Brown Argus (Aricia agestis)
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© Nick Greatorex-Davies
The Brown Argus is a species of open grassland and disturbed habitats. Until the late 1980s this was a declining butterfly that had become largely restricted to calcareous grassland and coastal dunes where its caterpillars fed mainly on Rockrose (Helianthemum nummularium) and Storksbill (Erodium cicutarium,) respectively. Now it occurs much more widely breeding on annual cranesbills (Geranium spp.) on such places as roadverges, setaside and old industrial sites (For further details on this species see http://www.butterfly-conservation.org/Butterfly/32/Butterfly.html?ButterflyId=14).
Status details :
Status since 1976 is Stable with a decrease of -26.6%
Status over the last 20 years is Stable with a increase of 2%
Status over the last 10 years is Stable with a increase of 10%
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 species has spread rapidly from the early 1990s particularly in central and eastern England colonising many sites away from its traditional chalk and limestone grassland sites where it is able to feed on alternative hostplants (geranium species). The species has also expanded its range considerably northwards to Yorkshire. This is very much reflected in the data as many UKBMS sites have been colonised during the monitoring period. This expansion in range, thought to be linked to climate change, is not reflected in a significant increase in abundance as with some other species, though it has steadily increased.
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, Brown Argus has been recorded from 507 transects in the Butterfly Monitoring Scheme. Of these, annual indices of abundance have been calculated from 537 sites, with an average index of 17 individuals per site.
For 274 of these sites, Brown Argus has been recorded well enough to calculate annual indices of abundance in more years, allowing trends to be calculated.
In 2020, 8771 individuals were recorded from 278 sites, producing annual indices at 219 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
|Extinct sites (2)|
|Colonised sites (22)|
|Stable sites (274)|
|Increasing sites (49)|
|Declining sites (26)|