Invertebrates and ancient trees
There are more than 2000 different invertebrate species in Britain (650 in Ireland) which are dependent on decaying wood in order to complete their life cycles. This represents about 7% of the entire invertebrate fauna – wood-decay is a major resource! That means more than 2000 different life styles, since each species has very particular and individual requirements. These statistics really bring home just how diverse a habitat wood-decay can be.
The key to understanding the ecology of these invertebrates is to develop an understanding of the two key processes involved:
- the aging process of woody plants, and
- the process of wood decay.
What do they feed on?
Very few invertebrates possess the necessary gut enzymes to break down the principal 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 feed directly on undecayed cellulose – include goat moth Cossus cossus, many longhorn beetles (Cerambycidae), bark beetles (Scolytidae) and the scarce beetle Lymexylon navale. In reality, these are still dependent on other organisms to digest their food, the key difference being that they carry the necessary cellulose-decaying bacteria in their guts.
The most important wood for wood-decay invertebrates is of course the living tree, for it is the living tissues that 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 a diverse age structure of living woody plants in order to ensure continuity of wood-decay habitats as well as sufficient trees to maintain the viability of the invertebrate populations. Tree density is another important factor as many species require open-grown trees – for a variety of reasons – while others favour the more humid conditions found in dense areas of trees.
Invertebrates of heartwood (and ripewood) decay
The single most important wood-decay resource for invertebrates is a large standing living tree with columns of decay in the heartwood (or ripewood).
In the early stages of decomposition, white-rotted heartwood is commonly fed upon by larvae of lesser stag beetle Dorcus parallelipipedus and rhinoceros beetle Sinodendron cylindricum (Lucanidae), 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 and black coloured net-winged beetles Platycis minutus and Pyropterus nigroruber (Lycidae) develop in relatively soft moist heartwood, especially beech and ash. The brassy tortoise beetle Thymalus limbatus (Trogossitidae) develops in the later stages of white-rot decay when the heartwood is dry and soft. The false darkling beetle Melandrya caraboides (Melandryidae) 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 (Oedemeridae) develop in relatively soft white-rotting heartwood of a great variety of broadleaves. The larvae of the spectacular tiger or feather-horned craneflies Ctenophora, Dictenidia and Tanyptera spp. (Tipulidae) also develop in soft moist white rot, generally where present as pockets of rot higher in the trees – known as rot-holes.
Brown rot (also known as red rot) supports a quite different fauna, including some of the most colourful members of the click beetle family, Elateridae. These include the rare Lacon querceus, which occurs in Windsor Forest, but nowhere else in Britain. Its larvae prey on the hairy fungus beetle Mycetophagus piceus (Mycetophagidae), which feeds directly on the mycelium of the chicken-of-the-woods fungus Laetiporus sulphureus, deep inside the decaying trunk. Another very rare heartwood species, the bright red cardinal click beetle Ampedus cardinalis, also develops in brown-rotten heartwood of old oaks, in large boughs as well as trunks. Like the Lacon it is an active predator, feeding on the larvae of developing beetles and flies.
The brown rot itself is bored by a number of small dark beetles, notably Dorcatoma chrysomelina and Anitys rubens (Anobiidae). The larvae of the forest windowfly Scenopinus niger (Scenopinidae) are specialist predators on beetle larvae, particularly in dry brown-rotting heartwood of various broadleaves, and are thought to be associated with anobiids, including the two above species in particular. As the brown rot begins to fall apart and crumble, accumulations of a fine dust begin to gather in nooks and crannies throughout the decayed trunk and are colonised by insects which specialize in moving though dry powdery media. The larvae of darkling beetles are particularly characteristic and occur here and in the wood mould which develops later, scavenging on any digestible organic remains. These larvae can be very numerous and so provide options for specialist predators. The larva of the very rare forest stiletto fly Pandivirilia melaleuca (Therevidae) is a particularly aggressive predator of this situation and is typically found with larvae of the darkling beetle Prionychus ater (Tenebrionidae).
