Common Blue (Polyommatus icarus)
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© Nick Greatorex-Davies
The Common Blue is widespread in Britain but has much declined with the destruction of much semi-natural herb-rich grassland through agricultural intensification. It lives in discrete colonies on herb-rich grassland and disturbed sites where such legumes as Bird's-foot Trefoil (Lotus corniculatus) and Black Medick (Medicago lupulina) grow on which the caterpillars feed. (For further details on this species see http://www.butterfly-conservation.org/).
Status details :
Status since 1976 is Stable with a decrease of -17.9%
Status over the last 20 years is Stable with a decrease of -28.0%
Status over the last 10 years is Stable with a increase of 30%
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 :
The all-sites collated indices show no significant trend in either generation and the population at moniored sites is calssed as stable. The data at site level indicate that numbers fluctuate considerably, often in a cyclic fashion over a period of years and often quite synchronously between sites within a region. This suggests a possible interaction with parasites. Numbers at the majority of sites have remained stable overall. However perhaps half this number of sites show declines, whereas about a quarter this number show increases. Many of those sites that show declines have a strong woodland component and loss of open areas through increasing shade seems a likely reason.
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, Common Blue has been recorded from 860 transects in the Butterfly Monitoring Scheme. Of these, annual indices of abundance have been calculated from 1013 sites, with an average index of 76 individuals per site.
For 474 of these sites, Common Blue has been recorded well enough to calculate annual indices of abundance in more years, allowing trends to be calculated.
In 2016, 58938 individuals were recorded from 584 sites, producing annual indices at 455 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 (6)|
|Extinct sites (10)|
|Stable sites (623)|
|Increasing sites (43)|
|Declining sites (74)|