Accredited official statistics

Status of priority species: distribution

Updated 10 December 2024

Applies to England

© Crown copyright 2024

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This publication is available at https://www.gov.uk/government/statistics/england-biodiversity-indicators/status-of-priority-species-distribution

Last updated: 2024

Latest data available: 2016

Introduction

This indicator measures change in the number of 1 kilometre grid squares across England in which priority species were recorded in any given year. This is referred to as the ‘occupancy index’ and is effectively equivalent to changes in the distribution of priority species for which data are available. Priority species are defined as those appearing on the priority species list for England (Natural Environmental and Rural Communities Act 2006 - Section 41). In England there are 940 species on the priority species list. The priority species were highlighted as being of conservation concern for a variety of reasons, including rapid decline in some of their populations. The indicator will increase when priority species become more widespread on average and decrease when species becomes less widespread on average.

This indicator should be read in conjunction with the priority species relative abundance indicator which provides data on those species for which abundance information is available.

Data for this indicator can be found in the published datafile. We also produce a similar indicator at the UK scale.

Type of indicator

State Indicator

Type of official statistics

Official Statistic

Assessment of change

Measure Assessment Time period Result
Priority species distribution Long term 1970 to 2016 Deteriorating
Priority species distribution Short term 2011 to 2016 Deteriorating

The long-term assessment is made on the smoothed time series of relative species distribution. It is based on a test of statistical significance that compares the change and the 95% credible intervals between 1970 and 2016. If the credible interval is entirely below 100 (in this case 79 and 88) the time series would be assessed as decreasing, if it was entirely above 100 the indicator would be assessed as increasing, and if the credible interval spanned 100 the indicator would be assessed as no significant change.

To assess the short-term trends, the same approach was applied to the most recent 5-year (2011 to 2016) period. If the credible interval for the most recent year (2016) is entirely below the value for 5-years previous (2011) the time-series would be assessed as decreasing, if it was entirely above the value for 5-years previous the indicator would be assessed as increasing, and if the credible interval spanned the value for 5-years previous the indicator would be assessed as no significant change.

Key results

Official lists of priority species have been published for each UK country. There are 2,890 species on the combined list, of which in England there are 940 priority species; actions to conserve them are included within the respective countries’ biodiversity or environment strategies. This indicator shows the average change in the 181 species for which distribution trends are available in England.

By 2016, the index of distribution of priority species in England decreased to 84, a decrease of 16% of the 1970 value (Figure 1). Over this long-term period, 24% of species showed a strong or weak increase and 38% showed a strong or weak decline (Figure 2).

More recently, between 2011 and 2016, the distribution index declined from 92 to 84, that is, by 9% of the 2011 value. Over this short-term period, 13% of species showed a strong or weak increase and 59% showed a strong or weak decline.

Figure 1: Change in distribution of 181 priority species in England, 1970 to 2016

Source: Biological records data collated by a range of national schemes and local data centres.

Notes about Figure 1:

  • The line graph shows the smoothed trend (solid line) with its 95% credible interval (shaded area). The width of the credible interval (CI) is in part determined by the proportion of species in the indicator for which data are available.
  • All species in the indicator are present on the priority species list for England (Natural Environmental and Rural Communities Act 2006 – Section 41).
  • This indicator is not directly comparable with the previous publications

Source: Biological records data collated by a range of national schemes and local data centres.

Notes about Figure 2:

  • The bar chart shows the percentage of species within the indicator that have increased, decreased or shown little change in distribution (measured as the proportion of occupied sites), based on set thresholds of change.
  • All species in the indicator are present on the priority species list for England (Natural Environmental and Rural Communities Act 2006 – Section 41).
  • This indicator is not directly comparable with the previous publication; for the 2021 publication, this indicator has been updated with England-only level data for the first time.

Further detail

The trends of the taxonomic groups included within a multi-species indicator are often obscured by its composite nature. Indicator lines have been generated for a number of sub groups using the same method so that the trends for these groups can be seen more clearly (see Figure 3). The moths have undergone the biggest decline with an index value in the final year 63% of the value in 1970. Similar strong declines in moths were noted in the priority species abudance indicator. The underpinning causes of this decrease are not completely understood. These are counterbalanced by increases in bryophytes and lichens, which had an index value of 172 in 2016, although with broad credible intervals. The other taxa group have fluctuated since the beginning of the time series in 1970, and in 2016 was 5% below its baseline value.

