the British saproxylic invertebrate fauna - People's Trust for ...

Proceedings of the second pan-European conference onSaproxylic BeetlesThe British saproxylic invertebrate faunaKeith Alexander, Ancient Tree Forum, 59 Sweetbrier Lane,Heavitree, Exeter EX1 3AQ, UK.AbstractThe compilation of a checklist of the fauna of saproxylicinvertebrates has recently made it possible to identify how manyspecies are known to be dependent on the process of wood decayin trees and shrubs in the countryside of Britain and Ireland(Alexander, 2002).The answer is close to 1800 species for Britain anda little over 600 for Ireland.The figures for Coleoptera are 700 forBritain and 259 for Ireland (see Table 1).The immense nature conservation importance of decaying woodis emphasised by the fact that:● the 1800 species represent a major proportion - about 6%- of the total fauna known from Britain; and● 38% of the saproxylic invertebrate species haveconservation status in Britain, with Coleoptera having thehighest figure of any taxonomic group - 54%.Despite this great importance, the biology of wood decay is verypoorly understood by most invertebrate specialists. A basicunderstanding of tree biology is needed if we are to successfullypromote the conservation of wood decay habitats and – our specialinterest - the associated fauna. Living trees contain dead woodnaturally and this has nothing to do with the health of the livingtissues. The heartwood decay succession is especially important tounderstand for the conservation of species such as Violet Click BeetleLimoniscus violaceus and Noble Chafer Gnorimus nobilis.IntroductionIn this paper I present the new annotated checklist of the Britishsaproxylic invertebrate fauna and some of the conclusions whichhave been drawn from the data.This leads on to a discussion of theimportance of entomologists developing at least a basicunderstanding of the process of wood decay.The ChecklistThere has long been a need for a checklist of the saproxylicinvertebrates. It has been needed for many reasons but especially to:● facilitate the recording of particular species on particularsites, to focus field workers on the significant habitatfeatures;● draw the attention of nature conservationists to theenormous variety of niches exploited in wooded habitatsand to emphasise the importance of wood decaysuccession;● stimulate ecological research of the fauna.A checklist has now been compiled for Britain and Ireland(Alexander, 2002) and this new publication was launched at theSymposium, with all delegates being provided with a complimentarycopy.The checklist is provided as “provisional” quite simply because ofthe enormity of the task. The publication represents the stage atwhich the compilation has reached by June 2002. Much literature isstill to be searched; many experts are still to be consulted. It is hopedthat the delegates at the Symposium will contribute in two ways:● provide additional information on the known biology orecology of the species listed, or to propose additionalspecies or deletions;● work towards generating a comparable list for their owncountries or regions.One of the shortcomings of attempts to list the European saproxylicspecies most in need of conservation – for the Berne Convention orHabitats & Species Directive, for example - has been a lack ofcomparable information across the whole continent. Speight (1989),for instance, focused on a few families of larger and more attractiveColeoptera for practical reasons rather than scientific ones, and thishas formed the basis for the species selected for listing by theCouncil of Europe and the European Commission.Wood decayThe definition of saproxylic used as a basis for this presentation isas follows:● the immature stages of the invertebrate develop in somepart of the wood decay succession or in its products;● the wood decay succession is itself defined as including allstages from the un-decayed wood right through to theTable 1:Comparison ofnumbers ofColeoptera,Diptera and totalInvertebrates onthe saproxylicchecklist, withinformation onconservation statusTaxonomicgroupingNumber ofsaproxylicspecies reportedfrom IrelandNumber of nativeGB saproxylicspecies(excludingextinct species)Number ofNationallyScarce*saproxylicspecies in GBNumber of RedData Book*saproxylicspecies in GB% of GBspecies withconservationstatusColeoptera 259 700 219 162 54Diptera 293 730 99 143 33All invertebrates 615+ 1792+ 354 327 38*Nationally Scarce – species believed to be confined in Britain to 100 or fewer 10km squares of the Ordnance Surveynational grid. Red Data Book – species believed to occur in fewer than 15 of the 10km squares© People’s Trust for Endangered Species, 2003 All rights reserved. Extracts of this publication can be taken for non-commercial, in-house use, subject to the sourcebeing acknowledged. Registered Address: PTES, 15 Cloisters House, 8 Battersea Park Road, London SW8 4BG, UK.Tel: +44 (0)20 7498 4533 email: enquiries@ptes.org web:www.ptes.org1

