The indicator shows changes in the population
size of eight widespread bat species, based on summer field surveys
and roost counts and winter hibernation counts. Population
change between 1999 and 2014 is analysed using a statistical model
developed by the Bat Conservation Trust. The composite index
of bat population size has increased significantly in the long
term. The short-term assessment of the indicator, which
considers change over the past five years (2008–2013), shows that
the trend has stabilised over this period. Assessments are
run to the penultimate year of the trend as the most recent
smoothed data point (2014) is likely to change as future years of
data are added. There was a dip in the index in 2014 but this
result should be treated as provisional for the reason outlined
Bat populations are considered to be a good indicator of the
broad state of wildlife and landscape quality because they utilise
a range of habitats across the landscape and are sensitive to
pressures in the urban, suburban and rural environment. All
bats and their roosts are protected by domestic and European
legislation. The UK is a signatory to the EUROBATs agreement,
set up under the Convention on Migratory Species, with the
intention of conserving all European bat populations. The
wider relevance of bats as biodiversity indicators is presented in
Jones et al. (2009).
Bat species make up a third of the UK’s mammal
fauna and occur in most lowland habitats across the UK. The
species used in this index (Table C8i) are widespread throughout a
variety of landscapes including urban areas, farmland, woodland,
and river/lake systems. All bats in the UK feed at night and prey
on insects. To thrive they require adequate roosting
opportunities (particularly for breeding and hibernating), foraging
habitat and connected landscape features, such as hedgerows and
tree lines, that assist them in commuting between roost sites and
feeding locations. Key pressures on bats (landscape change,
agricultural intensification, development, habitat fragmentation)
are also relevant to many other wildlife groups. Bats are
sensitive to pollution and factors affecting their insect prey
(e.g. pesticides, drainage, land management change). Climatic
shifts are predicted to affect bat populations through changes in
their yearly hibernation cycles, breeding success and food
Bats experienced major declines throughout
Western Europe during the latter half of the 20th century, which
have been attributed to agricultural intensification, habitat and
roost loss, deliberate killing, remedial timber treatment and
insecticide poisoning, and declines of their insect prey.
However bats were relatively understudied in the UK during the
period of greatest population loss, and the supporting evidence,
synthesised in Haysom et al. (2010), is fragmented.
- reports of the loss of large colonies of several species from
traditional roosting sites;
- a questionnaire survey documenting roost loss, declines in
abundance at roosts, and deliberate killing (Racey and Stebbings
- range contractions of lesser horseshoe bat (Rhinolophus
- a small number of published population trends (e.g. Stebbings
1988; Ransome 1989; Guest et al. 2002).
Figure C8ii is an example historical trend
showing decline of combined species of pipistrelle bats in
England. The base point of this graph is set to 100 in 1999
as sample sizes are small in the first few years of the series and
the analysis uses only sites with five or more years of data.
The dataset for Figure C8ii pre-dates the separation of
pipistrelles into common (Pipistrellus pipistrellus) and
soprano pipistrelle (Pipistrellus pygmaeus), and the
proportion of each species is unknown in the historical data.
The trend begins in 1977, the earliest year for which there are an
acceptable number of sites. Some caution is necessary in
interpreting the decline in Figure C8ii, as more recent pipistrelle
data suggests that trends from roost counts are negatively biased
relative to those from field surveys; it is not at present possible
to assess how much of the decline in Figure C8ii is genuine and how
much might be an artefact caused by this bias.
Figure C8ii. Historical declines in pipistrelle bat roost
counts, 1977 to 1999.
- The graph is a composite index of common pipistrelle, soprano
pipistrelle, and unidentified pipistrelle roost count data from
- Graph shows unsmoothed trend (dashed line) and smoothed trend
(solid line) with its 95% confidence interval (shaded).
Source: Bat Conservation Trust, using data
collected by Stebbings and published in Harris et al.
1995, plus more recent data.
In response to the reported declines in bat populations,
large-scale national monitoring was put in place through the
establishment of the National Bat Monitoring Programme (NBMP) in
the UK; the NBMP was established in 1996 with the first surveys
undertaken in 1997. It delivers trends for 11 of the UK’s 17
resident bat species, and since 1997 has deployed over 3,100
volunteers to record observations at almost 5,900 sites (Figure
Figure C8iii. Location of National Bat Monitoring
Programme monitoring sites.
The indicator has been compiled by the Bat
Conservation Trust using data collected annually from the
NBMP. It is a composite index which combines population trend
data for eight bat species. Surveys for these species include
visual and/or acoustic observations along predetermined transects
within 1km randomly selected survey grids or along 1km sections of
waterway, summer roost counts and counts at hibernation
sites. Most of the species are surveyed by two of the three
methods, all of which are included in the index. The index is
presented independent of habitat, but the predominant habitat types
represented in the combined dataset are woodland (broad-leaf and
conifer), farmland (arable and grassland), urban and waterway
(rivers, streams and canals).