Figure 3: Change in distribution of 181 priority species, by taxonomic group, 1970 to 2016

Source: Biological records data collated by a range of national schemes and local data centres.

Notes about Figure 3:

  • The graphs show the smoothed trend (solid line) with its 95% credible interval (shaded area) for each of the taxonomic groups included in the composite indicator. The width of the credible interval is in part determined by the proportion of species in the indicator for which data are available.
  • The figures in brackets show the number of species included in each measure.
  • Other taxa includes a number of insect groups, molluscs and spiders.
  • All species in the indicator are present on the priority species list for England (Natural Environmental and Rural Communities Act 2006 – Section 41).
  • This indicator is not directly comparable with the previous publication; for the 2021 publication, this indicator has been updated with England-only level data for the first time.

Priority species are defined as those appearing on the priority species list for England (Natural Environmental and Rural Communities Act 2006 - Section 41. In England there are 940 species on the priority species list. The priority species were highlighted as being of conservation concern for a variety of reasons, including rapid decline in some of their populations.

Currently, 181 of the 940 species on the priority species list for England have sufficient occupancy data available to be included in this indicator. These 181 species comprise of Bees (13), Bryophytes (9), Lichens (16), Moths (93), Spiders (9), and Wasps (7). Approximately 60 species of moths appear in both the relative abundance and distribution indicators. The species have not been selected as a representative sample of priority species and they cover only a limited range of taxonomic groups. The measure is therefore not fully representative of species in the wider countryside. See the Technical Annex for more detail.

The relative change in distribution of each of these species is measured by the number of 1 kilometre grid squares across England in which they were recorded –this is referred to as the ‘occupancy index’. Uncertainty in the species-specific annual occupancy estimates are incorporated into the overall indicator; details of how this was done are included in the Technical Annex.

The occupancy index will increase when a species becomes more widespread; it will decrease when a species becomes less widespread.

Relevance

Priority species are defined by the Secretary of State under Section 41 of the Natural Environment and Rural Communities Act 2006 as species which are of principal importance for the purpose of conserving biodiversity in England. The priority species list for England contains a total of 940 species. 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 the priority species abudance 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 priority species indicators, as well as other information on other priority species for which there are insufficient data for inclusion in the indicator.

International/domestic reporting

This indicator feeds into the Outcome Indicator Framework, a set of indicators describing environmental change related to the ten goals within the 25 year Environment Plan. As part of the Outcome Indicator Framework, this data contributes towards the evidence base used to prepare the annual progress report for the Environmental Improvement Plan. This indicator contributes to OIF indicator D6: Relative abundance and/or distribution of priority species in England.

Acknowledgements

Thank you to the many people who have contributed by providing data and to the many colleagues who have helped produce this indicator.

Technical annex

Background

The measure is a composite indicator of 181 species from 30 taxonomic groups, see Species List for a detailed breakdown of the species and groups in the indicator. The priority species identified in the Natural Environmental and Rural Communities Act 2006 – Section 41 for England 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 are 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.

Data sources

Biological records are observations of species at a particular location and at a particular time. Most records are made by volunteer recorders and, whilst these data may be collected following a specific protocol, the majority of records are opportunistic. As the intensity of recording varies in both space and time (Isaac et al. 2014), it can be difficult to extract robust trends in species’ distributions from unstructured data. Fortunately, a range of methods now exist for extracting signals of change using these data (for example, Szabo et al. 2010; Hill, 2012; Isaac et al. 2014). Of these methods - occupancy-detection models - are best-able to produce robust trends in occupancy (Isaac et al. 2014). Occupancy-detection models comprise 2 hierarchically coupled sub-models: an occupancy sub-model (that is, presence versus absence), and a detection sub-model (that is, detection versus non-detection). Together, these sub-models estimate the conditional probability that a species is detected when present. One distinctive feature of occupancy-detection models is that data need not be available for every year-site combination in order to infer a species’ occupancy (van Strien et al. 2013).

