Indicator description
The indicator shows changes in the population size of six
widespread bat species, based on summer field surveys and colony
counts and winter hibernation counts. Populations of these
bat species have been relatively stable during the last decade,
with the overall trend positive. Assessment is based on the
analysis of these trends, using a statistical model. Since 2000,
the populations have increased by 20 per cent, giving an
‘improving’ assessment.
Bats experienced major declines during the latter half of the
twentieth century. In response to these declines, large-scale
national monitoring was put in place so that future changes could
be detected. Bats have benefited from strict legal protection,
direct conservation action and public education, but remain
vulnerable to pressures such as landscape change and development. A
significant increase in the lesser horseshoe bat (Rhinolophus
hipposideros) population underpins this positive trend and has
been attributed to conservation measures and a series of mild
winters that have enhanced winter survival. The lesser horseshoe
bat increase has been sustained throughout the period of the
indicator, with more recent contributions to the positive trend to
a lesser extent from other species such as noctule (Nyctalus
noctula) and common pipistrelle (Pipistrellus
pipistrellus).
Relevance
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 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 entirely on insects and other
arthropods. In order to thrive they require adequate roosting
(particularly breeding and hibernating) opportunities, 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 availability.
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.
Background
The indicator has been compiled by the Bat Conservation Trust
(BCT) using data collected annually from the National Bat
Monitoring Programme (NBMP). This delivers trends for 11 of the
UK’s 17 resident bat species by deploying a network of over 2,200
volunteers to record observations at approximately 4,700
sites.
The indicator is a composite index which combines population trend
data for six widespread species. Surveys for these species include
summer roost (colony) counts, visual and/or acoustic observations
along predetermined transects within 1km randomly selected survey
grids or along 1km sections of waterway, 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). The locations of monitoring sites for
the six index species are shown in Figure 1c (iii).
For each species, Generalised Additive Modelling (GAM) is used to
calculate the trends in numbers over time. 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 six 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 1 (ii)).
Table 1c. Species used in the bat indicator
|
Species
|
Status
|
|
Eptesicus serotinus (serotine)
|
Habitats Directive Annex IV
|
|
Myotis daubentonii (Daubenton’s bat)
|
Habitats Directive Annex IV
|
|
Nyctalus noctula (noctule)
|
Habitats Directive Annex IV. UK BAP species from 2007
|
|
Pipistrellus pipistrellus (common pipistrelle)
|
Habitats Directive Annex IV. UK BAP species until 2007
|
|
Pipistrellus pygmaeus (soprano pipistrelle)
|
Habitats Directive Annex IV. UK BAP species
|
|
Rhinolophus hipposideros (lesser horseshoe bat)
|
Habitats Directive Annexes II & IV. UK BAP species
|
Figure 1c(ii). Generalised Additive Modelling trend in
bat populations, 1999 to 2010
Figure 1c (iii). Location of monitoring sites for the 6 index
species
Further development planned
A method to derive population trends for bats in woodland,
farmland and urban landscapes has been trialed. Efforts to extend
the survey network to deliver trends at country level are
ongoing.
Current assessment of the indicator is based on the analysis of
trends, using a statistical model which may be subject to further
development.