Status of priority species: distribution

Figure 1: Change in distribution of 476 UK priority species, 1970 to 2018

Figure 2: Long-term and short-term changes in 476 UK priority species’ distribution trends, 1970 to 2018

Figure 3: Change in distribution of 476 priority species, by taxonomic group, 1970 to 2018

Relevance

Priorities for species and habitat conservation are set at a country level through country biodiversity or environment strategies. Each country has an identified list of priority species, which are of high conservation concern due, for example, to restricted range or population declines. The indicator therefore includes a substantial number of species that, by definition, are becoming less widespread.

Measures of distribution are less sensitive to change than measures of abundance (see priority species abundance indicator). Nonetheless, if a threatened species that has been declining starts to recover, its distribution should stabilise, and may start to increase. If the proportion of species in the indicator that are stable or increasing grows, the indicator will start to decline less steeply. If the proportion declines, it will fall more steeply. Success can therefore be judged by reference to trends in both indicators, as well as other information on other priority species for which there are insufficient data for inclusion in the indicator.

International/domestic reporting

The UK Biodiversity Indicators have been reviewed in response to the latest goals and targets agreed under the Kunming-Montreal Global Biodiversity Framework (GBF) of the Convention on Biological Diversity (CBD). The indicators are being adapted, with some new indicators being developed, to better align them with the monitoring framework for the GBF. The suite of biodiversity indicators is therefore expected to change ready for forthcoming UK national reports to CBD in 2026 and 2029.

Web links for further information

• Aquatic Heteroptera Recording Scheme: Home page
• British Arachnological Society Spider Recording Scheme: Home page
• Bees, Wasps and Ants Recording Society: Identification guides to download
• British Bryological Society: Home page
• British Dragonfly Society: Recording Dragonflies and Damselflies in the British Isles
• British Lichen Society: Home page
• British Myriapod and Isopod Group: Centipede and Millipede recording schemes
• Bruchidae & Chrysomelidae Recording Scheme: Relevant BRC webpage
• Butterfly Conservation: Butterflies and Moths
• Conchological Society of Great Britain and Ireland: Home page
• Dipterists Forum: Cranefly,Empididae & Dolichopodidae, Fungus gnat Recording Schemes
• Gelechiid Recording Scheme: Home page
• Ground Beetle Recording Scheme: Home page
• Hoverfly Recording Scheme: Home page
• Hypogean Crustacea Recording Scheme: Home page
• Lacewings and allies: Recording schemes
• National Moth Recording Scheme: Home page
• Orthoptera Recording Scheme: Home page
• Riverfly Recording Schemes: Ephemeroptera, Plecoptera and Trichoptera: Home page
• Soldierflies and Allies Recording Scheme: Home page
• Staphylinidae Recording Scheme: Relevant BRC webpage
• Terrestrial Heteroptera Recording Scheme - Shield bugs and allied species: Home page
• UK Biodiversity Partnership: UK Biodiversity Action Plans
• UK Centre for Ecology & Hydrology – Biological Records Centre: Recording Schemes
• UK beetle recording: Weevil and Bark Beetle Recording Schemes
• Natural England: S41 List of priority species in England
• NatureScot Scottish Biodiversity List
• Wales Biodiversity Partnership Section 7 priority species in Wales
• Northern Ireland Environment Agency Northern Ireland Priority Species List

Background

The measure is a composite indicator of 476 species from 22 taxonomic groups (eight of the 30 groups originally modelled did not contain any species with sufficient data to be included in the final analysis), see Species List below for a detailed breakdown of the species and groups in the indicator. The priority species identified in each of the four UK countries were highlighted as being of conservation concern for a variety of reasons, including their scarcity, their iconic nature or a rapid decline in their population. They are not representative of wider species in general. They do however include a range of taxonomic groups and will respond to the range of environmental pressures that biodiversity policy aims to address, including land use change, climate change, invasive species and pollution. The short-term assessment of change can be used to assess the impact of recent conservation efforts and policy aimed at halting and reversing species declines. However, natural fluctuations (particularly in invertebrate populations) and short-term response to weather may have a strong influence on the short-term assessment.

Regardless of advances in statistical techniques and the increase in the number of biological records collected, there are likely to be species on the priority lists for which little monitoring or occurrence data is available. Reasons for this include rarity, difficulty of detection, or those for which monitoring methods are unreliable or unavailable. In order for the indicator to be representative of priority species, a method of assessing the changing status of these remaining data-poor species would need to be considered.

The indicator represents annual change in the geometric mean estimated occupancy across the constituent species. The index is set to a value of 100 in the start year (the baseline), so that changes subsequent to this represent proportional change in occupancy; if on average species’ trends doubled, the indicator would rise to 200, if they halved it would fall to a value of 50.

