Monday, 25 September 2017
How mobile are hoverflies and other Diptera?
When out walking I make a serious effort to record the insects that I come across. Obviously most, but not all, are hoverflies. On several occasions I have seen relatively uncommon saproxylic species well away from woodland. Two species immediately come to mind: Myolepta dubia, which I found in several places last year, and Mallota cimbiciformis which I found once in a tree-lined avenue. I recall, however, recording Mallota at Thrislington Plantation (an open grassland) a decade or more ago. At the time I thought it odd because there were no decent-sized trees for at least a mile in any direction.
Casting the mind wider, last year I found the snipe fly Chrysopilus erythrophthalmus ovipositing on an isolated Black Poplar where it had been felled and in the same area found the cranefly Gnophomyia viridipennis. Both species breed in decaying sap, a highly transient habitat. They must have arrived within a year of the tree being felled: so where did they come from?
Once one starts to think about insect mobility, the question then arises: do we actually know much about the mobility of insects of conservation interest? I think we are just starting to scrape the surface. Work on the Aspen Hoverfly Hammerschmidtia ferruginea has shown that it is actually comparatively mobile. Again, I have encountered this species substantial distances away from potential breeding sites. By inference, I think we can also assume that Callicera rufa is very mobile, given that it is now cropping up at a wide variety of English localities.
Conversely, there are other species that appear to be anything but mobile, or they are so specialised that those individuals that stray never find suitable habitat? Examples that come immediately to mind include Caliprobola speciosa that is seemingly confined to the New Forest and Windsor Great Park, and the Pine Hoverfly Blera fallax that is restricted to two closely approximated localities in Speyside.
Where are, and which are the intermediates?
When I worked in the Invertebrate Site Register Team back in the 1980s, the thinking was that saproxylic species were relatively immobile. After all, if they flew too far, they would end up in a wilderness of open countryside! That approach was clearly wrong! Or at least was it? It seems to me that we need to think in much more detail about insect mobility from a conservation management perspective. Many invertebrates utilise micro-features that are transient to some extent and so they must move as the situation demands. The crucial point is that the landscape needs to support sufficient of these micro-features as a constant asset, even if the individual features appear and disappear.
There are of course the flagships for meta-populations such as Marsh Fritillary Butterfly. But, who is thinking about strategic distribution of micro-habitats for other invertebrates? Conservation strategies tend to focus on big projects – major site developments. Perhaps one of the most useful programmes might be to work on relatively small features that are important and distinct micro-habitats? For many years I have felt that there was a case for locilised reduction in grazing pressure over base-rich seepage lines. Perhaps too, there is a case for developing a strategic vision of a landscape that will eventually be richer in veteran trees with associated micro-habitats?
I also wonder if there is a need to develop projects that investigate the availability of important invertebrate micro-habitats. Not large-scale mapping of macro-features, but a log of important micro-features such as trees with different types of rot and their distribution across the landscape. Can we make any correlation between densities of such attributes and the distribution of important saproxylic assemblages? If there is a link (and I'm sure there is one) then the best way to improve the overall resource of saproxylic species is strategic planting to secure veteran trees across hitherto uninhabited landscapes.