Indicator assessment
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. Since 2000
bat populations have increased by 21 per cent, giving an
‘improving’ assessment.
Description of trends
The indicator shows that populations of
widespread bat species have been relatively stable during the last
decade, with the overall trend positive. 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 population underpins this positive trend and has been
attributed to conservation measures and a series of mild winters
that have enhanced winter survival. The 2008 increase in the
indicator is influenced by high counts of noctules in the National
Bat Monitoring Programme (NBMP) field survey.
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 connective
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
10 of the UK’s 17 resident bat species by deploying a network of
over 1600 volunteers to record observations at approximately 3300
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, both 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 6 index
species are shown in Figure 1c(iii).
For each species, GAM (Generalised Additive
Modelling) 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 6 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 (Fig 1c(ii)).
Table 1c. Species used in the bat
indicator
|
Species
|
Status
|
|
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
|
|
Nyctalus noctula (Noctule)
|
Habitats Directive Annex IV. UK BAP species
from 2007
|
|
Eptesicus serotinus (Serotine)
|
Habitats Directive Annex IV
|
|
Myotis daubentonii (Daubenton’s
bat)
|
Habitats Directive Annex IV
|
Figure 1c(ii). Generalised Additive
Modelling trend in bat populations, 1999 - 2008
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 trialled.
Efforts to extend the survey network to deliver trends at country
level is ongoing.