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C8. Mammals of the wider countryside (bats)

Type: State indicator

Indicator Description

This indicator shows changes in the relative abundance of 11 of the UK’s 17 breeding bat species, based on data from transect surveys, roost counts and counts at hibernation sites. Whilst 11 species are included there are 10 species trends, as an aggregate trend is used for the whiskered bat (Myotis mystacinus) and Brandt’s bat (Myotis brandtii); these 2 species are difficult to distinguish in the field. Bat species make up a third of the UK’s mammal fauna and occur in most lowland habitats across the UK.

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  1. Summary
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Summary

The bat index has increased by 42% between 1999 and 2017. In the short term, between 2012 and 2017, the bat index has increased by 10%. 

The bat index is a composite of 10 species trends (11 species with 2 combined). Since 1999, 5 of the bat species trends included in the index have increased and 5 have shown no significant change. The UK’s rarer and more specialised bat species are not included in the index due to difficulties monitoring these species.

The increase in the index is underpinned by significant increases in populations of 3 species, greater horseshoe bat, lesser horseshoe bat and common pipistrelle. These increases indicate that some bat species are recovering after what are believed to have been major population declines during the 20th century. 

Figure C8i. Trends in bat populations, 1999 to 2018

A line graph showing how the trend for the UK bat index has changed between 1999 and 2018. The smoothed index has shown an increasing trend since 1999. The unsmoothed index shows considerable year-on-year variation. A bar chart showing the percentage of species trends within the UK bat index that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change over both the long term (since 1999) and short term (2012 to 2017). 

Notes:

  1. The line graph shows the unsmoothed trend (dashed line) and smoothed trend (solid line) with its 95% confidence interval (shaded).
  2. The figure in brackets shows the number of species in the index.
  3. This indicator includes measures for 11 species of bats; the index only includes 10 trends. This is because an aggregate trend is used for the whiskered bat (Myotis mystacinus) and Brandt’s bat (Myotis brandtii); these 2 species have been combined due to difficulties with distinguishing them in the field.
  4. The bar chart shows the percentage of species trends which, over the time periods of the long-term and short-term assessments, have shown a statistically significant increase or decline, or no significant change.
  5. Since 2018, this indicator has been extended to include 11 species instead of 8. The complete time series in the accompanying dataset has also been updated to reflect these changes.
  6. The model used to analyse some individual species trends has changed since the previous publication, and these results are therefore not directly comparable (see Background section for more details).

Source: Bat Conservation Trust.

The smoothed bat index increased every year between the 1999 baseline and 2008. Between 2008 and 2013 it was relatively stable, before increasing again between 2013 and 2017. However, the composite indicator masks variation between the species that contribute to it. The long-term increase in the indicator is primarily driven by strong increases in 3 species; greater horseshoe bat, lesser horseshoe bat and common pipistrelle and between 1999 and 2017, the combined survey trend for these species increased by 159%, 107% and 87% respectively. The combined trend for the whiskered bat and Brandt’s bat showed a weaker increase over the same period, as did the trend for noctule. The remaining 5 species showed no significant change. In the short term, between 2012 and 2017, 3 species have increased significantly and the rest show no significant short-term change. No species show a decline in either the long or short term, however it is not possible to produce separate trends for whiskered bat and Brandt’s bat, as they cannot be reliably distinguished in the field. It is therefore possible that an increase in one species could mask a decline in the other. It is also important to note that the UK’s rarer and more specialised bat species are not included in the index due to difficulties monitoring these species.

Long- and short-term assessments are run to the penultimate year of the trend as the most recent smoothed data point (2018) is likely to change as future years of data are added. The assessment of change in the latest year (2018) is based on unsmoothed data.

Assessment of change in widespread bat populations

 

Long term

Short term

Latest year

Bat populations


1999–2017


2012–2017

Decreased (2018)

Note: Long-term and short-term assessments are made on the basis of smoothed trends to the penultimate year (2017) by the Bat Conservation Trust. This is because the most recent smoothed data point (2018) is likely to change in next year’s update when additional data are included for 2019. As such, the latest year assessment is based on unsmoothed data

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Downloads

Download the Fiche, Datasheet and Technical background document from the JNCC Resource Hub

Last updated: September 2019

Latest data available: 2018

 

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Categories:

UK Biodiversity Indicators 2019

Published: .

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