Ancient Tree Forum

Red cardinal beetle		There are more than 1700 different invertebrate species in Britain and Ireland which are dependent on decaying wood in order to complete their life cycles (Alexander, in prep. The invertebrates of living & decaying timber in Britain & Ireland). This represents about 6% of the entire British invertebrate fauna - wood-decay is a major resource! That means more than 1700 different life styles, since each species has very particular requirements. These statistics really bring home just how diverse a habitat wood-decay can be. The keys to understanding the ecology of these invertebrates is to develop an understanding of the two key processes involved: the aging process of woody plants the process of wood decay.

What do they feed on?		Very few invertebrates possess the necessary gut enzymes to break down the principle components of wood - cellulose and lignin. Most rely on fungi and/or micro-organisms to convert these compounds into more digestible materials. The exceptions to this - species which can digest cellulose - include goat moth Cossus cossus, longhorn beetles (Cerambycidae), bark beetles (Scolytidae) and the very rare beetle Lymexlon navale. The most important wood for wood-decay invertebrates is of course the living tree, for it is the living tissues which generate the wood which will ultimately decay. Dead wood has a limited existence, it decays and is ultimately re-cycled. Conservation of wood-decay communities requires conservation of a diverse age structure of woody plants in order to ensure continuity of wood-decay habitats.

Invertebrates of Heartwood decay		The single most important wood-decay resource for invertebrates is a large standing living tree with columns of decay in the heartwood. In the early stages of decomposition, white-rotted heartwood is commonly fed upon by larvae of lesser stag beetle Dorcus parallelepipedus and rhinoceros beetle Sinodendron cylindricum, forming characteristic large and convoluted galleries. White-rot decay supports a very wide range of some of our most colourful insects. Larvae of the beautiful red net-winged beetle Platycis minutus develop in relatively soft moist heartwood, especially beech and ash. The brassy tortoise beetle Thymalus limbatus develops beneath loose bark on decaying broad-leaved timber, especially oak, and in the later stages of white-rot decay when the heartwood is dry and soft. Melandrya caraboides is a large black beetle with a metallic green sheen and develops in relatively soft moist dead heartwood of boughs, trunks and stumps of various broadleaves, but especially ash and beech. The larvae of the bright green beetle Ischnomera cyanea develop in relatively soft white-rotting heartwood of a great variety of broadleaves. The larvae of the spectacular tiger or feather-horned craneflies Ctenophora spp also develop in soft moist white-rot. Red-rot supports a quite different fauna, including some of the most colourful click beetles. The hairy fungus beetle Mycetophagus piceus feeds directly on the mycelium of the sulphur polypore fungus, deep inside the decaying trunk - it is itself eaten by larvae of the rare click beetle Lacon querceus in Windsor Forest, but nowhere else in Britain. Another very rare heartwood click beetle, the bright red Ampedus cardinalis, also develops in red-rotten heartwood of old oaks, in smaller boughs as well as trunks. Like the Lacon it is an active predator, feeding on the larvae of developing beetles and flies. The red-rot itself is bored by a number of small dark beetles, notably Dorcatoma chrysomelina and Anitys rubens. The larvae of the stiletto fly Pandivirilia melaleuca (Therevidae) is a particularly aggressive predator living in very dry powdery red-rotten heartwood of oak. Scenopinus niger is another scarce fly the larvae of which are specialist predators on dermestid and probably other beetle larvae in dry red-rotting heartwood of various broadleaves. The larvae of the rare noble chafer Gnorimus nobilis develop in wood mould within hollowing old trees; often associated with old fruit trees, but also in oaks and willows; it is a speciality of the lower Thames, Severn and Solent Basins.

Exposed heartwood		Many of these insects gain access to the hollowing interior of old trees through patches of heartwood which have been exposed to the air through physical damage to the bark, eg through lightning strikes or damage caused by the collapse of a neighbouring tree. The dull brown beetle Ptilinus pectinicornis bores in exposed dry heartwood of old broad-leaves, making the small pin-holes which are so common in areas of exposed heartwood. As with so many of these wood-decay beetles, the female attracts males by release of a pheromone and the males have feathery antennae designed to maximise the sensory area. The females bore breeding passages into the solid outer heartwood to lay eggs, preferring standing tree trunks over fallen ones; only a few females actually leave the old breeding site to initiate new infestation. The bright red and blue beetle Tillus elongatus is a specialist predator of Ptilinus pectinicornis larvae, entering the pin-holes in the exposed heartwood and exploring the galleries below for occupied burrows. Their larvae hunt nocturnally within the galleries and actively explore the trunk surfaces for new prey. Tomoxia bucephala is an example of a species with decay-feeding larvae which is unable to excavate access for itself but uses the vacated Ptilinus galleries to get in to the decay.