Final stages of heart-rot
Both white rot and brown rot ultimately decay and compost into a soil-like wood mould which accumulates in the bottom of the cavities hollowed out within the tree. This is the rarest of the wood decay habitats as it is the end product of trunk and bough hollowing by fungi – the tree has to become sufficiently old to have accumulated enough dysfunctional annual rings to be become suitable for colonisation by heartwood-decay fungi. Also, there has to have been sufficient time for the fungi to have decayed this wood and for the products to have composted, and then for the specialist insects to have colonised the tree – the odds are not good in modern landscapes full of people with chainsaws! 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. For this reason, the specialist insects concerned tend to be used as flagship species for conservation, e.g. the European protected hermit beetles Osmoderma species (Cetoniidae; not found in Britain or Ireland) and violet click beetle Limoniscus violaceus (Elateridae)
The violet click beetle is a particularly significant species as it is believed to be one of the final specialist wood-decay invertebrate species to colonise a tree – most larvae have been found in the soggy wood mould lying in the hollow bases of ancient trees, where rainwater is penetrating. It is only known from a restricted part of Windsor Forest and two areas of the north Cotswold wood-pastures. Another important wood mould species, the noble chafer Gnorimus nobilis (Cetoniidae), is primarily known from old fruit trees in traditional orchards, but has also been found in old open-grown oaks and willows; it is a speciality of the lower Thames, Severn (Forests of Dean & Wyre) and Solent (New Forest) Basins. The related species, variable chafer G. variabilis, is mainly known from Windsor Forest and Great Park.
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, providing habitats for darkling beetles.
In the later stages of wood decay, the conditions are not essentially very dissimilar to those found in other decaying organic matter. Thus, certain wood decay-inhabiting invertebrates may also be found in other decayed and composted vegetation. A good example is the click beetle Denticollis linearis, a widespread species, which develops not only in decaying timber but also in peat on moorland. Some specialist wood-decay species are even known to be attracted to garden compost heaps in the autumn and to overwinter there, although it is unclear whether they can successfully breed in this situation. Decaying timber eventually supports what is essentially a soil fauna, characterized more by millipedes, woodlice and centipedes.
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 (Formicidae). Wood is macerated by their jaws and hardened by secretions from the mandibular glands to create the carton nest itself and intricate passageways are developed through the partially decayed 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. Many of these appear to be extremely rare and localised, presumably for much the same reasons as suggested for the wood mould invertebrates – the probability of a tree being suitable is very low.
It has only recently been recognized that what were thought to be common or garden black ants L. niger nesting occasionally in moist decaying wood are actually a separate species L. platythorax which specializes in permanently moist situations – it is also known from Sphagnum bogs. The true L. niger is actually confined to relatively warm and dry situations.
Other guest species
Further invertebrate 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 (Histeridae) is usually found in the burrows of lesser stag beetle Dorcus parallelipipedus in moist crumbly decaying heartwood, although has also been recorded with rhinoceros beetle Sinodendron cylindricum and brown tree ant Lasius brunneus. Other beetles, notably the scydmaenids, are specialist predators of mites, inhabiting and scavenging amongst the galleries.
Many bee and wasp species exploit the exit holes of wood-boring insects and other cavities in timber as nest sites. Hole-nesting digger wasps (Sphecidae) are good examples. These have their own specialist parasites including certain sarcophagid flies such as the rare Macronychia polyodon and M. striginervis.
Most of the above discussion relates to hollowing trunks, but similar processes also occur in smaller cavities in the trunk and in the branches. These are generally referred to as rot-holes and are particularly favoured by Diptera (two-winged flies) for the moister conditions which often prevail. Such rot-holes are often open to the elements, particularly rain, thus creating different conditions to those found in the centre of the main trunk. The waterlogged decay and other debris 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., etc. The rare hoverflies Myolepta potens and Pocota personata are good examples. As in the decaying heartwood, these invertebrates include species which feed directly on the wet decaying mulch; others are scavengers, predators and/or parasites.
Water-filled rot-holes even support a specialist freshwater fauna including the copepod crustacean 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 the 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.
The decay already discussed relates to the activities of the fungal mycelium – the wood–feeding part of the fungus. The fruiting bodies themselves provide specialist habitats for another huge array of invertebrate species.
The hard black fruiting bodies of the fungi Daldinia concentrica and Hypoxylon spp. are – remarkably – favoured by certain insects. The precise species of fungus appears unimportant, rather the hard black medium provided: the scarce fungus weevil Platyrhinus resinosus (Anthribidae) develops in Daldinia concentrica fruiting on the dead bark of ash trees as well as Hypoxylon fragiforme fruiting on dead beech bark, and other beetles behave similarly, e.g. Biphyllus lunatus (Biphyllidae), Litargus connexus and Mycetophagus atomarius (Mycetophagidae).