Occurrence records were extracted at the 1 kilometre grid square scale and with a temporal precision of one day. Data were collated through the Biological Records Centre and include data from the following recording schemes: 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; Bruchidae & Chrysomelidae Recording Scheme; Conchological Society of Great Britain and Ireland; Cranefly Recording Scheme; British Dragonfly Society; Empididae, Hybotidae & Dolichopodidae Recording Scheme; Fungus Gnat Recording Scheme; Gelechiid Recording Scheme; Grasshopper Recording Scheme; Ground Beetle Recording Scheme; Hoverfly Recording Scheme; Lacewings and Allies Recording Scheme; National Moth Recording Scheme; Riverfly Recording Schemes: Ephemeroptera, Plecoptera and Trichoptera; Soldierbeetles and Allies Recording Scheme; Soldierflies and Allies Recording Scheme; Terrestrial Heteroptera Recording Schemes; UK Ladybird Survey; Weevil and Bark Beetle Recording Scheme.

Data from between 1970 and 2016 were extracted as this represents the core period of recording for many of the taxonomic groups. However, some datasets do not cover the whole period. Since the 2018 indicator the Biological Records Centre has received updates of the scheme data from some taxonomic groups (Table 1). This has enabled the improvement of model estimates for certain years (particularly 2016). Note that approximately 60 of the 105 moth species also appear in the priority species abudance indicator.

Table 1: Summary of species’ time-series included in the priority distribution indicator. Only species in taxonomic groups for which the Biological Records Centre receives data are presented.

Taxonomic group Number of species on S41 list Number of species on S41 list with sufficient data for Priority Indicator Models updated since Outhwaite et al. (2018)
Ants 5 3 YES
Aquatic Bugs 1 1 NO
Bees 17 13 YES
Bryophytes 77 9 NO
Carabids 13 6 NO
Centipedes 1 0 NO
Craneflies 5 1 NO
Dragonflies 2 1 YES
Empidid & Dolichopodid 4 3 NO
Ephemeroptera 2 1 NO
Fungus Gnats 2 0 NO
Gelechiids 2 0 NO
Hoverflies 5 1 NO
Ladybirds 0 0 NO
Leaf and Seed Beetles 11 4 NO
Lichens 86 16 NO
Millipedes 3 0 NO
Molluscs 7 4 NO
Moths 105 93 NO
Neuropterida 1 0 NO
Orthoptera 3 2 NO
Plant Bugs 1 0 NO
Plecoptera 1 1 NO
Shield Bugs 0 0 NO
Soldier Beetles 0 0 NO
Soldierflies 5 4 NO
Spiders 24 9 NO
Trichoptera 3 0 NO
Wasps 5 7 YES
Weevils 2 2 NO
Total 393 181 NA

An occupancy-detection model, following van Strien et al. (2013) and Isaac et al. (2014), with improvements based on Outhwaite et al. (2018), was applied to all species from those taxonomic groups for which data were available. For each 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 based on multiple iterations (here 999) of model fitting. A detailed description of the occupancy model can be found in Outhwaite et al. (2019).

Methodology

From the occupancy model for each species we extract the proportion of occupied sites within England. We used only those species with at least 10 records in England and which passed data availability thresholds (Pocock et al. 2019), to ensure reliable inference. Although, due to the size of the dataset for moths, model quality tests were unavailable, so only those moth species with greater than or equal to 10 records in England and greater than or equal to 50 records across all regions (Outhwaite et al. 2019) were included. Given these data requirements, 181 species contributed to the Priority Species Indicator (Table 1).

To create the composite index, a new hierarchical modelling method for calculating multi-species indicators within a state-space formulation was used (Freeman et al. 2020), as for the priority species abudance indicator. The method produces an estimate of the annual geometric mean occupancy across species. The resulting index is 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 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 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 3.

The Freeman method combines the individual species abundance trends taking account of the confidence intervals around the individual trends. However, because the method is Bayesian, it produces credible intervals to show the variability around the combined index, as well as in the trends of individual species.

