Chalk Hill Blue (Polyommatus coridon)
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© Nick Greatorex-Davies
The Chalkhill Blue, as its name suggests, is restricted to chalk and limestone grassland where its foodplant Horseshoe Vetch (Hippocrepis comosa) grows. Though still widespread in these habitats it is far less common than it once was because of the destruction of herb-rich calcareous grassland due to agricultural intensification. (For further details on this species see http://www.butterfly-conservation.org/).
Status details :
Status since 1976 is Stable with a decrease of -7.1%
Status over the last 20 years is Stable with a decrease of -0.2%
Status over the last 10 years is Stable with a decrease of -8.7%
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 butterfly has shown big fluctuations in its index over the monitoring period. There is no significant long-term trend and the population on UKBMS sites is considered stable. A similar number of sites have shown increases as decreases with the majority of monitored sites showing no trend at all and being classed as stable. Because the data show mixed results, changes probably largely reflect the local management of sites.
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, Chalk Hill Blue has been recorded from 323 transects in the Butterfly Monitoring Scheme. Of these, annual indices of abundance have been calculated from 295 sites, with an average index of 164 individuals per site.
For 133 of these sites, Chalk Hill Blue has been recorded well enough to calculate annual indices of abundance in more years, allowing trends to be calculated.
In 2018, 43473 individuals were recorded from 125 sites, producing annual indices at 101 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)|
|Stable sites (89)|
|Increasing sites (25)|
|Declining sites (40)|