More typical annual bracket fungi also have their specialist fauna. In lowland Britain, the beetles Triplax russica (Erotylidae) and Orchesia micans (Melandryidae) develop in the brackets of Inonotus hispidus but T. russica associates with Fomes fomentarius in northern Britain and O. micans is able to use Inonotus radiatus in the moister conditions of western Britain and in Ireland. The beetle Abdera flexuosa (Melandryidae) is the typical species of Inonotus radiatus throughout its range. Chicken-of-the-woods Laetiporus sulphureus is the key larval habitat for the beetles Eledona agricola (Tenebrionidae) and Hallomenus binotatus (Melandryidae). The fruit bodies of these bracket fungi are fairly persistent, 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, notably the fly Tephrochlamys flavipes (Heleomyzidae). The soft fruit bodies of oyster mushroom Pleurotus ostreatus are favoured by the bright red and blue or black Triplax beetles (Erotylidae) and many Diptera such as Brachypeza species of fungus gnats (Mycetophilidae).
More persistent annual brackets and the perennial brackets of other fungi support a host of further species, including many which require longer time intervals to complete their life cycles. The larvae of the small moth Morophaga choragella (Tineidae) feed in galleries excavated within brackets, especially those of Inonotus and Ganoderma spp., pupating either in the fungus or in deadwood below. Elodia ambulatoria is a rare tachinid fly which specialises in parasitism of such tineid moth larvae, apparently targeting Morophaga choragella in particular. The small black beetles Dorcatoma dresdensis and D. serra (Anobiidae) both develop in longer-lived bracket fungi, although D. serra seems to specialize in the persistent annual species of Inonotus dryadeus in Britain while D. dresdensis is mainly found in the hard perennial brackets of Ganoderma spp. and Phellinus spp., including P. pomaceus in traditional orchards.
Amongst the many interesting variations on the theme are the larvae of the fly Agathomyia wankowiczii (Platypezidae) which develop in galls formed uniquely on the undersides of Ganoderma applanatum brackets. It is presumed to be a recent establishment in Britain, from the European continent, as the galls have been noticed only in recent years and yet are very conspicuous.
Two slug species are particularly associated with old growth situations – ash-black slug Limax cinereoniger and slender slug Malacolimax tenellus – and feed especially on the fruiting bodies and spores of fungi. The latter is the rarer of the two and a beautiful animal especially when feeding, for example, on black buttons Bulgaria inquinans, when the contrast of the bright yellow slug and the deep matt black of the fungus is very attractive. Both these slugs can be found away from trees – and slender slug is often found feeding on Russula toadstools – but both need the presence of large hulks of decaying wood as permanently moist refugia when the air is dry and cold.
Heart-rotters as keystone species
Basically, the heartwood-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 hollow trees?
Exposed wood on trunks and branches
Many of these insects gain access to the hollowing interior of old trees through patches of wood which have been exposed to the air through physical damage to the bark, e.g. through lightning strikes or damage caused by the collapse of a neighbouring tree. The exterior of this dead wood is sapwood in origin and tends to be decayed by white-rot fungi, although brown-rot fungi deep in the heartwood (or ripewood) within can fruit through it.
Ptilinus pectinicornis (Anobiidae) – a dull brown beetle but with dramatic feathered antennae – bores in this exposed dry sapwood on old broad-leaved trees, making the small pin-holes which are so common in such areas of exposed sapwood. As with so many of these wood-decay beetles, the female attracts males by release of a pheromone and it is the males that have the feathery antennae, which have evolved to maximise the sensory surface area and thereby make detection of the female easier. The females bore breeding passages into the hard outer sapwood to lay their eggs, preferring standing tree trunks over fallen ones; only a few females actually leave the old breeding site to initiate new colonisation. The bright red and blue beetle Tillus elongatus (Cleridae) is a specialist predator of Ptilinus pectinicornis larvae, entering the pin-holes in the exposed sapwood and exploring the galleries below for occupied burrows. Their larvae hunt nocturnally within the galleries and actively explore the outer trunk surfaces for new prey. Tomoxia bucephala (Mordellidae) 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 find its way into the decay.
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 hosts with them and inoculate the freshly dead timber themselves! The beetle Hylecoetus dermestoides (Lymexylidae) 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 and the cambial layer
The outer covering 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 (Cleridae) is a specialist predator of bark beetles and other wood-borers, and larvae of certain long-headed flies Medetera spp. (Dolichopodidae) 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 sapwood and smooth bark on the trunks, where they court, mate and catch their prey.