fully decayed wood mould which is barely distinguishablefrom soil or peat.The process of wood decay is a very specialist subject, with fewpeople expert. It is not surprising that few invertebrate specialistshave even a basic understanding of the fungal decay of wood.Thereare notably few mycologists who do! In Britain we are fortunate inhaving two key academics - Alan Rayner and Lynne Boddy (Boddy,1994; Boddy & Rayner, 1983; Rayner, 1993, 1996; Rayner & Boddy,1988) – as well as the environmental campaigner Ted Green (Green,1991, 1993), who have done much to make their knowledge availableoutside of the specialist literature.Despite this, a widespread lack of understanding has seriousconsequences for conservation and ill-informed conservationpractices are rife.The ageing process in trees and shrubsAll woody plants appear to go through much the same stages in theirdevelopment. Each year a layer of new tissue is generated by thecambial layer – the annual ring. As the tree or shrub grows, thediameter of the stem increases and so the circumference of theannual ring necessarily increases. The actual depth of each growthring is influenced by many things, particularly the quality of thegrowing season. But some general trends can be identified:● the volume of new tissue contained in each successivering steadily increases as the canopy of foliage-bearingbranches increases in size and develops to its maximumextent;● once the canopy has reached maximum development forthe species and the situation in which the individual isgrowing, then the volume of new tissue in each successiveannual ring remains relatively constant;● eventually a situation is reached whereby the new ringhas to encompass such a large girth of stem, and yet drawon a no-longer increasing leaf area for carbohydrates, thatit must fragment, and cease to be a continuous ring ofliving tissue;● in parallel to this change to the main stem or trunk, thecanopy itself becomes unsustainable (the reasons are notunderstood), the upper branches begin to die back andthe canopy starts to thin;● as light increasingly reaches the lower parts of the tree,through the declining high canopy, so new and vigorousgrowth occurs throughout the lower canopy – a processwhich has been called “retrenchment” or “growingdownwards”;● eventually a new, lower canopy is established – this stagein the tree’s growth is termed the “ancient” tree andancient trees may live a very long time, often manycenturies.The pattern of growth and development described above relates toopen-grown trees or shrubs, which have space for optimaldevelopment and are not affected by competition from other treesor shrubs. Clearly, where canopy competition is occurring, growingdownwards will lead to the early death of the individual as itbecomes over-shaded.The process offungal decay of woodThe annual growth ringsbuild up. As theythemselves age thenthey begin to die off.Thepattern of death of thegrowth rings variesfrom species to species.In oak, the rings aregenetically preprogrammedto die afterabout 20 or so years andso a definite “heartwood”is produced – a deadwoody core to the tree. Inbeech, the rings die moreerratically and the core ofthe tree has a mix of deadAn ancient sweet chestnut Castanea sativasplit in half during a storm, revealingcomplete hollowing of the trunk by the fungusLaetiporus sulphureus, and accumulateddebris in the base - this "wood mould" is therarest habitat for saproxylic beetles acrossEurope. Photo by KNA Alexander.and living rings, and so this species is said to not have a trueheartwood.These two distinct patterns of ring death may not be thewhole story as this aspect of tree biology is also not well understood.Whichever pattern is occurring in a particular tree or shrub, thekey thing from a saproxylic point of view is that each individual treeor shrub increasingly accumulates dead tissues within the core oftheir stems.These tissues arguably serve no function to the tree orshrub – there are apparently valid engineering arguments that saythat, on the one hand, a solid trunk is better for tree survival, and, onthe other, a hollow stem is better. What is certain however is thatthese dead tissues will be colonised by wood-decay fungi at somestage, in many - but not all - cases still while they are envelopedwithin the living outer ring of the tree’s tissues.Fungal decay of wood can take a variety of routes.The two which aremost significant to saproxylic entomologists are red-rot (referred toas brown-rot by arborists) and white-rot. The main components ofwood are lignin and cellulose.All of the wood-decay fungi break downcellulose but not all can break down the lignin. If the lignin is leftbehind then it remains as the characteristic hard, but brittle, red-rotstructures.Where lignin and cellulose are both broken down then thedecay produces a paler and softer material, termed white-rot.Eventually the whole dead core of tree rings is broken down, leavinga hollow cavity, and debris accumulates in its base – the dark woodmould,comprising a complex mixture of composted and compostingdegraded woody tissues, invertebrate faecal pellets, etc. Where thecavities have been used by nesting or roosting birds, or bats, thenother debris may have been introduced, including nesting materials,vertebrate faeces and urine, discarded feathers, and even the remainsof the vertebrates themselves which have died.The vertebrates mayalso have brought in other organisms, eg discarded owl pellets willinclude small mammals remains and fragments of other food,including, in the case of little owl, a variety of large invertebrates.More than one fungus – species or individual – may be activewithin the trunk of a particular tree or shrub at any one time.Thesemay be breaking down tissues in parallel, or exploiting material indifferent stages of decay.Once a stem has been hollowed, the living outer tissues arecontinuing to grow - new growth rings continue to be laid down eachyear. The living tissues are not being harmed by the decay fungi.Eventually these “new” growth rings will begin to die and a new cycleof fungal colonisation begins.2

