riverfly Recording Schemes: Plecoptera;
• Riverfly Recording Schemes: Trichoptera;
• Soldier Beetles, Jewel Beetles and Glow-worms Recording Scheme;
• Soldierflies and Allies Recording Scheme;
• Staphylinidae Recording Scheme;
• Terrestrial Heteroptera Recording Schemes;
• UK Ladybird Survey;
• Weevil and Bark Beetle Recording Scheme.

The great majority of the contributing data sources above are collected through the efforts of expert and dedicated volunteer surveyors and recorders, without whom our knowledge of biodiversity in Scotland would be much poorer, and this indicator would not be possible.

This is an Experimental Official Statistics publication for Scotland and we would welcome comments and feedback. Please contact [email protected].

References

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