Accredited official statistics

Status of priority species: relative abundance

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/4a-status-of-priority-species-relative-abundance

Last updated: 2024

Latest data available: 2022

Introduction

This indicator shows changes in the relative abundance of priority species in England 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, and this indicator shows the average change in the 149 species for which abundance trends are available. The priority species were highlighted as being of conservation concern for a variety of reasons, including rapid decline in some of their populations. Presented in this indicator are values of abundance relative to the starting year (set to a value of 100), rather than absolute abundance. Changes to this value reflect the average change in species abundance; if on average species experienced a doubling in abundance, the indicator would rise to 200, if they halved it would fall to a value of 50.

The method for calculating this indicator has been updated in line with work done to develop the Indicators of species abundance in England publication. As a result, the method is not yet finalised and two smoothing options are presented. Note that the number of species is fewer (149 versus 153) than in the previous publication of the priority species indicator: this is because the modelled trend for four moth species did not meet the quality assurance thresholds for inclusion see Technical Annex.

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

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

Type of indicator

State indicator

Type of official statistics

Official statistics in development – indicator under development: The England biodiversity indicators project team would welcome feedback on the novel methods used in the development of this indicator. For more information, please visit the UK Statistics Authority’s website on Types of official statistics – UK Statistics Authority.

Assessment of change

Measure Assessment Time period Result
Priority species Long term 1970 to 2022 Deteriorating
Priority species Medium term 2012 to 2022 Little or no overall change
Priority species Short term 2017 to 2022 Little or no overall 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 149 species for which abundance trends are available in England.

By 2022, the index of change in relative abundance of priority species in England had declined to 21% of its baseline value in 1970 (Figure 1). Over this long-term period, 13% of species showed a strong or weak increase and 68% showed a strong or weak decline.

More recently, between 2017 and 2022, the relative abundance index did not change significantly. Over this short-term period, 51% of species showed a strong or weak increase and 38% showed a strong or weak decline.

Figure 1: Change in the relative abundance of 149 priority species in England, 1970 to 2022

Source: Bat Conservation Trust, British Trust for Ornithology, Butterfly Conservation, Defra, Joint Nature Conservation Committee, People’s Trust for Endangered Species, Rothamsted Research, Royal Society for the Protection of Birds, UK Centre for Ecology & Hydrology.

Notes about Figure 1

  • Figure 1 shows the two options for the smoothed trend (solid line) with their 95% credible intervals (shaded area).
  • The width of the credible interval is in part determined by the proportion of species in the indicator for which data are available.
  • Index values represent change from the baseline value in 1970, the credible interval widens as the index gets further from the 1970 value and confidence in the estimate of change relative to the baseline falls.
  • The credible intervals capture uncertainty in the trends of individual species that contribute to the index. They do not capture uncertainty associated with the spatial locations of sample points, nor about the degree to which the species represent wider biodiversity. The credible intervals partially capture uncertainty in the species abundance estimates.
  • Several different levels of smoothing were considered, with Option 1 being smoothed on a ten-year timescale and Option 2 being smoothed on a three-year timescale see discussion of smoothing in Technical Annex.

Source: Bat Conservation Trust, British Trust for Ornithology, Butterfly Conservation, Defra, Joint Nature Conservation Committee, People’s Trust for Endangered Species, Rothamsted Research, Royal Society for the Protection of Birds, UK Centre for Ecology & Hydrology.

Notes about Figure 2

  • Figure 2 shows the percentage of species within the indicator that have increased (weakly or strongly), decreased (weakly or strongly) or shown little change in abundance based on set thresholds of change see Methodology for more detail
  • Due to rounding, the data labels may not sum exactly to 100%.
  • There are five species for which a short term assessment of change isn’t available.