For each species, Generalised Additive
Modelling (GAM) is used to calculate the trends in numbers over
time following Fewster et al. (2000). The models
include terms for factors that can influence the apparent
population means (e.g. bat acoustic detector model, temperature,
etc.), so their effect can be taken into account. For easier
interpretation the means are then converted to an index that is set
to 100 for the selected baseline year of data. The species
indices are revised when new data become available or when improved
modelling methods are developed and applied retrospectively to
earlier years. To generate the overall composite bat
indicator, each of the eight species has been given equal
weighting, and the annual index figure is the geometric mean in
that year. The GAM models produce smoothed trends with
confidence intervals which are the basis of the indicator
assessment (Figure C8i). A similar method was used to produce
the smoothed trend for historical pipistrelle data (Figure C8ii).
The survey methods and statistical analysis used by the NBMP
to produce individual species trends are described in Barlow et
Bats have benefited from strict legal
protection, direct conservation action and public education
(Mitchell-Jones 1993; Haysom et al. 2010), but remain
vulnerable to pressures such as landscape change, climate change,
development and emerging threats such as new building practices,
wind turbines, and light pollution (Haysom et al. 2010;
Kunz et al. 2007; Rebelo et al. 2009; Stone
et al. 2009, 2012). A significant increase in the
lesser horseshoe bat population underpins the overall positive
trend of the indicator since 1999 and has been attributed to
conservation measures and a series of mainly mild winters that have
enhanced winter survival. The positive direction of the trend
is in line with a prototype European indicator of trends in bat
populations, developed from counts at hibernation sites in nine
European countries including the UK (Haysom et al.
Table C8i. Long-term and short-term percentage change in
the species used in the bat indicator.
brown long-eared bat
lesser horseshoe bat
Annexes II & IV
* Denotes a statistically significant change (based on smoothed
Note: To better capture patterns in the data,
long-term and short-term assessments are made on the basis of
smoothed data, with analysis of the underlying trend undertaken by
Bat Conservation Trust.
Further development planned
The indicator will be periodically re-evaluated in relation to
the availability of suitable data. Efforts to extend the NBMP
survey network to deliver trends and indicators at country level
Goals and targets
Aichi Targets for which this is a primary indicator
Strategic Goal C. To improve the status of
biodiversity by safeguarding ecosystems, species and genetic
Target 12: By 2020, the extinction
of known threatened species has been prevented and their
conservation status, particularly of those most in
decline, has been improved and sustained.
Aichi Targets for which this is a relevant indicator
Strategic Goal B. Reduce the direct pressures
on biodiversity and promote sustainable use.
Target 5: By 2020, the rate of loss of all
natural habitats, including forests, is at least halved and where
feasible brought close to zero, and degradation and
fragmentation is significantly reduced.
Strategic Goal C. To improve the status of
biodiversity by safeguarding ecosystems, species and genetic
Target 11: By 2020, at least 17 per cent of
terrestrial and inland water, and 10 per cent of coastal and marine
areas, especially areas of particular importance for biodiversity
and ecosystem services, are conserved through effectively and
equitably managed, ecologically representative and well connected
systems of protected areas and other effective area-based
conservation measures, and integrated into the wider landscape and
Web links for further information
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Lechiara, N.L., Racey, P.A., Walsh, A.L. & Langton, S.D. (2015)
Citizen science reveals trends in bat populations: the National Bat
Monitoring Programme in Great Britain. Biological
Conservation, 182, 14–26.
Fewster, R.M., Buckland, S.T., Siriwardena, G.M., Baillie, S.R.
& Wilson, J.D. (2000) Analysis of population trends for
farmland birds using generalized additive models. Ecology,
Guest, P., Jones, K.E. & Tovey, J. (2002) Bats in
Greater London: unique evidence of a decline over 15 years.
British Wildlife, 13, 1–5.
Harris, S., Morris, P., Wray, S. & Yalden, D. (1995) A
review of British mammals: population estimates and conservation
status of British mammals other than cetaceans. Peterborough,
Haysom, K.A., Jones, G., Merrett, D. &
Racey, P.A. (2010) Bats. In: N. Maclean, ed.
Silent Summer: The State of Wildlife in Britain and
Ireland. Cambridge University Press, 259–280.
Haysom, K.A., Dekker, J., van der Meij, T.
& van Strien, A. (2014) European bat population trends. A
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energy development on bats: questions, research needs, and
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twenty-six years. Biological Journal of the Linnean
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of European Bats. London: Christopher Helm.
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at a cost to biodiversity? Impacts of LED lighting on bats.
Global Change Biology, 18, 2458–2465.