Geoff Wilkinson
Heavy raindrops pelted the window as I stared moodily out at grey clouds condensed into a mournful, brooding gloom hung over a landscape ravaged by winter. The fire danced merrily in the hearth as, huddled to its warmth, my thoughts drifted back to the summer that was. At times not an awful lot different! Though it did have its moments. Rare days of splendour when the sky, without blemish of cloud, was an immense radiance of unstained blue with the sun shining high and bright. On such days it was my appointed task as acolyte of Ellen Rotheray, a PhD student from Stirling University, to sit by various rotting aspen logs in Strathspey and await the appearance of Hammerschmidtia ferruginea, also known as the Aspen Hoverfly. But why engage in this strange behaviour as I was frequently asked by passer-by’s?
In Britain the aspen hoverfly is endangered and is a flagship species representing 12 other rare flies dependent on dead aspen. It is a UK Biodiversity Action Plan species and its conservation is also beneficial to those other flies. It is a species restricted to Scotland, as it is predominately a creature of northerly climes, found at seven localities within fragments of aspen woodland in the Scottish Highlands.
The larvae dwell under the bark of rotting fallen aspen trees and branches where they filter their bacterial food from the bilious brown decaying sap in much the same fashion as baleen whales feed on plankton in our seas. However, this is a temporary habitat as fallen logs, within a few years, develop cracks and the sap dries up. Therefore consistent inputs of dead wood is required to maintain populations. Although aspen is a common enough tree rarely is it found in areas of 5 hectares or more. Most of these large woodlands are home to the aspen hoverfly suggesting that inputs of dead wood is high and regular enough at these localities to maintain a continuity of larval habitat. Should there be a break in this continuity then the population faces extinction.
The reasons why dead wood input has decreased in aspen woodlands is mostly to do with the loss of trees due to the suppression of natural regeneration by livestock or rabbits over the centuries that would otherwise have replaced dead or fallen trees. In effect such woodlands are dying on their feet. Although regeneration has been encouraged at some sites in recent years there will be a lag time before they will provide the much needed dead wood which raises the potential running out. Therefore there has been some selective felling of trees and branches, which are then left to rot to provide larval habitat until the woodland naturally provides.
Clearly there is also a need to increase the aspen woodland resource to better secure this species in Scotland. In creating new woodlands there will be potentially large areas of hostile habitat, like agriculture and roads, separating them from existing populations in other woodlands. So the questions are: how far can an aspen hoverfly disperse from its natal home and will they travel over hostile areas to reach suitable habitat? The answers to these questions have implications for future conservation measures. If the hoverfly is unable to disperse far or travel across hostile habitat to new woodlands then schemes such as reintroduction would then need to be considered at much financial cost and effort.
Helping to find answers to these questions was behind my ascetic vigil next to rotting aspen logs. To test the powers of aspen hoverfly dispersal rotting aspen logs of similar size were distributed (with much sweat and cursing) at 1 km points along 3 transects extending out, from Insh Marshes RSPB reserve, to 3 km with one extra log at 4 km along the NE transect (see map). Each log was attended for approximately an hour each by a group of Ellen’s apostles. 1066 aspen hoverflies were captured as they emerged from aspen logs at Insh and marked with enamel paint before their liberation at a central point. Then log watching began in earnest.
It was with great mirth and excitement that we had re-sightings of paint-marked aspen hoverflies along all the transects. By the end of the flight period we had notched up 28 re-sighted individuals with most dispersers found along the north-east transect and one even made it the full 4 km! The next step is to lengthen these transects to see how much further these insects are willing to travel. From this experiment we can say with reasonable confidence that these flies are able to disperse across hostile landscapes and locate areas of suitable habitat which bodes well for their natural dispersal to new aspen woodlands when they are created.
Being apart of this project has been an important part of my apprenticeship and has given me valuable insight into conservation of endangered insects. It presented a much coveted opportunity to work out in the field in a beautiful part of the world. In addition I gained many a tick and midge bite, but I guess you have to take the rough with the smooth!