Further detail

The headline indicator (Figure 1) masks variation between the taxonomic groups. Figure 3 shows the index for each taxonomic group separately, generated using the same methods as the headline indicator. The relative abundance measure comprises 44 birds species, 21 butterflies, 7 mammals and 77 moths. The moths have undergone the biggest decline with an index value in the final year (2022) that was only 15% of its value in 1970. Butterflies and birds have also experienced strong declines in 2022, with butterflies having an index value that was 42% of its value in 1976, and birds have an index value of 28% relative to its value in 1970. The mammals index is the only taxonomic group out of the four to have not changed significantly from its baseline value in 1998.

Figure 3: Change in relative species abundance by taxonomic group, 1970 to 2022

Source: Bat Conservation Trust, British Trust for Ornithology, Butterfly Conservation, Defra, Joint Nature Conservation Committee, People’s Trust for Endangered Species, Rothamsted Research, Royal Society for the Protection of Birds, UK Centre for Ecology & Hydrology.

Notes about Figure 3

  • Figure 3 shows the two options for the smoothed trend (solid line) together with their 95% credible intervals (shaded area) for each of the four taxonomic groups included in the composite indicator.
  • Index values represent change from the baseline value for each group; in 1970 for birds, 1976 for butterflies, 1998 for mammals and 1970 for moths. The credible interval widens as the index gets further from the baseline value and confidence in the estimate of change relative to the baseline falls.
  • The width of the credible interval is in part determined by the number of species in the indicator for which data are available.
  • Included in Figure 3 are 44 bird species, 21 butterfly species, 7 mammal species and 77 moth species.
  • The credible intervals capture uncertainty in the trends of individual species that contribute to the index. They do not capture uncertainty associated with the spatial locations of sample points, nor about the degree to which the species represent wider biodiversity. The credible intervals partially capture uncertainty in the species abundance estimates.

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 abundant.

Measures of abundance are more sensitive to change than measures of distribution (see the priority species distribution 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.

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.

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

For a more in-depth discussion of the methodology and data used in this indicator, please see Indicators of species abundance in England. A brief summary of these is presented in the following.

Source data

Much of the data on species abundance is collected through well-established volunteer-based recording schemes, many of which are run through partnerships between government bodies, NGOs and research organisations, or through statutory monitoring schemes. 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 Species included.

Three criteria were used to assess whether data was suitable for inclusion in the indicator: that the scheme uses a standarised approach with protocols and appropriate analytical methods, has spatially replicated survey design across England and a taxonomic resolution ideally to species level. More details on these criteria can be found in the full statistical release

Robust English population time-series were sought for as many priority species as possible to produce the indicator for Priority Species in England. The measure is a composite indicator of 149 species from 4 broad taxonomic groups. The majority of the data in this indicator have previously been published and many of the datasets are currently used elsewhere within the England biodiversity indicators. Table 1 presents the taxonomic coverage and data sources contributing to the indicator. Further information on the analytical methods used by each scheme can be found here.

Table 1: Summary of information on the data collection schemes included in the indicators.

Name of scheme Taxonomic coverage Number of species in priority species indicator Timespan included in indicators
Breeding Bird Survey (BBS) Birds 25 1970-2022
Breeding Birds Survey (BBS) Mammals Mammals 1 1995-2022
National Bat Monitoring Programme (NBMP) Mammals 5 1998-2021
National Dormouse Monitoring Programme (NDMP) Mammals (single species) 1 1995-2022
Priority Moths Moths 11 1995-2021
Rare Breeding Birds Panel (RBBP) Birds 8 1970-2021
Rothamsted Insect Survey Light Trap Moths 66 1970-2022
Seabird Monitoring Programme (SMP) Birds 1 1986-2019
Statutory Conservation Agency and RSPB Annual Breeding Bird Scheme (SCARRABS) Birds 6 1971-2016
UK Butterfly Monitoring Scheme (UKBMS) Butterflies 21 1976-2022
Wetland Bird Survey (WeBS) Birds 4 1975-2021

Notes about Table 1:

  • The Breeding Bird Survey began in 1994 and incorporates the Waterways Breeding Bird Survey and the Heronries Census. Prior to this, data came from the Common Bird Census (CBC).

