How exciting was that!?
The Willow Emerald benefits from urbanisation, as it favours permanent garden and park ponds, surrounded by trees or bushes. Its is a late flying species, making the end of the dragonfly/damselfly season more exciting. It is the only Odonata species that oviposits into live wood, usually thin branches overhanging water, where eggs induce a diagnostic, gall-like reaction in the wood in a pattern of parallel lines.
 After a few sporadic records, the Willow Emerald became a regular breeding species in the UK in 2009, where many breeding colonies were discovered in Suffolk. Since then, it has steadily increased in range west and north, and this year it crossed the Humber for the first time.
 Today, there was a forecast of sunny spells and light WNW wind, and I decided to got on a damselfly twitch. I arrived at the park at 9:00 and walked to the eastern side of the lake, where the area around the boardwalk is favoured by dragonflies and damselflies. The first sunny spell took about an hour to arrive. When it did, Migrant Hawker males became active, with up to 5 males sharing the area, patrolling and resting over the large patch of marginal vegetation (above), a single female making a short appearance.
A female Common Darter (above) sat on the railings of the boardwalk, the first record of this species in the park this year. After walking up and down for a while searching for the Willow Emerald and with another large cloud looming, I moved onto the western side of the park to search for Small Red-eyed Damselflies. No luck, not a single damselfly on the west side of the main lake or boating lake.
 After a hot drink in the cafe I returned to the boardwalk. More searching of trees and marginal vegetation and walking up and down the boardwalk. The Migrant Hawkers were active so I watched them for a while. It was 12:20, the temperature quite pleasant in the sun, barely a breeze. Two male Common Darters were in attendance, chasing. After three hours in the park, I thought I had to content myself with a tandem pair of Common Darters, which were looking for an oviposition site. Maybe the Willow Emerald had succumbed to predation, of moved on. Another cloud was coming. I thought I'd stay for the next sunny spell. Then, a lovely, large sturdy green damselfly flitted about, checked the passing pair of hesitant darters in tandem, and sat on a leaf near the boardwalk: yes! the male Willow Emerald! It gave the impression of a sizeable insect, it is indeed as long or a bit longer than a common darter, and a stronger flyer than the common emerald. It sat on exposed leaves over the water, moving every now and then to another perch. It sat on alder leaves, on branched burr reed flower heads and leaves. I could take plenty of photos, as I watched it for about 20 min. A lovely damselfly tick!

Cumulative number of East Yorkshire dragonfly and damselfly species and the first record of each colonist species (data compiled from Paul Ashton's Dragonflies of South East Yorkshire, 2013).

Figure 1 Changes over time in the proportion of the illustrated melanic and non-melanic morphs of the two-spot ladybird beetle along a transect of ca. 115 km in length in the Netherlands (bottom-left). Samples were collected in each of the 5 years indicated at 16 more or less evenly spaced localities from west to east. Colouring of years matches the histograms for melanic frequency in the individual samples from each locality. The panel on the bottom-right shows deviations in average temperature from a ‘normal’ season/year at De Bilt (red spot on map). From left to right, columns represent data for winter (Wi), spring (Sp), summer (Su), autumn (Au) and the overall year (Tot), respectively, and from top to bottom for different years beginning before the period of ladybird sampling. The colour of each block indicates the extent to which the average temperature in the particular season/year deviated from ‘normal’; white, no deviation, blue, cooler than normal (dark blue more extreme than light blue), red, warmer than normal (dark red more extreme than light red)(from Brakefield & de Jong, 2011)

The story has a second dark aspect. The researchers had trouble reaching acceptable sample sizes in the 2004 sampling season. They even failed to find 2 spot ladybirds in two localities where they previously had been abundant. They attribute the decrease in numbers of the 2 spot ladybird to the impact of the invasive harlequin ladybird, which reached Holland in 2002. Not only the melanism cline is gone, but the 2 spot seems to be dissapearing as well.

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.


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