The indicator shows changes in the population size of eight bat
species, which occur in the wider countryside, based on summer
field surveys and roost counts and winter hibernation counts.
Population change between 1999 and 2013 is analysed using a
statistical model developed by the Bat Conservation Trust.
Bat populations have increased significantly in the long
term. The short-term assessment of the indicator, which
considers change over the past five years (2007–2012), shows that
the trend has stabilised over this period. Assessments run to
the penultimate year of the trend as the most recent smoothed data
point (2013) is likely to change as future years of data are
The summer of 2013 was warmer and drier compared to the very
poor season in 2012. Possibly as a result of this, most species
showed a slight recovery from the dip in 2012.
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. Evidence includes:
- 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 by deploying a network of over 3,500
volunteers to record observations at over 5,900 sites (Figure
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
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).
A paper documenting the survey methods and statistical
analysis used by the NBMP to produce individual species trends is
currently under review for publication (Barlow et al. In
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 recently
published prototype European indicator of trends in bat
populations, developed from counts at hibernation sites in nine
European countries including UK (Haysom et al. 2014).
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.
Figure C8iii. Location of National Bat Monitoring
Programme monitoring sites.
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
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.
Target 7: By 2020, areas under agriculture,
aquaculture and forestry are managed sustainably, ensuring
conservation of biodiversity.
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
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
Target 13: By 2020, the genetic diversity of
cultivated plants and farmed and domesticated animals and of wild
relatives, including other socio-economically as well as culturally
valuable species, is maintained, and strategies have been developed
and implemented for minimizing genetic erosion and safeguarding
their genetic diversity.
Strategic Goal D. Enhance the benefits to all
from biodiversity and ecosystems.
Target 14: By 2020, ecosystems that provide
essential services, including services related to water, and
contribute to health, livelihoods and well-being, are restored and
safeguarded, taking into account the needs of women, indigenous and
local communities, and the poor and vulnerable.
Target 15: By 2020, ecosystem resilience and
the contribution of biodiversity to carbon stocks has been
enhanced, through conservation and restoration, including
restoration of at least 15 per cent of degraded ecosystems, thereby
contributing to climate change mitigation and adaptation and to
Web links for further information
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Lechiara, N.L., Racey, P.A., Walsh, A.L. & Langton, S.D. (In
press) Citizen science reveals trends in bat populations: the
National Bat Monitoring Programme in Great Britain. Biological
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. pp 259–280 in: Maclean, N. (ed.) Silent Summer: The
State of Wildlife in Britain and Ireland. Cambridge
Haysom, K.A., Dekker, J., van der Meij, T. & van Strien, A.
(2014) European bat population trends. A prototype biodiversity
indicator. EEA Technical Report No. 19/2013. European Environment
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G.D., Larkin, R.P., Strickland, M.D., Thresher, R.W. & Tuttle,
M.D. (2007) Ecological impacts of wind energy development on bats:
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bats studied near Bristol over the past twenty-six years.
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of climate change on European bats in relation to their
biogeographic patterns. Global Change Biology,
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Stone, E.L., Jones, G. & Harris, S. (2009) Street
lighting disturbs commuting bats. Current Biology,
Stone, E.L., Jones, G. & Harris, S. (2012) Conserving energy
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Global Change Biology, 18, 2458–2465.