Larvae of the oak jewel beetle Agrilus biguttatus (Buprestidae) tunnel in and under thick bark of dying or freshly dead oak stems and branches. 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 an upsurge in Chronic Oak Decline, which boosted the species ever further to the point that the beetle began to be implicated in this worrying oak condition, albeit on very questionable grounds. As one of the first readily visible species on the scene it was a case of guilty by association – no reliable evidence has been forthcoming. The role of this beetle in disease has been raised again more recently as Acute Oak Decline has become just the latest condition affecting our native oaks. Again, there is no scientific evidence that the beetle is directly involved; it merely follows after the pathogen has created its freshly dead habitat.
The cambial layer is rich in nutrients in comparison with the bark or the wood below, and accordingly supports a rich assemblage of specialist invertebrates once dead – or even just dying. In particular this is where most of our longhorn beetles (Cerambycidae) develop.
Larvae of the predatory awl fly Xylophagus ater develop beneath bark on dead branchwood of a wide variety of broadleaved trees, very soon after the branch has died and when insect activity is greatest. They feed on the larvae of larger beetles such as longhorn beetles (Cerambycidae), cardinal beetles (Pyrochroidae) and possibly other insects although it is not known quite how they overcome their large prey – the prey are by no means defenceless since many have large jaws used for chewing hard wood! It is suspected that the Xylophagus larva uses its large toughened beak to puncture the waterproof coating of a potential prey larva, then retiring and waiting for the prey to lose vigour as it dehydrates, and then returning for the kill once the victim is more or less moribund and unable to defend itself. 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 defenceless prey beneath the bark – a remarkable thing about snakefly larvae is that they can move backwards as fast as forwards, which is a useful ability when meeting large-jawed longhorn beetles beneath the bark or being attacked by a woodpecker.
Britain’s largest false scorpion Dendrochernes cyrneus is another predator, which lives beneath loose dead bark on tree trunks and larger branches. It is mainly a southern species and prefers timber warmed 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 (see below) as well as the saproxylics!
Four species of cobweb beetle (Dermestidae) 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, the most widespread of these species, is to be found on just about any ancient tree throughout much of the British countryside. In contrast Trinodes hirtus is a great rarity found only on ancient oaks in old wood-pastures.
One of the key distinctions between open-grown and woodland trees is in their crown-shape.. In woodland conditions the impetus is to place all resources into growing taller in order to optimize sunlight-gathering by the foliage. Except in very shade-tolerant species, the lower branches generally remain small and soon die. However, an open-grown tree has light all around and grows into a characteristically domed shape, with the lower branches becoming large and spreading. As the tree grows larger and taller, these branches become increasingly shaded by the upper branches and die. Owing to the shaded position of these dead branches in the lower crown, they remain attached in humid air. This is a unique situation and it should be no surprise that there is a characteristic community of fungi and invertebrates exploiting it. A good example is the beetle Phloiophilus edwardsii (Phloiophilidae) which develops exclusively in the fruit bodies of the fungus Peniophora sp. – this fungus is similarly more or less exclusive to this situation. Once the branch breaks and falls to the ground it will be colonised by different fungi and Phloiophilus is obliged to fly off and find new dead aerial branches. Peniophora quercina is the main host and is most frequent on oak branches, although other Peniophora spp. on different tree species may also be used. Other beetle species such as Tetratoma desmaresti (Tetratomidae) and Abdera biflexuosa (Melandryidae) are also confined to aerial dead branches on open-grown oak trees but are thought to be associated with Stereum spp. fungi. The rare Tetratoma ancora is particularly associated with standing dead hazel poles in shady woodland – a similar environment but a variation on the theme.
Decaying wood lying in water
Decaying wood lying in water provides yet another range of niches to be exploited by invertebrates. The beetle Cyanostolus aeneus (Monotomidae) occurs only under bark on trunks and boughs which have been saturated with water in 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. An important group of craneflies (Limoniidae) are also associated with log-jams – accumulations of dead branches and twigs along water courses. These are not of course associated with ancient trees.
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 (Lucanidae), the larvae of which develop in moist decaying wood – white rot – below the soil surface, especially the decaying roots of old stumps, but also in the bases of fence posts and other situations. The tanner beetle Prionus coriarius (Cerambycidae) is a large and dramatic longhorn beetle which develops in large decaying tree roots in moist soils, especially along river floodplains. The rare metallic green hoverfly Caliprobola speciosa (Syrphidae) develops in wet-rot in underground roots of beech stumps, and many Criorhina spp. and Xylota spp. hoverflies also specialise in decaying roots. The larger roots of ancient trees clearly provide valuable habitats for such species.
