Part 2: Nematocera and Aschiza not dealt with by Falk (1991) - JNCC

Diptera recording scheme meetings, intensive surveys of particular sites, from many individual recorders
and remarkable results have been obtained from the series of surveys using passive sampling techniques
(water traps, pitfall traps and Malaise traps) that were carried out in the 1980s by the former NCC and
other recorders more recently.
The body of information on which the distribution of British fungus gnats is based is now considerable,
although it is fully appreciated how wide the gaps in our knowledge are. It can be stated that there are
now records of some species for more than 1500 hectads. More than 100 hectads have in excess of 50
species known. There are at least 40 sites scattered throughout Britain for which more than 100 species
have been recorded and about 200 or more species are known for each of the seven principal ancient
forest areas that have been well surveyed (Burnham Beeches, Epping Forest, New Forest, Savernake
Forest, Windsor Forest and Wyre Forest in England; Rannoch in Scotland). Altogether, there are some
75,000 species/site records from Britain of which about 18,000 are from Scotland.
The larval biology of these families was summarised under Section 3.5 above, from which it is apparent
that most species are dependent on undisturbed forest habitats, although a smaller number of species are
more associated with fungi in other habitats such as wetlands or uplands. An association with fungal
tissue is the predominant biological requirement. Few species are monophagous, although oligophagy is
common and most species are confined either to lignicolous or terrestrial fungi, and in the case of the
more polyphagous species there is usually a limit imposed by the texture of the fungus.
The threats to the continued existence of fungus gnats on sites are varied, but there are two major
problems affecting those dependent on woodland fungi. The first concerns the treatment of standing or
fallen dead trees or dead wood on the ground. Many sites have suffered from excessive tidiness during
management and even in sites managed for conservation there has frequently been a failure to recognise
the importance of dead wood for insects and fungi. The clearance and replanting (often where natural
regeneration would have been more effective) that has taken place on many sites affected by storm
damage in recent years has been particularly deleterious. Even where dead wood is left it is often in
exposed positions where it becomes too dry for fungal growth to take place.
The second problem concerns the widely reported decline of the terrestrial mycorrhizal fungi in recent
years, in which a variety of factors have no doubt been involved and even acid rain has been implicated.
Successive years of summer droughts in the last decade of the 20th century have taken their toll and there
has been a significant reduction in the autumn flush (usually the first half of October) of adult fungus
gnats, with species associated with terrestrial fungi predominating, in many regions. Until 1993 the
spring flush (usually late May to early June), with species associated with lignicolous fungi
predominating, held up quite well but a sharp decline was apparent in some areas there too in 1994. In
storm damaged areas the opening out of woodland has evidently had an effect, and damage to soil
structure in the clearance and replanting operations mentioned above will have exacerbated the situation.
In general, increased drainage will have been damaging, and is likely to have contributed to the decline
of many species of woodland insects. Mycologists have also been slow to accept that the removal of
large quantities of particular species from a site for culinary purposes must have an effect on the survival
of the fungi as well as the insects developing in them.
Nevertheless, fungus gnats have shown themselves to be resilient to environmental changes and it is
expected that most of the species dealt with in this Review will be found to occur more widely than is
presently known when more extensive recording has been carried out. It is clear that they are adapted to a
food resource which has always experienced natural fluctuations in availability.
The greatest number of site records of fungus gnats have been based on adults obtained by sweeping with
a net in suitable harbourages such as wooded stream banks, sheltered slopes and hollows in woodland
and the vicinity of decaying trees or large bodies of dead wood. As mentioned many of the recent
records, including a significant number of additions to the British list and other previously little known
species, have been obtained by passive sampling techniques. Malaise trapping has been most successful,
with large catches from the Berkshire/Oxfordshire fens and carr woodlands, Savernake Forest in
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