Data sources and species-specific time series

Biological records data are used to produce an indicator of change based on annual estimates of the proportion of occupied sites (“occupancy”) for a set of priority species in the UK. Biological records are observations of species in a known place in space and time.These data are curated by the Biological Records Centre. They include data from the following recording schemes: Aquatic Heteroptera Recording Scheme, Bees, Wasps andAnts Recording Society, British Arachnological Society Spider Recording Scheme, British Bryological Society, British Isles Neuropterida Recording Scheme, British Lichen Society, Centipede Recording Scheme, British Myriapod and Isopod Group, Millipede RecordingScheme, Bruchidae & Chrysomelidae Recording Scheme, Conchological Society of Great Britain and Ireland, Cranefly Recording Scheme, British Dragonfly Society, Empididae & Dolichopodidae Recording Scheme, Fungus Gnat Recording Scheme, Gelechiid Recording Scheme, Ground Beetle Recording Scheme, Hoverfly Recording Scheme, National MothRecording Scheme, Orthoptera Recording Scheme, Riverfly Recording Schemes: Ephemeroptera, Plecoptera and Trichoptera, Soldierflies and Allies Recording Scheme, Staphylinidae Recording Scheme, Terrestrial Heteroptera Recording Scheme - Shield bugs and allied species and the Weevil and Bark Beetle Recording Scheme + Scolytidae.

Most records are made by volunteer recorders and whilst these data may be collected following a specific protocol, the majority of records in these datasets are opportunistic. The intensity of recording varies in both space and time (Isaac et al. 2014), which is a challenge for estimating robust quantitative trends. Fortunately, a range of methods now exist for producing such trends using unstructured biological records data (e.g., Szabo et al., 2010;Hill, 2012; Isaac et al., 2014). Bayesian occupancy models have been shown to be more robust and more powerful than these other methods when analysing this kind of data (Isaac et al., 2014), specifically because the occupancy model explicitly models the data collection process and produces annual estimates for each species of the proportion of occupied sites (van Strien et al., 2013).

By using occupancy models to analyse occurrence records, greater taxonomic coverage was achieved for the 2015 C4b indicator (Eaton et al. 2015). Further improvements to the occupancy modelling framework (Outhwaite et al. 2018) enabled the inclusion of more taxonomic groups (for example, lichens, craneflies and weevils) in the 2020 indicator and more species from groups included in previous indicators. In 2021, new rules-of-thumb for assessing the data available for each species (Pocock et al. 2019) have been applied for 2021, giving greater confidence in the trend lines produced.

Annual occupancy estimates are available for 5,293 UK bryophytes, lichens, and invertebrates in 31 taxonomic groups with sufficient data (Outhwaite et al. 2019). For each 1 by 1 kilometre site-year combination, the model estimates presence or absence for the species in question given variation in detection probability: from this the proportion of occupied sites, ‘occupancy’ was estimated for each year. The models are analysed in a Bayesian framework, meaning that, in addition to point estimates of occupancy, credible intervals (a measure of uncertainty) can be generated for each species’ time-series. A detailed description of the occupancy model can be found in Outhwaite et al. (2019). These occupancy models are updated as and when new, (validated) data are received from recording schemes. Although continued improvements to the modelling process mean that more species can now be included, estimates cannot currently be established for all priority species. Only recording schemes with sufficient data and for which the scheme has validated the assumptions of occupancy models.

Annual estimates of occupancy from between 1970 and 2018 were used for the multi-species indicator as this represents a core period of recording for many of the taxonomic groups. However, some datasets finished at different years within this time period. Since the 2020 C4b indicator the Biological Records Centre has received substantial data updates for Aquatic bugs, Empid & Dolichopodid Flies, Ephemeroptera, Plecoptera and Trichoptera (Table 1). This has enabled the improvement of model estimates for recent years and the addition of new species to the indicator.

A change from the approach used to select species-specific trends for the 2020 indicator is the adoption of new criteria (rules-of-thumb), based on the suitability of the underlying data for producing occupancy trends with acceptable precision. This data-driven approach is considered to be more objective than the previous threshold of 50 records (Pocock et al. 2019). Rarely recorded species (less than 1 record in every 100 visits) were excluded if there were fewer than 3.1 records across the 10% of the best recorded years. More frequently recorded species were excluded if there were fewer than 6.7 records across the 10% of the best recorded years (Pocock et al. 2019). Exclusion criteria are based on classification trees, selected to balance the rates at which species are excluded when not meeting precision thresholds and included when meeting the precision thresholds. These model quality tests were unavailable for the moth dataset, so moth species with fewer than 50 records across the UK (Outhwaite et al. 2019, Powney et al. 2019) were excluded. The change in threshold creates a turnover of just a few species for all groups except Bryophytes and Lichens, which would gain 75 and 50 species, respectively. It is felt that further investigation and consultation with the scheme is required to assess whether the addition of 125 species to the indicator would be justified, therefore the new rules of thumb were not applied to Bryophytes and Lichens this year. They were instead excluded if there were fewer than 50 records across the UK.

Changing the criteria for species inclusion has led to the addition of 60 species and the loss of 12 species, compared with the 2020 publication. In addition, 13 new species have been added following a re-evaluation of mismatches between species names in the priority lists and those in the UK Species Inventory. In total, the 2021 indicator comprises 476 species that met these criteria for inclusion. This represents a net increase of 61 species compared with the 2020 indicator.