A further assemblage of invertebrates is associated with exposed and/or fermenting sap from damaged or freshly dead trees or branches. Few of these have particular associations with ancient or veteran trees but one rarity, the hoverfly Hammerschmidtia ferruginea, is associated with old aspen trees in the Caledonian forest area of Scotland – the larvae develop in accumulations of sap beneath the intact bark of recently dead fallen aspen trunks. The trunk needs to have snapped apparently – or felled – which means that fungal colonization up from the stump is not possible.
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 recognised. 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 be 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 (Aderidae), for instance, which develops in red-rot in old oaks.
Today’s fauna is unique in time. The fauna of the ancient Wildwood of Britain and Ireland may be expected to have been particularly species-rich. Extinction has been a continuing process, owing to a variety of factors, in particular fluctuations in climate and the activities of people since prehistoric times. The fossil record includes many species which nowadays can be seen only in continental Europe: Rhysodes sulcatus (Rhysodidae) is a relict species of primary, wholly undisturbed forest, i.e. before it was disturbed by human activity, and is most recently known in Britain at c3000 BP. The adults feed exclusively on slime moulds (Myxomycetes) and therefore need large quantities of dead wood in a humid microclimate – this fits well with the unproven hypothesis of closed-canopy conditions at that time. However, Prostomis mandibularis (Prostomidae) is of particular interest as it is the commonest species found as a sub-fossil. Adults and larvae of this species live amongst moist decayed heartwood of large old tree trunks, including oak, cherry, beech and pine; it uses live standing trees as well as dead fallen trunks. It is widely regarded as an old growth species across its range, now occurring in ancient wood pastures within the cultural landscape – its requirements conform more to the open wood pasture hypothesis for the structure of the Wildwood. It has not only been lost from Britain and Ireland in the prehistoric period, but has continued to decline across northern and central continental Europe and is now most frequently found – albeit rarely – across southern Europe today. These two species demonstrate the need to work at the species level when attempting to interpret sub-fossil remains.
There is evidence for the presence in Britain of another species of stag beetle, Platycerus caraboides (Lucanidae), up until the Bronze Age. It develops in white-rotted branches and trunks of diameter greater than five cm lying on the ground, always broad-leaved trees but the species of tree is less important.
Even many of our currently rare species were once much more widespread here. The weevil Dryophthorus corticalis (Rhynchophoridae) is found as a sub-fossil as far north as Thorne Moor in Yorkshire and west in the Somerset Levels, but is today confined in Britain to Windsor Forest and Great Park. It lives at the interface of hard undecayed timber with internal brown rot, its larvae are wood-feeders, and it is primarily known from old open-grown trees. In Britain today it is confined to old oaks but it formerly also occurred in large old pines. Clearly large old open-grown oaks and pines with heartwood decay were a widespread feature of the Wildwood period.
An interesting quirk is the story of the lime bark beetle Ernoporus caucasicus (Scolytinae) which was originally described in Britain from sub-fossil remains found in the Somerset Levels long before anyone noticed that it was still alive and well, and widespread across the English Midlands!
Moving into historic times, the rare chafer Gnorimus variabilis (Scarabaeidae) 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 and Great Park and one other medieval deer park site, in Sussex.
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 far away. Good examples are the weevil Euophryum confine (Curculionidae) and the small fungus beetle Cis bilamellatus (Ciidae) which originate from New Zealand. The jewel beetle Agrilus sulcicollis (Buprestidae) is one of the latest arrivals, having been expanding its range across Europe in recent years and was first noticed in Britain in 1993 – 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 climate change – are now becoming able to live out of doors. An example is Alphitobius diaperinus, known for many years as the Lesser Mealworm Beetle because it was exploiting stored products and especially deep litter poultry houses. Its other name of Black Fungus Beetle is, however, now becoming more appropriate as it colonises old trees in the countryside.
Alexander KNA 2002 The invertebrates of living & decaying timber in Britain and Ireland – a provisional annotated checklist. English Nature Research Report No. 467.
Alexander KNA 1999 The invertebrates of Britain’s wood pastures. British Wildlife 11: 108-117
Alexander KNA, Butler JE & Green EE 2006 The value of different tree and shrub species to wildlife. British Wildlife 18: 18-28.
Dajoz R 2000 Insects and Forests. Paris: Lavoisier publishing.
Speight MCD 1989 Saproxylic invertebrates and their conservation. Council of Europe: Nature and Environment Series No 42.