Later stages of heart-rot		In both red- and white-rot the end product is a black wood mould which accumulates in the bottom of the hollow trunk as the fungus works its way into the upper trunk and main boughs. Some of Britain's rarest insects develop in this medium of relatively constant temperature and humidity, protected from the outside world by the surrounding living trunk tissues. The darkling beetle Prionychus ater is one of the most widespread specialists here. Amongst the rarer species are many of the bright red Ampedus click beetles and the famous violet click beetle Limoniscus violaceus - rare throughout its European range and one of Britain's very few legally protected beetles. The larvae of most of these species appear to develop in hollow trees which have been occupied by cavity-nesting birds such as jackdaw, stock dove or owls. The decayed heartwood is not rich in nutrients, and inputs of bird droppings, feathers, bones, etc, may provide an important source of minerals, etc, which promote successful development. Similar conditions can sometimes develop beneath loose bark on the trunks and main boughs and Prionychus ater can also be found developing in this situation, as well as the much rarer Prionychus melanarius. The later stages in the decay process of timber are not essentially dissimilar to other decaying organic matter and certain wood-decay inhabiting invertebrates may also be found in other decaying matter. A good example is the click beetle Denticollis linearis, a widespread species developing in decaying timber, but it also develops in peat on moorland. Decaying timber eventually supports what is essentially a soil fauna, dominated by millipedes, woodlice and centipedes.

Heartwood-nesting ants		There are two ant species which form their nests in the decaying heartwood of trees: brown tree ant Lasius brunneus and jet ant Lasius fuliginosus. Wood is macerated by their jaws and hardened by secretions from the mandibular glands to create the nest itself and intricate passageways are developed through the heartwood to access points in the outer trunk from whence the workers forage over the leaf canopy for food. These ants are of considerable interest for the wide range of other insect species which live specifically in their nests and which are associated with their runs. Good examples are the rove beetles (Staphylinidae) of the genus Zyras which live in the runs and nests of jet ant.

Other guest species		Other species appear to be associated rather more with the galleries of wood-decay insects than with the wood-decay itself. The rare beetle Aeletes atomarius is usually found in the burrows of lesser stag beetle Dorcus parallelopipedus in moist crumbly decaying heartwood, although also recorded with Sinodendron cylindricum and brown tree ant Lasius brunneus. Many bees and wasps exploit the exit holes of wood-boring insects and other cavities in timber as nest sites. Hole-nesting digger wasps are good examples. These have their own specialist parasites including certain Sarcophagidae flies such as the rare Macronychia polyodon and Macronychia striginervis.

Rot-holes		Most of the above discussion relates to hollowing trunks, but similar processes also occur in smaller cavities in the trunk and in the branchwood. These are termed rot-holes and are particularly favoured by Diptera for the moister conditions which prevail. Such rot-holes are open to the elements and particularly rain, creating different conditions to those found in the centre of the main trunk. The gunge which accumulates in these cavities is favoured by a whole host of hoverflies (Syrphidae), moth flies (Psychodidae), wood gnats (Mycetobiidae), long-headed flies (Dolichopodidae), etc. One rare hoverfly Pocota personata is known to develop mainly in rot-holes which are high in the canopy. As with the decaying heartwood, these include species which feed directly on the decaying mulch, others are scavengers, predators and parasites. Water-filled rot-holes even support a specialist freshwater fauna including the copepod Moraria arboricola, non-biting midges (Chironomidae) such as Metriocnemus martinii, and mosquitoes and gnats such as Anopheles plumbeus. The last develops in water-filled holes on mature trees; the eggs are laid on the sides of tree holes just above waterline and hatch only when flooded. The pale orange beetle Prionocyphon serricornis also develops in water-logged hollows in old trees, especially favouring those hollows amongst roots at the base of the trunk; the larvae are aquatic, feeding on the detritus from the dead leaves which accumulate in the cavities.