Table 1: Summary of species’ time-series included in the Priority Species Bayesian measure

Methodology

The composite indicator was produced using a novel hierarchical modelling method for calculating multi-species indicators developed by UKCEH (Freeman et al. 2020), which offers some advantages over the geometric mean method used to produce the 2020 indicator. It can be applied to multiple data types, improving the comparability between metrics derived from occupancy and abundance data and can account for the uncertainty associated with the underlying species-specific time series as well as uncertainty in the indicator arising from the subset of species that are included. Case studies with four taxonomic groups show it to be robust to missing values, especially when these are non-random, for example when declining species are more likely to be missing observations in recent years or if recent colonists are absent earlier in the time series. Imputing missing values is informed by between-year changes in species for which data is available, assuming shared environmental responses. Additionally, a smoothing process is used to reduce the impact of between-year fluctuations - such as those caused by variation in weather - making underlying trends easier to detect. The smoothing parameter (number of knots) was set to the number of years divided by three following Fewster et al. (2000).

The Bayesian occupancy approach enables an estimation of species occurrence even though the data used in this indicator were collected without a standardised survey design (van Strien et al., 2013; Isaac et al., 2014a and b). For each species, records were extracted at the one kilometre grid square scale with records on different days being treated separately, and an annual time-series of the proportion of sites occupied was calculated. Each species-specific time-series was scaled so the first value in 1970 was set to 100. The annual index was estimated as the arithmetic mean of the scaled species-specific occupancy estimates. Each species was given equal weighting within the indicator. Uncertainty in the species-specific annual occupancy estimates is represented by the 95% credible intervals. See the technical background document for further detail on production of the indicator.

A hierarchical, state-space model based on species annual growth rates was used to estimate the multispecies indicator (Freeman et al. 2020). The hierarchical structure allows estimation of uncertainty from different sources including that associated with the underlying species-specific index values and from having an incomplete set of trends for priority species with which to estimate the indicator. A smoothing process reduces the impact of between-year fluctuations - such as those caused by variation in weather - making underlying trends easier to detect.

Assessment of change

Species were grouped into one of 5 categories based on both their short-term (over the most recent 5 years of data) and long-term (all years) mean annual change in occupancy (Table 2).

Asymmetric percentage change thresholds are used to define these classes as they refer to proportional change, where a doubling of a species index (an increase of 100%) is counterbalanced by a halving (a decrease of 50%).

The threshold values for each category were based on those of the wild bird indicator; whether an individual species is increasing or decreasing has been decided by its rate of annual change over the time period (long or short) of interest. If the rate of annual change would lead to an occupancy increase or decrease of between 25 per cent and 49 per cent over 25 years, the species is said to have shown a ‘weak increase’ or a ‘weak decline’ respectively. If the rate of annual change would lead to a population increase or decrease of 50 per cent or more over 25 years, the species is said to have shown a ‘strong increase’ or a ‘strong decline’ respectively. These thresholds are used in the Birds of Conservation Concern (PDF 1.6MB) status assessment for birds in the UK.

Table 2: Thresholds used to define individual species’ trends

Species list

The species list was derived from the biodiversity lists of the 4 UK countries. A link to the list for each country is provided below:

• England – Section 41 Species
• Northern Ireland – Northern Ireland Priority Species List
• Scotland – Scottish Biodiversity List
• Wales - Section 7 Priority species

Table 3: The biodiversity lists of the 4 countries of the UK

Table 4: Taxonomic breakdown of combined 4 country biodiversity list

The species lists are unchanged from those used in the 2020 indicator analysis. The criteria for inclusion in each of the 4 biodiversity lists are derived from those used to identify the UK Biodiversity Action Plan (UK BAP) priority species list, most recently in 2007, but there has been some divergence in approaches, see Table 3. For example, the Scottish biodiversity list and the Northern Ireland priority species list both have criteria based on rarity alone, whereas the UK BAP criteria did not consider rarity; rare species were only listed if they were considered threatened or declining.

For the purposes of this indicator, an inclusive approach has been taken, whereby a species only has to be included in one of the country lists to be included on the combined list. The Scottish Biodiversity list has a final criterion based on the importance of species to people, however, species listed as a result of this criterion were not considered here. The taxonomic composition of the combined Four Country List (FCL) is shown in Table 4.

Some countries have included a small number of taxa below the species level (that is, sub-species) on their biodiversity lists. Such infra-specific taxa were only retained on the combined 4 country biodiversity list if the associated species was not included. For example, a sub-species of the grass rivulet moth (Perizoma albulata) is included on the Scottish biodiversity list, but it is a full species on the Northern Ireland priority species list, thus on the combined list only the full species was retained.

For a full species list, please see the published datafile.

Development plan

Since our previous publication we have adapted the language and visualisations used in this indicator. We are keen to hear from our users about these changes, as well as our published development plan, please email us.