Just as you think the butterfly season is coming to an end, some second generation individuals appear. I was very happy to come across these second brood Brown Argus (Aricia agestis) butterflies in Bedfordshire on 13 Aug 2019.
In 1993, the Brown Argus was described as a scarce or local species, which had declined by as much as 40% – in terms of sites – throughout its range in Britain (Bourn and Thomas, 1993). There were, however, large and erratic fluctuations in its numbers, suggesting that the factors governing its abundance were complex. Since the late 1980s however, not long ago really, the decline has turned around, and it now seems to be doing much better. The UK Butterfly Monitoring Scheme, reports that over the last 10 years, its status is Stable with a increase of 10%. Good news for a change!
One of the reasons for its increased abundance, is climate change. This species is at the northern limit of its distribution in the UK and as average temperatures have increased in recent decades, Brown Argus populations have shifted north, colonising many new areas: such as the North York Moors for example. On average, it is thought to have moved 79 kilometers northward in Britain in 20 years (Pateman et al., 2012). The first beachhead landing in Yorkshire, of the expanding population, was at the tip of Spurn NNR! (Frost, 2005). The Brown Argus (A. agestis) has now started to hybridise with the Northern Brown Argus (A. artaxerxes), which mostly occurs in northern Britain, but they are largely separate species, I understand.
Another reason for the upturn in fortunes of the Brown Argus, is that it has expanded its diet to include different host plants, particularly annual cranesbill species (Geranium spp.) such as Dovesfoot cranesbill (below), which tend to occur in more open open, disturbed ground
habitats. Individuals with the propensity to feed on these different species must have originally existed in the population, but have presumably been selected as the climate and habitats changed. So the Brown Argus it is clearly on to a winning strategy, capitalising on both climate and landscape changes! Previously, it had been largely dependent on a yellow rockrose, Helianthemum nummularium, which is much lass abundant than the cranesbill species.
Dovesfoot Cranesbill (Geranium molle) Wiki in Germany. Aiwok [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D
In summary, climate change and host plant shifts have come together:
“to generate an unexpectedly rapid transformation in the metapopulation dynamics of the butterfly from a highly localized distribution associated with southerly facing rockrose-containing calcareous grasslands to widespread use of virtually any grassland with rockrose or Geranicaeae host plants.” (Pateman et al., 2012).
I photographed these butterflies on a small meadow of undisturbed grassland, surrounded by woodland, on Felmersham gravel pits. The males and females are similar, but can usually been separated on the basis of the males being slightly smaller, with more pointed fore-wings and slightly smaller orange marginal lunules (see below). I find it easier to separate them on their undersides, than their upper sides!
Bourn, N. A. D. & Thomas, J. A. (1993). The ecology and conservation of the brown argus butterfly Aricia agestis in Britain. Biological Conservation, 63(1), 67-74.
Buckley, J., Butlin, R. K. & Bridle, J. R. (2012). Evidence for evolutionary change associated with the recent range expansion of the British butterfly, Aricia agestis, in response to climate change. Molecular Ecology, 21(2), 267-280.
Frost, H. M. (2005). The butterflies of Yorkshire. Butterfly Conservation.
Pateman, R. M., Hill, J. K., Roy, D. B., Fox, R. & Thomas, C. D. (2012). Temperature-dependent alterations in host use drive rapid range expansion in a butterfly. Science, 336(6084), 1028-1030.