Each species in the indicator was weighted equally. When creating a species indicator weighting may be used to try to address biases in a dataset, for example, if one taxonomic group is represented by far more species than another, the latter could be given a higher weight so that both taxonomic groups contribute equally to the overall indicator. Complicated weighting can, however, make the meaning and communication of the indicator less transparent. The main bias on the data is that some taxonomic groups are not represented at all, which cannot be addressed by weighting. For this reason, and to ensure clarity of communication, equal weighting was used.

Assessment of change

Formal assessment of change is made on the basis of credible intervals for the time period; if the indicator value for the first year falls outside of the credible intervals for the final year then the indicator is deemed to have changed over that time period. This was done for three time periods; long-term (from the beginning of the time series to 2022), medium-term (the most recent 10 years) and short-term (the most recent 5 years).

To illustrate the variation in trends among individual species, an assessment of change is made for each species. Species are categorised into one of five categories on the basis of defined thresholds (Table 2). The five trend thresholds are based on average annual rates of change over the assessment period and are derived from the rates of decline used to assign species to the red and amber lists of Birds of Conservation Concern (Eaton et al., 2015). 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%).

Category Threshold Long term change
Strong increase An increase of more than 2.81% per annum Equivalent to an increase of more than 100% over 25 years
Weak increase An increase of between 1.16% and 2.81% per annum Equivalent to an increase of between 33% and 100% over 25 years
Little change Change is between +1.16 % and -1.14% per annum Equivalent to a change of between +33% and -25% over 25 years
Weak decrease A decrease of between 1.14% and 2.73% per annum Equivalent to a decrease of between 25% to 50% over 25 years
Strong decrease A decrease of more than 2.73% per annum Equivalent to a decrease of more than 50% over 25 years

Species list

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.

References

  • Freeman, S. N., Isaac, N. J. B., Besbeas, P., Dennis, E., B. and Morgan, B, J., T. (2020) A Generic Method for Estimating and Smoothing Multispecies Biodiversity Indicators Using Intermittent Data. Journal of Agricultural, Biological and Environmental Statistics, 26, 71 to 89. doi.org/10.1007/s13253-020-00410-6
  • Hill, M.H. (2012) Local frequency as a key to interpreting species occurrence data when recording effort is not known. Methods in Ecology and Evolution, 3(1), 195 to 205.
  • Isaac, N. J. B., van Strien, A. J., August, T. A., de Zeeuw, M. P. and Roy, D. B. (2014). Statistics for citizen science: extracting signals of change from noisy ecological data. Methods in Ecology and Evolution. https://doi.org/10.1111/2041-210X.12254
  • Outhwaite, C.L., Chandler, R.E., Powney, G.D., Collen, B., Gregory, R.D. & Isaac, N.J.B. (2018). Prior specification in Bayesian occupancy modelling improves analysis of species occurrence data. Ecological Indicators, 93, 333 to 343.
  • Outhwaite, C.L., Powney, G.D., August, T.A., Chandler, R.E., Rorke, S., Pescott, O.L., Harvey, M., Roy, H.E., Fox, R., Roy, D.B. & Alexander, K. (2019). Annual estimates of occupancy for bryophytes, lichens and invertebrates in the UK, 1970–2015. Scientific Data, 6, 1 to 12.
  • Pocock, M.J., Logie, M.W., Isaac, N.J., Outhwaite, C.L. and August, T. (2019). Rapid assessment of the suitability of multi-species citizen science datasets for occupancy trend analysis. BioRxiv, page 813626.
  • Szabo, J.K., Vesk, P.A., Baxter, P.W.J. & Possingham, H.P. (2010) Regional avian species declines estimated from volunteer-collected long-term data using List Length Analysis. Ecological Applications, 20, 2157 to 2169.
  • Van Strien, A. J., van Swaay, C. A. M. and Termaat, T. (2013). Opportunistic citizen science data of animal species produce reliable estimates of distribution trends if analysed with occupancy models. Journal of Applied Ecology, 50(6), 1450 to 1458. https://doi.org/10.1111/1365-2664.12158
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