Species included

The species considered for inclusion in the England Priority Species Indicator are those on the S41 list. Species on the S41 list are those on the 2007 UK Biodiversity Action Plan (UK BAP) list that are present in England with the addition of Hen Harrier. There are a small number of taxa below the species level (that is, sub-species) on the S41 lists. Such infra-specific taxa were only retained if the associated species was not included. This led to the removal of three sub-species and reduced the total taxa on the S41 list from 943 to 940. However, not all species on that list have suitable data available. The species in the priority species indicator are those species for which annual estimates of abundance are available, derived from national-scale monitoring schemes. Currently it contains data on 149 species. Four species of moth were excluded from the priority species indicator this year, due to the same failure of quality assurance checks in the all-species indicator. These species are: Haworth’s minor (Celaena haworthii), grey mountain carpet (Entephria caesiata), crescent (Helotropha leucostigma), and heath rustic (Xestia agathina).

There are 12 Section 41 species included in the all-species index that have not yet been included in this version of the priority species index (8 fish, 1 bumblebee, 1 vascular plant and 2 freshwater invertebrates). To retain consistency and comparability with the previous release of this indicator, we have not added them to this release. The impact of this on the results is small and these species will be included in future version of the indicator. Table 2 gives a summary of the relationship between the number of species on the S41 list and the number of these for which population time-series are available, compared to the number of species in the UK.

Table 2: Taxonomic groups included in each of the all-species and priority species indicators and their representativeness of UK biodiversity

Higher group Group Number of species in the UK Species on S41 Species on S41 with data included in priority species indicator
Vertebrates Amphibians 7 4 -
Vertebrates Birds 218 49 44
Vertebrates Fish 82 48 -
Vertebrates Mammals 49 34 7
Vertebrates Reptiles 6 8 -
Invertebrates Coleoptera (beetles) 4,093 75 -
Invertebrates Butterflies 59 23 21
Invertebrates Hymenoptera (bees, ants, wasps) 7,154 31 -
Invertebrates Moths 2,345 142 77
Invertebrates Diptera (flies) 7,099 28 -
Invertebrates Other insects 3,197 23 -
Invertebrates Non-insect invertebrates 5,369 76 -
Plants Vascular plants 1,497 149 -
Plants Bryophytes 1,056 77 -
Plants Chlorophyta 2,342 15 -
Fungi Non-lichenised fungi 15,100 64 -
Fungi Lichens 2,354 94 -
TOTAL   - 940 149

Notes about Table 2:

  • An indicative list of species was only available for the whole of the UK (Burns et al., 2018), rather than for England specifically. The total number of multicellular eukaryotic freshwater and terrestrial species found in the UK is approximately 55,000, as reported in Burns et al., 2018.

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

Methodology

Pre-smoothing

Species abundance of many organisms tends to fluctuate from one year to the next. These fluctuations make it difficult to reveal the underlying trends. For this reason, some schemes include a statistical smoothing to remove short term stochastic variation. We therefore applied a smoothing term to each species time series, except those for which a smoothed trend was already available (bats and most of the birds). We applied a thin plate spline with 0.3 degrees of freedom for each data point (Fewster et al., 2000) and did this on the log scale. The resultant smoothed trends were then taken forward to the next stage of analysis.

To create the composite index, we used a method specifically developed for creating multispecies indicators from heterogeneous data (Freeman et al., 2020). The resulting index is an estimate of the geometric mean abundance. This is a relatively newly developed method and offers some advantages over older techniques: it is 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.

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. For this a penalised spline was used with the number of “knots” set to one of two values. Firstly, as has been done for previous iterations of the priority species indicator and as is standard elsewhere (Fewster et al., 2000), we used the total number of years of data divided by 3. Secondly, in order to reveal a more stable long-term trend in the data, we used the total number of years of data divided by 10. These two values were selected to demonstrate the range of plausible indicator values for the purposes assessing meaningful change in species abundance over time.

