Aspen, a vital resource for saproxylic flies

The Biodiversity and Management of Aspen woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001

Graham Rotheray
National Museums of Scotland, Chambers Street, Edinburgh EH1 1JF. E-mail: ger@nms.ac.uk

Mature Aspen woodland © Ernest and Valerie Emmett Only large and mature Aspen stands, such as this one on the RSPB's Insh Marshes Reserve, can produce enough deadwood to support the full saproxylic invertebrate community.

Saproxylic organisms are those that depend on dead wood at some stage in their life cycle. They vary from woodpeckers to fungi, but the most biodiverse groups are Coleoptera (beetles) and flies (Diptera). Over most of Europe saproxylic organisms are under threat, due to the removal of woodland cover and impoverishment of what remains (Speight 1989).

Over the past 12 years, members of the Malloch Society have been involved in a study of saproxylic Diptera in Scottish woodlands. In comparison with Coleoptera, Diptera are poorly known (Rotheray et al. 2001), and part of our aim was to redress this imbalance. Our emphasis was on finding breeding sites and rearing larvae.

During the study we visited over 300 woodlands throughout Scotland. We obtained 2061 records of 258 species in 32 families. Two hundred and six species were reared, many for the first time. We reared 53 red-listed species. In addition, we recorded nine species new to Britain and 10 new to science, which further demonstrates how poorly known this fauna is (Rotheray et al. 2001).

Most records came from common and relatively widespread boreal trees such as Silver birch, Betula pubescens E., Scots pine, Pinus sylvestris L. and also from Ash, Fraxinus excelsior L. However, another tree species was also important, Aspen, Populus tremula L. For example, we found that for red-listed and other significant species (defined here as 'new to Britain', 'new to science'), Aspen was the third most important tree species after Birch and Pine of 22 tree species examined. It had three Red Data Book (RDB) category 1 "endangered" species reared from it, including the UK BAP Priority species, Aspen hoverfly Hammerschmidtia ferruginea (Fallen) (Diptera, Syrphidae) (Table 1). No other tree species had as many RDB 1 species asso-ciated with it (Rotheray et al. 2001). Altogether a group of 39 species were reared from Aspen, of which 14 were red-listed or otherwise significant.

Many of the red-listed and significant Diptera associated with Aspen appear to be confined to it. We did not rear them from any other tree species. Possibly these species will use other Willows and Poplars (Salicaceae) in different geographical regions, but this does not appear to be the case in Scotland. Thus, Aspen has a rich, specialised and unique fauna of saproxylic Diptera associated with it.

The most important microhabitats used for breeding by saproxylic Diptera include tree-holes, exudations of tree sap, decaying sap under bark and decaying sapwood and heartwood. With the exception of exuding tree sap which is associated with live trees, all these microhabitats are found in stumps and live and dead trees and branches. Although some tree species exhibit a tendency to have more of one type of microhabitat than others, (e.g. tree-holes in Beech, decaying sapwood in Birch), all were features of most of the 22 tree species examined, including Aspen.

For saproxylic Diptera associated with Aspen, the most important microhabitat was decaying sap under bark. When a tree or branch dies one of the first stages in the decay process is bacterial decomposition of the cambial layers between the bark and the sapwood. In Aspen this process results in the gradual build-up of a dark, oily, pungent-smelling layer under the bark, and it often includes the inner layers of the bark. This decay process is common to all trees, but in no other species did we find a layer as thick and wet as in Aspen.

This layer develops patchily at first but will eventually, under suitable conditions of shade and light and perhaps other as yet unknown features such as the state of fungal decay within the wood, encompass the entire underside of the bark. Eventually the bark separates from the sapwood, cracks and this lets in air. Bacterial decomposition of the sap ends at this point and the oily layer dries out and becomes unsuitable. The dynamics of this decay process are unclear, but the initial build-up takes about two years and lasts for another three years or so. The thickness of the oily layer depends on the thickness of the branch or tree. In branches below about 10cm, the oily layer of decay is too thin to provide a breeding site for most of the important Diptera.

It is within this oily layer of trees and branches above 10cm diameter that most of the important saproxylic Diptera dependent on Aspen breed. Their larvae either feed directly on the bacteria or acting as predators feeding on other insect larvae. A key characteristic of this particular micro-habitat is that it is dynamic and does not last long. Thus, for populations of saproxylic Diptera, a continuous input of fresh fallen or dead wood is required.

A particular feature of the important saproxylic Diptera associated with Aspen is their geographical distribution. Most of them are confined to just 14 sites in north-eastern Scotland. These sites contain large stands of Aspen, above 4.5 hectares. Although Aspen is widespread in Scotland it is only in the north-east that such large stands exist. The survival of these Diptera may thus be explained. It is only in these large stands that there is enough Aspen to provide a sufficient input of new fallen or deadwood for breeding.

