Sunday, 18 October 2015

Dipterists Forum Autumn Field Meeting 2015

This last week was the final Dipterists Forum field meeting of the year (10-17 October): a combined visit to Dorset and Hampshire. These meetings focus on craneflies and fungus gnats, with the additional hope of Platypezids and Heliomyzids.

The first 4 days were based at Dorchester and concentrated on the Isle of Purbeck. The main localities visited were Studland NNR and Arne Heath. The second half of the week we transferred to Ringwood and focussed on the New Forest.

As in many recent years, we had nice sunny weather but this year was a little cooler. The temperature meant that we did not get started until 11 am each day, and by 3.30 there was very little moving. It also meant that we generally only managed to visit 3 localities each day.

The numbers of flies seen were remarkably low and it is difficult to be sure why. It was certainly cool, and although there was sunshine it was difficult to find warm places where flies gather. Perhaps numbers were genuinely low? If so, why might this have been?  Fly larvae need damp conditions: periods of drought and high temperatures can kill delicate larvae. We can be fairly sure that long periods of drought have played a part in declining fly numbers over the past 40 years. Many wetland species took a long time to recover after the droughts in the mid-1970s and were hit again in the early 1990s and early 2000s.

The low catch of craneflies seems to reflect an ongoing trend in which the numbers and diversity are declining year-on-year. My own hauls of fungus gnats were equally pathetic – I rarely managed to assemble a sample of more than 20 specimens in an hour’s sweeping and we rarely exceeded a site list of 20 species all week. When you bear in mind that we had a trip to Sussex a few years back, in which we recorded the better part of 200 species and assembled several site lists exceeding 90 species, this year has not been a good one despite visiting one of the most important saproxylic sites in the country. Numbers of Platypezids were a little better, with seven or eight species recorded. With these I did a little better than in previous years but still only managed two species and a handful of specimens.

Data for the hoverfly Rhingia campestris show how this species is substantially knocked out by periods of drought, with the summer generation failing to materialise after intense hot spells in July. One possible explanation for the lack of flies this autumn is therefore that the intense hot spell in July was responsible for the death of many fly larvae. That cannot be the whole story, however, because we saw good numbers of Helemyzidae (not many species) at several sites in The Forest. Another possibility is that gnat numbers were depressed because there were relatively few fungal fruiting bodies. But, then again, there were patches of honey fungus, Armillaria sp., but very few gnats in attendance. So, perhaps it was just too cold?

When I sought permission to visit the New Forest I was aware that there was concern about my mention of fungi – until I clarified that we were interested in fungus-feeding flies. There are big problems with commercial mushrooming, with many areas stripped of all fruiting bodies. So, unsurprisingly, any mention of fungi will inevitably raise concern. In the event, our visit was not a problem in this respect and we hope that the data produced will be helpful. In many places fungi were notable by their absence, but then again we did not see large numbers of fungi anywhere on our travels this autumn (the best meeting I can remember was in Radnorshire about ten years ago).

The reasons for low fly numbers must remain unresolved. There are a number of possible factors but none can completely explain our experience, which goes to show how much more there is to learn about the factors that influence invertebrate abundance.

Despite low numbers of flies there were a few high points. One of the most noteworthy records was that of Scathophaga scybillaria – a highly localised species of Scathophagidae with long dusky wings that might be mistaken for a big S. stercoraria. Prior to last year I had never seen this species but we found it fairly abundantly on bogs on the Lleyn Peninsula during the summer field meeting based at Bangor. It was great to see further examples in the New Forest (caught by Alan Stubbs). Malcolm Smart caught a specimen of the Keroplatid Keroplates testacea – a very large fungus gnat that at one time was very noteworthy but in recent years has proven to be far commoner and widespread than previously thought. Alan Stubbs also recorded the Mycetophilid Greenomyia mongolica which is a recent addition to the GB list. One further interesting observation was that a few of the ponies were heavily covered in eggs of the bot fly Gasterophilus equinus - illustrated.

Gasterophilus equinus eggs attached to hairs on the upper foreleg of a New Forest pony. The ponies lick these off and ingest them where the larvae develop in their gut before passing out in the dung.