Fungal fruiting		The decay already discussed relates to the activities of the fungal mycelium - the feeding part of the fungus. The fruiting bodies themselves provide specialist habitats for another huge array of invertebrate species. The larvae of the small moth Morophaga choragella (Tineidae) feed in galleries excavated within the fruiting bodies of various wood-rotting fungi, especially Inonotus and Ganoderma spp, pupating either in the fungus or in deadwood. Elodia ambulatoria is a rare tachinid fly which specialises in parasitism of tineid moth larvae, mainly Morophaga choragella. The hard black fruiting bodies of the fungi Daldinia concentrica and Hypoxylon spp are remarkably favoured by insects. The precise species of fungus appears not important, rather the hard black medium provided: the scarce fungus weevil Platyrhinus resinosus develops in Daldinia concentrica on ash trees as well as Hypoxylon fragiforme on beech, and other beetles behave similarly, eg Biphyllus lunatus, Litargus connexus, and Mycetophagus atomarius. More typical bracket fungi also have their specialist fauna, eg Inonotus hispidus is where the beetles Triplax russica and Orchesia micans develop, I. radiatus for Abdera flexuosa, and sulphur polypore appears to be the key larval habitat for the beetles Eledona agricola and Hallomenus binotatus. These bracket fungi are fairly long-lived, but even the short-lived brackets of beefsteak fungus Fistulina hepatica are used by a number of species which can develop from egg to adult relatively quickly. The soft-bodied fruits of oyster mushroom Pleurotus ostreatus, for instance, are favoured by the bright red and blue or black Triplax beetles. Amongst the many interesting variations on the theme is the small black beetle Dorcatoma ambjoerni which has only been found in the fruiting bodies of the bracket fungus Inonotus cuticularis actually inside hollow beeches rather than growing externally. The larvae of the platypezid fly Agathomyia wankowiczii develop in galls under brackets of Ganoderma applanatum. So far only the galls have been found in Britain and it is currently known from only six localities in the south-east. It is presumed to be a recent establishment from the continent as the galls have only been noticed in recent years and yet are very conspicuous. Two slugs are particularly associated with old wood-pastures - ash-black slug Limax cinereoniger and slender slug Limax tenellus. The latter is the rarer of the two and a beautiful animal especially when feeding on say black buttons Bulgaria inquinans, when the contrast of the bright yellow slug and the deep matt black of the fungus is very attractive.

Heart-rotters as keystone species		Basically, the heart-rotting fungi are keystone species - a large number of other species are completely dependent on the conditions which they create. And not just invertebrates. Where would woodpeckers and bats be without hollowing trees?

Ambrosia beetles		The above species are all opportunistic, colonising suitable wood after fungi have become established. A small group of beetles go one step further and carry their fungal food with them and inoculate the freshly dead timber themselves! The beetle Hylecoetus dermestoides develops in dead timber and root stumps of hard and softwoods. The eggs are laid in batches in wood crevices, in rough bark or in boreholes. The fungal spores are in the eggshells and the larvae feed on the ambrosia fungus which develops on the walls of larval galleries. The other ambrosia beetles known from Britain are all in the bark beetle family (Scolytidae).

Bark		The outer parts of the trunk and branches - the bark - also has a whole succession of invertebrates associated with it. Freshly dead or even dying bark is rapidly colonised by bark beetles (Scolytidae) and with them come a series of scavengers which exploit the beetle galleries, plus predators and parasites. The ant beetle Thanasimus formicarius is a specialist predator of bark beetles, and larvae of the long-headed flies (Dolichopodidae) Medetera spp are also found in the burrows of bark beetles and other beetles on whose larvae and pupae they feed. The adult flies are very characteristic of the surfaces of exposed heartwood on the trunks where they court, mate and catch their prey. Larvae of the oak jewel beetle Agrilus pannonicus tunnel in and under thick oak bark where dying and dead. Its main refugia are ancient woodlands and pasture-woodlands, but this beetle spreads more widely on occasion. Numbers were already building up in the south-east when the Great Storm of 1987 gave the species a bonanza of freshly dead oaks. This event was closely followed by Oak Dieback Disease, which has boosted the species ever further to the point that it is now being implicated in this worrying oak condition, albeit on very questionable grounds. Larvae of Wood Snipe Fly Xylophagus ater develop beneath bark on branchwood of a wide variety of dead broadleaves in the early stages of decay. They feed on the larvae of larger beetles such as longhorn beetles and possibly other insects although it is not known quite how they overcome their large prey - these are by no means defenceless since they have large jaws used for breaking up hard wood! It is suspected that they use their large toughened beak to puncture the waterproof coating of a potential prey larva and then retire and wait for the prey to loose vigour as it dehydrates and then to come back for the kill once the victim is more or less moribund. It has even been suggested that the dense rings of wood-dust with which longhorn beetles surround themselves before pupating is intended as a defensive stockade to protect this vulnerable stage specifically from Xylophagus. The larvae of snakeflies such as Xanthostigma xanthostigma are more conventional predators which pursue more defenseless prey beneath the bark. Many fungi exploit dead wood on the outer surfaces of the tree or on smaller branchwood and it is not surprising that there are many further invertebrates which specialise on them. Good examples are species such as the beetles Phloiophilus edwardsii and Tetratoma ancora which develop in the fruits of the fungus Phlebia merismoides. A number of scarce species develop particularly in the dead lower limbs of trees, branches which have become shaded out by the tree's own canopy, presumably feeding on fungi such as Stereum spp. Examples include the beetles Tetratoma desmaresti and Abdera biflexuosa. Britain's largest false scorpion Dendrochernes cyrneus lives beneath loose bark on tree trunks. It is mainly a southern species and prefers timber heated by the sun. It not only feeds on small wood-decay invertebrates but also explores the outside surfaces of the trunk on calm warm summer evenings - exploiting the epiphyte communities as well as the saproxylics!

