The little Brown Argus complex tale of range expansion

Climate change is making a measurable impact on the distribution of many organisms. The populations of some species are disapearing or becoming more fragmented, increasing extinction risk, others are actually expanding their range north with the increasing temperatures. Insects - and particularly butterflies - are very sensitive to changes in climate, and amongst the winners of climate change is a lovely, tiny butterfly, the Brown Argus, Aricia agestis. In the last few days, I have spotted this butterfly in two new locations in East Yorkshire, including my local wildlife garden, where it is unlikely I would have overlooked it, indicating recent colonisation.

Distribution of Brown Argus (Aricia agestis) in Britain. Black circles show that the species was present in 1970–1982; open circles show newly colonised areas (1995–1999 records, not present in 1970–1982). Circles represent 10 km grid cells (from Menéndez et al 2008)

This species has increased markedly in range in the U.K in the last 30 years, expanding around 10 km per year since the early 1990s, reverting a previously declining trend. This could be seen as a direct response to climate change. But the story is not that simple, the positive response to temperature has been facilitated by changes in the interactions of this butterfly with other organisms. Before the expansion, the predominant caterpillar foodplant of the Brown Argus was the Common Rock Rose, Helianthemum nummularium, a plant that grows on sheltered, south facing, sunny hillsides in chalk and limestone grasslands. Some southern populations used several species from the geranium family, especially Geranium and Erodium. These plants grow on lowland, in cooler habitats than the Rock Rose, so only after temperature increased were these populations able to colonise and exploit these areas. Chris Thomas and collaborators established that expanding populations had a preference to lay their eggs on the more available geraniums, even when they came from populations in hills using Rock Roses. This diet/habitat shift allowed the butterfly to recolonise distant Rock Rose areas, using lowland geranium habitat as stepping stones, which single dispersing butterflies would have been unlikely to reach. A niche model - based on preferred temperature and established food plant - of the predicted range expansion would have grossly underestimated the recent expansion of this butterfly.
 A further complexity stems from an "enemy release" effect. Invasive populations - often translocated by man from distant areas - are hypothesized to expand unchecked as they have left behind their natural enemies: specialist predators, parasites or parasitoids might be absent, and generalist ones might not have the right "search image" for them. Rosa Menéndez and her collaborators tested if this applied to an expanding native population, where the distance travelled from the nearest population is smaller, and there are related species whose parasites might also infect them. They used the Brown Argus and its parasitoids as models. The Brown Argus shares its range with a very common and related species, the Common Blue, Polyommatus icarus, and four parasitoids use both species as host, therefore there is potential for the parasitoids to use the expanding butterfly. They compared the rates of parasitism of old established populations and newly colonised populations. Although both were parasitised by a similar number of parasitoid species (old, six parasite species, new, five), the new Brown Argus populations had an overall lower parasitism rate than the established ones.

         Population

Observed parasitism (%) of Aricia agestis caterpillars (i.e. sum of parasitism by 

all parasitoid species) during the first generation in 2004 populations that differ in the 

position within the butterfly range (established vs. new parts of the range). Values are mean + SE and numbers within bars show sample sizes (numbers of caterpillars collected) (from Menéndez et al 2008)

The northward expansion of the Brown Argus is therefore not a direct response to temperature, but the result of a complex interaction including both a diet shift and a partial release of their parasites. This complexity of the biological interactions of each species makes it even more challenging to predict the responses of species to climate change.

References
MENÉNDEZ, R., GONZÁLEZ-MEGÍAS, A., LEWIS, O., SHAW, M., & THOMAS, C. (2008). Escape from natural enemies during climate-driven range expansion: a case study Ecological Entomology, 33 (3), 413-421 DOI: 10.1111/j.1365-2311.2008.00985.x
Thomas CD, Bodsworth EJ, Wilson RJ, Simmons AD, Davies ZG, Musche M, & Conradt L (2001). Ecological and evolutionary processes at expanding range margins. Nature, 411 (6837), 577-81 PMID: 11385570