The overall trend shows the balance across all the species included in the indicator. Individual species within each measure may be increasing or decreasing in abundance (Figure 2). Estimates will be revised when new data or improved methodologies are developed and will, if necessary, be applied retrospectively to earlier years. Further details about the methods used to create the species indicator can be found in the full statistical release.

Confidence and uncertainty

The credible intervals around the multispecies index represent confidence in the degree to which average abundance in any given year is different from the baseline year (1970). They do not provide a clear guidance on the degree to which pairs of years (for example, 2000 versus 2022) differ.

The credible intervals capture uncertainty in the trends of individual species that contribute to the index. They do not capture uncertainty associated with the spatial locations of sample points, nor about the degree to which the species represent wider biodiversity. The credible intervals partially capture uncertainty in the species abundance estimates, inasmuch as the method includes a term to estimate measurement error. However, our approach does not explicitly propagate information about relative uncertainty of different species or years.

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 4). 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

Development plan

Developments planned for this indicator follow broadly the plans for the full statistical release. Over the short term, these plans include:

  • We will work towards selecting the most appropriate smoothing option and producing a single indicator of species abundance, and priority species abundance, in England. We particularly welcome user feedback on the two options presented in this indicator.
  • A review of potential data sources was conducted as part of the development work supporting Schedule 2 of The Environmental Targets (Biodiversity) (England) Regulations 2023, and suggests that data may be available for additional priority species that could be added to the priority species indicator. These data have not been included in this indicator but we intend to include them in future releases if data allows.
  • We will develop an indicator for the abundance of priority-species at the UK scale.
  • We will develop an indicator of priority-species distribution in England to complement the abundance indicator.

Longer term development plans:

  • We will review on an ongoing basis new species abundance data that may become available
  • We will continue to review the data that feeds into the indicator. This will include ongoing review of the status of monitoring schemes (including the schemes that provide data that is used in the current indicator, as well as those that may provide new abundance data in future).
  • We will continue to review the representativeness of the indicator. This will include reviewing how well the indicator represents the species groups that are already included in the indicator, as well as identifying opportunities to improve our evidence where there are specific gaps.
  • In this indicator we have broken down the trend by taxonomic group only. In future, we will explore further options for breakdowns that may be useful for users of the statistic (for example, separate trends for generalist and specialist species).
  • We will review our methods for assessing change over short and medium time-scales in the indicators and, if appropriate, refine them further.
  • We will continue to improve the quality of the raw data, representation of the indicator, and methodology, in line with our commitment to the Code of Practice for Statistics.

References

  • Burns, F., Eaton, M.A., Hayhow, D.B., Outhwaite, C.L., Al Fulaij, N., August, T.A., Boughey, K.L., Brereton, T., Brown, A., Bullock, D.J., Gent, T., Haysom, K.A., Isaac, N.J.B., Johns, D.G., Macadam, C.R., Mathews, F., Noble, D.G., Powney, G.D., Sims, D.W., Smart, S.M., Stroh, P., Walker, K.J., Webb, J.R., Webb, T.J., and Gregory, R.D. (2018). An assessment of the state of nature in the United Kingdom: A review of findings, methods and impact. Ecological Indicators, 94(1), 226 to 236. doi.org/10.1016/j.ecolind.2018.06.033
  • Eaton, M. A., Burns, F., Isaac, N. J. B., Gregory, R. D., August, T. A., Barlow, K. E., … Williams, J. (2015). The priority species indicator: measuring the trends in threatened species in the UK. Biodiversity, 16(2–3), 108–119. https://doi.org/10.1080/14888386.2015.1068222
  • Fewster, R M., S T. Buckland, G M. Siriwardena, S R. Baillie, and J D. Wilson (2000). Analysis of Population Trends for Farmland Birds Using Generalized Additive Models. Ecology 81, 1970–84.
  • 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
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