Cobweb beetles		Four species of cobweb beetle have larvae which live in the crevices beneath dead bark on the trunks of large old living trees, or under the dry loose bark of dead standing trees, where they are associated with the webs of bark-frequenting spiders. They feed on the remains of insects eaten and left over by the spiders. The larvae are covered with long bristles which protect them from the jaws of the spiders. They pupate within the larval skin, which splits along the back and affords protection for this vulnerable stage in their life cycle. Ctesias serra is the most widespread and to be found on just about any ancient tree in the countryside. In contrast Trinodes hirtus is a rarity found only on ancient oaks in old wood-pastures

Deadwood lying in water		Deadwood lying in water provides yet another range of niches to be exploited by invertebrates. The beetle Cyanostolus aeneus only occurs under bark on trunks and boughs which have been saturated with water, occurring along rivers and streams subject to spates. Larvae of the non-biting midge (Chironomidae) Orthocladius lignicola specialise in submerged rotten wood, and the rare hoverfly Chalcosyrphus eunotus develops in deadwood which is semi-submerged in freshwater.

Decaying roots		So far discussion has focused on the aerial parts of the trees. Decay in the roots is a further important habitat for invertebrates. The most famous species is the Stag Beetle Lucanus cervus, the larvae of which develop in moist decaying wood below the soil surface, especially the decaying roots of old stumps, but also in the base of fenceposts. The rare metallic green hoverfly Caliprobola speciosa develops in wet-rot in underground roots of beech stumps, and Criorhina spp and Xylota spp hoverflies also specialise in decaying roots.

Importance of nectar sources		The requirement of the adults of some of the insects which develop in wood-decay for access to blossom is widely appreciated. Nectar provides an energy-rich food which can rapidly be assimilated and used to fuel flight, and pollen is a protein-rich food which aids egg production. Flowering trees and shrubs are by far the most important sources, although other plants can also be very popular, notably hogweed and Angelica. Hawthorn provides the classic insect blossom, partly due to its flowering in late spring when so many wood-decay insects are in the adult stage. But really, blossom can important right through the season, and the presence of species such as sallow, holly, privet, rowan, crab apple, wild pear, guelder rose, bramble, and so on, are all beneficial. Even elder, with its poor reputation amongst entomologists, can be important for a select few species - it is particularly favoured by the nationally scarce beetle Aderus oculatus, for instance, which develops in red-rot in old oaks.

Changing fauna		Today's fauna is unique in time. The fauna of the ancient Wildwood of Britain and Ireland would have been particularly species-rich. Extinction has been a continuing process, in relation to a variety of factors but particularly fluctuating climate and as a result of the activities of people since prehistoric times. The fossil record includes many species which nowadays require visits to continental Europe to see them: Rhysodes sulcatus is a relict species of primary, wholly undisturbed forest, ie before it has been disturbed by human activity, and is most recently known in Britain at c3000 BP. The click beetle Porthmidius austriacus appears to have become extinct during the Neolithic period. There is evidence for the presence in Britain of another species of stag beetle Platycerus caraboides up until the Bronze Age. And so on. Even many of our currently rare species were once much more widespread here. The rare chafer Gnorimus variabilis, for instance, used to occur in old trees on Tooting Common, London, now so long gone that none of the residents can remember there ever having been old trees there! It is currently confined in Britain to Windsor Forest An interesting quirk is the story of the lime bark beetle Ernoporus caucasicus which was described in Britain from fossil remains found in the Somerset Levels long before anyone noticed that it was still alive and well and widespread across the Midlands! But it is not all decline. Species have been colonising too, although mostly in response to the activities of people! Many species have been accidentally introduced through commerce and others through the introduction of exotic plants for gardens and hothouse collections. Some of today's commonest wood-decay insects came originally from long away. Good examples are the weevil Euophryum confine and the small fungus beetle Cis bilamellatus which originate from New Zealand. The jewel beetle Agrilus sulcicollis is one of the latest arrivals, having been expanding its range across Europe in recent years and was first noticed in Britain in 199 - did it fly the channel or did it hitch a lift on a timber lorry? Some species have established themselves in Britain firstly within buildings, but with global warming are not being able to live out of doors. An example is Alphitobius diaperinus, known as the Lesser Mealworm Beetle for many years as it has been exploiting stored products and especially deep litter poultry houses, but now its other name of Black Fungus Beetle is becoming more appropriate as it colonises old trees in the countryside.

The invertebrate fauna of wood-decay