Few of these vital stands are protected and some have sustained damage in the past few years. One particular problem is grazing by rabbits and deer which often remove the bark of fallen wood thus ruining the breeding site. Another problem is competition from faster growing conifers and removal of fallen wood by people. An additional potential threat is the plan to release beaver into Scotland with their preference for eating Aspen (Batty - this volume).

Measures are required to protect these core Aspen stands if this rich community of saproxylic Diptera is to be conserved. In the short-term, continual inputs of wood can be created by felling one or two trees per year. Aspen stands are often characterised by wind-blown trees lying on their sides, but still alive attached by their root plate. Trees such as these are perhaps good candidates for felling to provide an increase in breeding potential. Over the long-term, Aspen stands can be protected from grazing with fencing as has been started at the Royal Society for the Protection of Birds (RSPB) Reserve at Invertromie (Prescott, this volume). Such protection should enable the fast-growing Aspen to recover and offers the potential to extend Aspen stands and link them up to create a fully functioning ecological unit, (McGowan, this volume).

This is all the more urgent, given recent assessments of the abundance of Aspen dependent saproxylic Diptera. These reveal them to be at a low point in numbers due to the lack of suitable fallen wood in many stands.

Some may argue, why bother to conserve insects such as these flies in the first place? It should be understood that just because these organisms are flies does not mean they lack significance. Scottish Aspen is unique for the richness of its associated saproxylic flies. Many are rare in not only a British but also a European context. Some are accorded the highest level of threat within the RDB and one is a UK Biodiveristy Action Plan Priority species. If such a wealth of uniqueness and rarity is not worth saving, what is? However, there is another aspect to the saproxylic Diptera associated with Aspen that is important. When Aspen spread north following the retreat of the ice about 10-11,000 years ago, many insects that depended on it also moved north. Some of these have separated from populations of the same species further south to varying degrees. Some vary just in ecology, like the hoverfly Brachyopa pilosa. In southern Britain, this species breeds in association with oak, Beech and Poplar, but in Scotland it appears to be restricted to Aspen. It seems that this species has undergone a change in Scotland and is therefore special and adds to the biodiversity of the British Isles. Other insects appear to have gone one further step and speciated with the new species becoming dependent on Aspen. For example, we discovered a new species of scatopsid (small black flies) confined to Aspen, Ectaetia christiei. This new species is very similar to a widespread southern species, Ectaetia clavipes to which it is most closely related (Rotheray and Horsfield 1997). All of these features make Scottish Aspen and its saproxylic Diptera special and important to European natural history. This deserves to be recognised as such and treated accordingly.

Acknowledgements

I am very grateful to fellow members of the Malloch Society who helped discover the significance of Scottish Aspen for saproxylic Diptera. These include Geoff Hancock, Steve Hewitt, David Horsfield, Iain MacGowan, David Robertson and Kenn Watt. I am also grateful to Scottish Natural Heritage, World Wide Fund for Nature and to the RSPB for financial support. All of us are very grateful to Tom Prescott of the RSPB who has already done much to ensure the survival of Aspen and its dependent flora and fauna in Scotland.

References

Graham Rotheray, Geoff Hancock, Steve Hewitt, David Horsfield, Iain MacGowan, David Robertson and Kenn Watt. 2001. The biodiversity and conservation of saproxylic Diptera in Scotland. Journal of Insect Conservation In Press.

Rotheray, GE Horsfield D. 1997. Ectaetia christiei sp. n., a Scottish species similar to Ectaetia flavipes (Diptera, Scatopsidae). Dipterists Digest 4: 41-4.

Speight, MCD. 1989. Saproxylic Invertebrates and their Conservation. Nature and Environment Series, No. 42. Strasbourg: Council of Europe.

Falk S., 1991. A Review of the scarce and threatened flies of Great Britain (Part 1) Research and Survey in Nature Conservation No.39, Nature Conservancy Council, Peterborough.

Hyman P.S and Parsons M.S., 1992. A review of the scarce and threatened Coleoptera of Great Britain (Part 1), UK Nature Conservation No. 3, Joint Nature Conservation Committee, Peterborough.

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For further information about aspen, please go to the Aspen Information Resource

Pages about Dead Wood Ecosystems on this site

• Dead Wood, from Ecological Features of the Caledonian Forest
• A Dead Wood Survey of Glen Affric
• Aspen, a vital resource for saproxylic flies, from The Biodiversity and Management of Aspen woodlands
• Aspen Saproxylic Insect Survey in our target area