National statistics

4a Status of priority species: relative abundance

Updated 3 April 2024

Applies to England

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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

Data last updated: November 2023

Latest data available: 2021

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. 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 the population of priority species grows on average and decrease when the population declines.

This indicator should be read in conjunction with 4b which provides data on those England priority species for which distribution information is available.

Type of indicator

State Indicator

Assessment of Change in the relative abundance of priority species in England

  • Long term (1970 to 2021): Deteriorating
  • Short term (2016 to 2021): Little or no change
  • Latest year (2020 to 2021): Decreased

Note: Analysis of the underlying trends is undertaken by the data providers.

Status of priority species: relative abundance

Trend description for Figure 4a1

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 153 species for which abundance trends are available in England.

By 2021, the index of relative abundance of priority species in England had declined to 19.8% of its base-line value in 1970 (Figure 4a1). Over this long-term period, 12% of species showed a strong or weak increase and 73% showed a strong or weak decline.

More recently, between 2016 and 2021, the relative abundance index did not change significantly, changing from 19.0 in 2016 to 19.8 in 2021. Over this short-term period, 52% of species showed a strong or weak increase and 30% showed a strong or weak decline.

Figure 4a1: Change in the relative abundance of priority species in England, 1970 to 2021

Figure 4a2: Long-term and short-term changes in 153 priority species’ trends for in England, 1970 to 2021

Download the data for Figures 4a1 and 4a2 in ods format

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 Figures 4a1 and 4a2

  1. Figure 4a1 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; the CI narrows as data becomes available for groups such as bats in the 1990s.
  2. Figure 4a2 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.
  3. There are 5 species for which a short term assessment of change isn’t available
  4. All species in the indicator are present on the priority species list for England (Natural Environmental and Rural Communities Act 2006 – Section 41).

Further information

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 indicator 4b). 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 4a and 4b, as well as other information on other priority species for which there are insufficient data for inclusion in the indicator.

The indicator shows progress with commitments to improve the status of our wildlife and habitats. It is relevant to outcomes 1 and 3 in Biodiversity 2020: A strategy for England’s wildlife and ecosystem services (see Annex A). It is also relevant to a number of international targets (see Annex B of the aforementioned publication for further details).

The UK and England Biodiversity Indicators are currently being assessed alongside the Environment Improvement Plan Targets, and the new Kunming-Montreal Global Biodiversity Framework Targets, when this work has been completed the references to Biodiversity 2020 and the Aichi Global Biodiversity Framework Targets will be updated.

Background

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.

Of the 940 species on the priority species list for England, 153 have robust quantitative time-series data on relative abundance. The relative abundance measure comprises Birds (44), Butterflies (21), Mammals (7) and Moths (81). Approximately 60 species of moths appear in both the relative abundance (4a) and distribution (4b) indicators. The abundance datasets are generated largely from data collected by national 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 the technical background document for more detail.

A recent review of potential data sources has been conducted as part of the development work supporting the Statutory Instrument of the Environment Act 2021. This review suggests that data may be available for other species/taxonomic groups represented on the S41 list. These data have not been included in this update of Indicator 4a but should be considered for future updates.

The relative abundance of a species will increase when the population of the species grows; it will decrease when the population of the species declines.

Between 1970 and 2021, the index of relative abundance of priority species in England fell from 100 to 20 (95% credible interval 18, 22). This change was assessed as a statistically significant decline. The long-term assessment is made on the smoothed time series of relative species abundance. It is based on a test of statistical significance that compares the change and the 95% credible intervals between 1970 and 2021. If the credible interval is entirely below 100 (in this case 18, 22) 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 5year (2016 to 2021) period. If the credible interval for the most recent year (2021) is entirely below the value for 5-years previous (2016) 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. Between 2016 and 2021, the 153 priority species in the relative abundance measure on average showed no statistically significant change.

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 153 species from 4 broad taxonomic groups, see the technical background document for a detailed breakdown of the species and groups included. 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. 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, 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.

To create the composite index, a hierarchical modelling method for calculating multispecies indicators within a state-space formulation was used (Freeman et al. 2020). This method 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. The resulting index is an estimate of the geometric mean abundance, set to a value of 100 in the start year (the baseline). Changes subsequent to this reflect the average change in species abundance; 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 total 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.

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 4a1). 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 species that are included in the indicator, and the methods used to create the priority species indicator can be found in the technical background document.

The headline indicator (Figure 4a1) masks variation between the taxonomic groups. Figure 4a3 shows the index for each taxonomic group separately, generated using the same methods as the headline indicator. The moths have undergone the biggest decline with an index value in the final year (2021) that was only 13% of its value in 1970. Similar strong declines in moths were noted in 4b. Butterflies and birds have also experienced strong declines in 2021, with butterflies having an index value that was 31% of its value in 1976, and birds have an index value of 32% relative to its value in 1970. The mammals index (comprising 5 species of bat, Hazel Dormouse and one species of hare) is the only taxonomic group out of the 4 to experience an increase in its index value of 11% relative to its value in 1998. Within this average, some species have declined strongly, such as Hazel Dormice, whereas some bat species are slowly recovering from previous declines.

Figure 4a3: Change in relative species abundance by taxonomic group, 1970 to 2021

Download the data for Figure 4a3 in ods format

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 4a3:

  1. Figure 4a3 shows the smoothed trend (solid line) together with the 95% credible interval (shaded area) for each of the 4 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.
  2. Included in Figure 4a3 are 44 bird species, 21 butterfly species, 7 mammal species and 81 moth species.
  3. All species in the indicator are present on the priority species list for England (Natural Environmental and Rural Communities Act 2006 – Section 41).

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
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