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Brown Argus (Aricia agestis)
© 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/).
Family: Lycaenidae
Status: Stable
Status details:
Status since 1976 is Stable with an increase of 28%
Status over the last 20 years is Stable with an increase of 87.5%
Status over the last 10 years is Stable with an increase of 8%
Back to the topLog 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. However this species spread rapidly from the early 1990s particularly in central and eastern England colonising many sites away from its traditional chalk and limestone grassland sites. The species has also expanded its range considerably northwards to Yorkshire. This is very much reflected in the data as many BMS sites have been colonised during the monitoring period. However this expansion in range is not reflected in an increase in abundance as with some other species. This range expansion is thought to be linked with climate change.
Distribution...
This map shows the distribution between 1995 and 1999. Data is derived from the Butterflies for the New Millenium dataset via the NBN Gateway (www.searchnbn.net).
View the distribution on the NBN GatewayPhenology...
Phenology plot
Phenology plot
This chart shows the average number of butterflies seen on transects between Arpil and October. The black line gives average counts over the full BMS series (1976 to date) and the red line gives the average for the last year.
Abundance...
Abundance
This map shows symbols for the mean abundance at transect sites, with the size of symbol reflecting the level of abundance. Means are over all years.
Coverage
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 6 or more years, allowing trends to be calculated.
In 2006, 8771 individuals were recorded from 278 sites, producing annual indices at 219 of these.
This map shows the trend in abundance at particular transect sites. Trends (increasing, declining or stable) are assessed at sites where the species has more than five years of annual index data. Click on the green question mark next to the species name above for more details on how trends are calculated.
Links to lists of sites with particular statuses
• Extinct sites (1)
• Colonised sites (16)
- Stable sites (132)
- Declining sites (10)
- Increasing sites (33)
