Status of priority species: relative abundance

Figure 1: Trend in the relative abundance of priority species in the UK, 1970 to 2021

Figure 2: Long-term and short-term changes in 149 priority species’ abundance trends for in England, 1970 to 2021

Figure 3: Change in relative species abundance by taxonomic group, 1970 to 2021

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 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 priority species indicators (abundance and distribution), 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

• Bat Conservation Trust: The National Bat Monitoring Programme
• British Trust for Ornithology: Wild Bird Indicators
• Butterfly Conservation: Butterflies and Moths
• UK Butterfly Monitoring Scheme: Butterflies as indicators
• BTO: Seabird Monitoring Programme
• People’s Trust for Endangered Species: National Dormouse Monitoring Programme
• JNCC: UK Biodiversity Action Plans
• NatureScot: Scottish Biodiversity List
• Wales Biodiversity Partnership: Section 7 priority species in Wales
• Natural England: S41 List of priority species in England
• Northern Ireland Environment Agency: Northern Ireland Priority Species List

Background

The measure is a composite indicator of 228 species from four broad taxonomic groups, see the published datasheet for a detailed breakdown of the species and groups included. 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, 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 more representative of priority species, a method of assessing the changing status of these remaining data-poor species would need to be considered.

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.

Data sources and species-specific time series

Robust population time series were sought for as many species on the combined 4 country biodiversity list as possible. The majority of these data have previously been published and many are used as part of the UK biodiversity indicator set currently; details of these analyses and the rules for species inclusion into the data sets are given in the following sections.

Tables 1 and 2 provide a summary of the relative abundance datasets included in the indicator. They show the analytical methods used to generate the species time series in each dataset.

Although these vary in detail, the underlying method is similar. These datasets are generated largely from data collected by national monitoring schemes. In these schemes data are collected in a robust and consistent manner and the geographical coverage is good, with statistical approaches used to correct for biases in coverage.

These datasets are ideal for producing population time series for widespread species; however, in some cases the sample size is insufficient to generate time series for cryptic, rarer or more range restricted species.

Each scheme has a set of criteria to determine whether time series can be generated for each species and if they are sufficiently robust to be included in the published results of the scheme. Table 3 gives an overview of the quality of the data derived from each scheme. Further information about each monitoring scheme and the data analysis and results can be found in the references given at the end of this paper.

Bird time series are well documented and several data sources are available (Table 1). Some bird species are represented in more than one dataset. The order of the rows in Table 1 shows the hierarchy used, from top to bottom, to ensure that the most appropriate and robust data for each species was included in the indicator.

The majority of species time series start around 1970 and the date of the last available update is 2021. The Rothamsted moth data starts in 1968, but to avoid over representing these time series in the overall indicator, data were only used from 1970 onwards, and the time series were expressed as a proportion of the 1970 value. Some datasets begin later than 1970, for example the butterfly time series begin in 1976. The method of incorporating this variation in time period into the indicator is discussed in the Methodology section below. Some datasets do not continue until 2021.

The steep decline in many moth species has an effect on the indicator as a whole. The impact of this on the assessment was considered in the 2013 indicator publication: if moths were excluded from the indicator the short-term decrease assessed in 2013 between 2005 and 2010 was not significant, and the indicator would have been assessed as ‘no change’. Over 10 years, from 2000 to 2010, the indicator in 2013 without the moth data would have been slightly positive, but not sufficiently so to be assessed as an increase. This analysis has not been repeated, but it is likely that moths are having a very similar impact on the indicator.

Table 1: Summary of the analysis methods and criteria for species selection for bird datasets

Table 2: Summary of the analysis methods and criteria for species selection for other taxonomic groups

Table 3: Overview of monitoring schemes (based on a 2013 assessment)

Methodology

Table 4 gives a summary of the relationship between the number of species on the combined 4 country biodiversity list (FCL) and the number of these for which population time series are available.

As far as possible, previously published methods of indicator creation were used, both because these are well-established, are likely to have undergone peer review and allow comparison of this indicator with existing species indicators for birds (C5), butterflies (C6) and bats (C8). These methods are described briefly below and references are given for further information.

Table 4: Summary of species time series included in the Species Indicator

The methodology for producing the indicator involves converting the time series for each species in the indicator into an index. Each time series is scaled as a percentage of its value in its first year (i.e. the first year has an index value of 100 regardless of when a species was first included in the indicator). This enables all species to be brought together on an equal basis – common species and rarer species are thereby given equal weighting.

To create the composite index, a hierarchical modelling method for calculating multi-species 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. It can be applied to multiple data types, improving the comparability between metrics derived from occupancy and abundance data. 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, consistent with shared environmental responses.

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 three. Six species’ time-series contained zeros, and eleven contained extremely low modelled values that represented zero counts; a total of 71 instances. As the model accepts the natural logarithm of the time-series values, 1% of the average value of the time series to the whole series of those species’ time series containing zeros (Loh et al., 2005). Zero counts prior to the first positive count were omitted.

The Freeman method combines the individual species’ abundance trends, taking account of the confidence intervals around the individual trends. However, the method is Bayesian, and therefore produces credible intervals to show the variability around the combined index.

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 1). 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.

Assessment of change

To illustrate the interspecific variation in trends, bar-charts are published alongside the indicators. These show the percentage of species showing different trends – strong increase, weak increase, little change, weak decrease, strong decrease – over two time periods (Table 5).

The long-term period is that since the start of the indicator (1970 in most cases) although for species entering into the indicator in subsequent years the period is shorter (the longest available trend is used, as long as it exceeds that used within the short-term change measure). The short-term period is the last five years of data (currently 2016 to 2021 or to the final year available). Where species’ time-series end prior to the short-term period no assessment was made. To estimate the long and short-term change for each species, annual species growth estimates were extracted from the indicator model, averaged across the relevant time-period and returned to the measurement scale.

The 5 trend class 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. 2009). 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%).

Table 5: 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

The species lists are unchanged from those used in the 2021 indicator analysis. The criteria for inclusion in each of the 4 biodiversity lists are derived largely 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 6. For example, the Scottish biodiversity list and the Northern Ireland (NI) 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 designated under this criterion were not considered here. The taxonomic composition of the combined four country list (FCL) is shown in Table 7.

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 four country biodiversity list if the associated species was not included. For example, a sub-species of the willow tit (Poecile montanus) is included on the Welsh list, but it is a full species on the Scottish Biodiversity list, thus on the combined list only the full species was retained.

Table 6: The biodiversity lists of the 4 countries of the UK

Table 7: Taxonomic breakdown of combined 4 country biodiversity 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.