The individual tree actual benefits from the fungal decay of its deadheartwood tissues. Resources are locked up in these dead tissues.Fungal decay makes them accessible again to the tree as well as tothe fungi and invertebrates. Roots may grow inwards, into the debriswithin the cavities and make use of the breakdown products.The fungal and invertebrate communities of wood decayThere are many thousands of species of fungi that exploit the deadwoody tissues of trees and shrubs. Heartwood decay – as describedabove – is just one part of the story. Other fungi are active on deadbranches within the canopy, others on the bark, others on deadroots. A small minority of fungi are actually pathogens or parasitesand feed on living woody tissues, the former causing death of tissueseither locally or to the whole organism.Thus a large and wide variety of fungal species are active in thedecay process, and there are successions of decay. Each tree or shrubspecies has a particular suite of fungal species associated with them,although few individual species of fungi are confined to one particulartree or shrub species.The array of niches in the wood-decay processfor invertebrates is immense and, until now, only guestimates ofspecies-richness were possible.The debris accumulating within tree cavities is the end point of thedecay succession and therefore – in a natural tree population – is therarest component. It takes a long period of time for a species suchas an oak to reach a large size and for fungal decay to reach thehollow cavity stage. Over such a long space of time much can happento kill the tree prematurely. In modern Europe there are manypressures on trees and large old hollow individuals are now verymuch the exception over large areas of countryside. It is thereforeno coincidence that some of our rarest and most threatenedsaproxylic beetles are those which develop in the debris of cavitiesin old trees. Limoniscus violaceus, Gnorimus nobilis and G. variabilis areexcellent examples which are exceedingly localised in modern Britainand which need special conservation measures. Osmoderma eremita isanother example so localised in Europe that it is absent from Britain.Conclusions from the checklistThe compilation of the checklist has made it possible to identify howmany of our species are known to be dependent on the process ofwood decay – the succession from the living wood through to thefully degraded wood mould.The answer is close to 1800 species forBritain and a little over 600 for Ireland. The figures for Coleopteraare 700 for Britain and 259 for Ireland.The immense nature conservation importance of decaying woodis emphasised by the fact that, overall, 38% of the saproxylicinvertebrate species have conservation status in Britain, withA sawn section of an old orchard plum tree Prunus domesticashowing advanced hollowing by the fungus Laetiporus sulphureus andlarvae of Noble Chafer Gnorimus nobilis in the wood mould debris withinthe cavity. Photo by Colin Twissell.Violet click beetle Limoniscus violaceus, a species which develops in the woodmould debris in the base of hollow trees. Photo by RS Key.Coleoptera having the highest figure of any taxonomic group - 54%.Very few of the invertebrate species listed can feed directly on thedead wood.The majority are dependent on fungi to break down thecellulose, lignin and other constituents into simpler, more digestiblesubstances.Well-known exceptions to this include the Scolytidae andCerambycidae which carry micro-organisms in their guts which canbreak these complex substances down.Key messages from this paper are:● ageing is a natural process;● heartwood decay and hollowing are also natural processes;● dead wood and decay are a normal part of a healthy tree;● saproxylic specialists need to be promoting thesemessages within the conservation movement if we wantour interests to be conserved.AcknowledgementsMy understanding of tree biology and fungal decay has been gained entirelythrough my association with the Ancient Tree Forum and I would like toacknowledge the inspiration generated by some of the key people, notablyTed Green, Nev Fay, Jill Butler, Caroline Davis,Vikki Forbes and Helen Read.I would also like to thank Roger Key and Jon Webb of English Nature fortheir support and encouragement with the checklist project.ReferencesAlexander, K.N.A. (2002) The invertebrates of living and decayingtimber in Britain and Ireland – a provisional annotated checklist. EnglishNature Research Reports No. 467.Boddy, L. (1994) Wood decomposition and the role of fungi:implications for woodland conservation and amenity treemanagement, pages 7-29, In: Spencer, J., & Feest, A., eds, Therehabilitation of storm damaged woods. University of Bristol:Department of Continuing Education.Boddy, L. & Rayner, A.D.M. (1983) Origins of decay in livingdeciduous trees: The role of moisture content and a re-appraisal ofthe expanded concept of tree decay. New Phytologist 94: 623-641.Green,T. (1991) Simply fungi. pp 26-27. In: H.J. Read (ed) Pollard andveteran tree management. Corporation of London.Green,T. (1993) Inonotus hispidus on apple trees. Mycologist 7 (2): 80-83.Rayner, A.D.M. (1993) The fundamental importance of fungi inwoodlands. British Wildlife 4: 205-215.Rayner,A.D.M. (1996) The tree as a fungal community. pp6-9. In: H.J.Read (ed.) Pollard and veteran tree management II. Corporation ofLondon.Rayner, ADM, & Boddy, L. (1988) Fungal decomposition of wood: itsbiology and ecology.John Wiley & Sons.Speight, M.C.D. (1989) Saproxylic invertebrates and their conservation.Nature and Environment Series No. 42. Council of Europe, Strasburg.3