White-letter Hairstreak (Satyrium w-album)
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© Eddie John
The White-letter Hairstreak is a very elusive butterfly that spends most of its time high up in the canopy of Elms (Ulmus spp.), on which it breeds, in woodlands and hedgerows. The butterfly is still widespread but far less common than it was before the ravages of Dutch Elm Disease. (For further details on this species see http://www.butterfly-conservation.org/).
Status details :
Status since 1976 is Rapid decline with a decrease of -93.4%
Status over the last 20 years is Rapid decline with a decrease of -49.6%
Status over the last 10 years is Stable with a decrease of -59.3%
Log collated index plot
This chart shows the index of abundance (LCI = Log Collated Index) over time. It shows fluctuations in
populations from year to year, and is scaled so that the average index over the whole series is equal to 2
(horizontal line). For greater detail about how this index is derived, click on the green question mark above.
Trend description :
This normally elusive canopy species is poorly monitored by the transect method and it is only occasionally recorded on most transects where there are populations. Nevertheless, this data shows that there have been significant declines at monitored sites since 1976. At sites such as Northward Hill where the butterfly used to regularly be recorded in double figures, numbers have plummeted to single figures. Although Dutch Elm Disease has played a major role in these declines, White-letter Hairstreak has been shown to survive on Elm suckers and young regrowth, and other factors such as habitat detioration, and in particular hedgerow management, are likely to have contributed greatly.
This map shows the distribution between 1995 and 2016. Data is derived from the Butterflies for the New Millenium dataset via the NBN Gateway
This chart shows the average number of butterflies seen on transects between Arpil and October across all sites (fitted values from a Generalised Additive Model). The blue line gives average counts over the full BMS series (1976 to date) and the red line gives the average for the last year.
This map shows symbols for the mean abundance at transect sites, with the colour of the symbol reflecting the level of abundance. Means are over all years. Grey background squares are the occupied cells as shown by the Butterflies for the New Millenium over the previous ten year period.
In total, White-letter Hairstreak has been recorded from 173 transects in the Butterfly Monitoring Scheme. Of these, annual indices of abundance have been calculated from 239 sites, with an average index of 2 individuals per site.
For 98 of these sites, White-letter Hairstreak has been recorded well enough to calculate annual indices of abundance in more years, allowing trends to be calculated.
In 2018, 166 individuals were recorded from 44 sites, producing annual indices at 34 of these.
Links to counts
This map shows the trend in abundance at particular transect sites for which data has been received within the last five years. Trends (increasing, declining or stable) are assessed at sites where the species has more than five years of annual index data. Use the option boxes below to view plots for individual sites.
Links to lists of sites with particular statuses
|Colonised sites (3)|
|Extinct sites (3)|
|Stable sites (25)|
|Increasing sites (5)|
|Declining sites (9)|