Site indices, as a relative measure of the actual population size, for UK butterfly species calculated from data from the UK Butterfly Monitoring Scheme (UKBMS). Site indices are a relative rather than an absolute measure of the size of a population, and have been shown to relate closely to other, more intensive, measures of population size such as mark, release, recapture (MRR) methods. The site index can be thought of as a relative measure of the actual population size, being a more or less constant proportion of the number of butterflies present. The proportion seen is likely to vary according to species; some butterfly species are more conspicuous and thus more easily detected, whereas others are much less easy to see.
Site indices are only calculated at sites with sufficient monitoring visits throughout the season, or for targeted reduced effort surveys (timed observations, larval web counts and egg counts) where counts are generally obtained as close to the peak of the flight period as possible and are subsequently adjusted for the time of year and size of the site (area of suitable habitat type for a given species). Wider Countryside Butterfly Survey (WCBS) sites are thus excluded because they are based on very few visits from which accurate indices of abundance cannot currently be calculated. For transect sites a statistical model (a General Additive Model, ‘GAM’) is used to impute missing values and to calculate a site index. Each year most transect sites (over 90%) produce an index for at least one species and in recent years site indices are calculated for almost 1,500 sites across the UK.
Site indices are subsequently collated to contribute to the overall ‘Collated Index’ for each species, which are relative measures of the abundance of each species across a geographical area, for example, across the whole UK or at country level in England, Scotland, Wales or Northern Ireland. Individual site indices are important in informing conservation management as not all sites show the same patterns for each species and likely reflect a combination of local climate and habitat management at the site.
Although the Centre for Ecology & Hydrology (CEH) and Butterfly Conservation (BC) are responsible for the calculation and interpretation of site indices, the collection of the data used in its creation is ultimately reliant on a large volunteer community. The UKBMS is run by BC, the CEH and the British Trust for Ornithology (BTO), in partnership with the Joint Nature Conservation Committee (JNCC), and supported and steered by Forestry Commission (FC), Natural England (NE), Natural Resources Wales (NRW), Northern Ireland Environment Agency (NIEA), and Scottish Natural Heritage (SNH). The UKBMS is indebted to all volunteers who contribute data to the scheme.
Publication date: 2016-11-14
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Automatic checks are applied when data are entered into the Transect Walker program to alert recorders to potential data entry errors, e.g. abnormally high counts of a species or a record of species outside of its recognised flight period. The recorder is given the option to proceed with the record or to alter it appropriately. Each site data belongs to a region for which a transect coordinator is responsible. The regional coordinator has good knowledge of the sites they receive records for and checks the records for any questionable records. Data entered online is also checked by the regional transect coordinators and can be done so continuously throughout the season. Following these preliminary validation checks, the data undergoes a series of further automated and manual validation procedures including queries to check for records of species that are: recorded out of their known distribution range (using data from Butterfly Conservation’s Butterflies for the New Millennium (BNM) and existing UKBMS data), recorded out of their normal flight period, recorded for the first time at a site, have extreme abundances or abundances that are markedly different to normal for a given site at a given time of year.