Observations on the New Forest

We visited a wide range of localities around the New Forest, giving a helpful overview of its composition and management. On the few previous occasions that I have visited The Forest, I have been struck by the level of grazing pressure. Much of The Forest is so heavily grazed that the turf would make a fine bowling green (making allowances for pony dung!). Even so, I don’t think I have ever previously seen Devil’s Bit Scabious 3 cm high and in flower! This management might maintain the plant composition, but from a Dipterist’s perspective it makes much of The Forest very uninteresting. Nice flushes with 2cm high turf is of precious little interest, so huge areas of The Forest have been effectively sterilised from that perspective.

In several places we noted the same grazing pressure within inclosures and wooded areas. Heather and bilberry a few centimetres high do not provide cover for flies and, if they do survive, they are extremely difficult to sample. It was darned hard work securing a reasonable sample in localities that are reputedly rich in wildlife. In the end I got to the point where I ran out of enthusiasm for sweeping over-grazed habitat and ceased sampling.
Pony lawn as Shepherds Gutter. This turf has some very nice wet flushed areas with Devil's-bit scabious but spot the flowers.

Miniature Devil's-bit scabious!

 A few years ago, Dipterists Forum visited The Forest in May and similar concerns about grazing pressure were expressed. The issue generated a certain amount of debate but we found ourselves out on a limb, with other specialist groups arguing that there had been no change in recent decades and that all the speciality species were still present. I find that very hard to believe. Perhaps grazing pressure has remained constant; in which case I suspect that the species of interest that may once have occurred have long-since disappeared, or are confined to very small parts of The Forest. I don’t think that this factor can be attributed to the loss of Eristalis cryptarum but we just don’t know what else once occurred and has since been lost!

It is worth bearing in mind, however, that assessing the condition of the New Forest is highly subjective. Whilst at Denny Wood, Keith Alexander commented that he thought it was in excellent condition as an example of wood pasture. That might be true for Coleoptera associated with ancient trees, but even then there are indications of a problem with grazing.

Veteran beech and oak at Denny wood.

Beech in various states of development at Rufus Stone
At the moment there are a good number of veteran beech trees within The Forest  but I was surprised, even shocked, to note how many had been blown over in the last two to three years. I suspect this period will have resulted in quite serious depletion of veteran beeches. The loss of a significant component of this one tree species in a short time-frame is arguably very serious, especially when one bears in mind that it is very important for a wide range of saproxylic Diptera. I therefore suggest that the debate about grazing pressure is not over. There is a need for action to address the rapid loss of beech, and to think about the wider implications of current grazing pressure.

These beech trees are often tall and straight-stemmed, indicating that they grew in high forest situations. They are much the same age and must have germinated within a matter of years of each other. Surely, if large numbers of beech trees survived around the same time that is indicative that the prevailing conditions suited their survival? Currently, one sees good numbers of young (1-2 year old) saplings but no older ones: they are grazed away pretty rapidly. So, I suggest that there must have been periods of lower grazing pressure that have allowed recruitment of new trees. In which case, grazing pressure has not been constant. Looking at the age structure in many of the wooded areas, one gets the impression that there has been little recruitment for between 30 and 50 years (possibly less severe at Mark Ash and really quite good at Pondhead Inclosure), which means that at some time in the future there will be a significant gap in the availability of veteran beeches and the saproxylic resources they provide.

Doubtless, some of the Inclosures and wetter bogs remain rich for Diptera and other insects, but The Forest is a huge area. If the majority is grazed to within an inch of its life, the ratio of rich wildlife to surface area rapidly diminishes to levels that one sees in a wider landscape context. Instead of a patchwork of nice habitat amongst an agricultural setting, there is a patchwork of interesting locations amongst a sea of short turf. Neither is particularly ideal and the same issues of island biogeography are likely to apply. Isolated populations are more at risk than populations that are connected by good habitat. Unfortunately, the data on distribution and composition of Diptera in The Forest are fragmentary and lack continuity of effort.

It is therefore neigh-on impossible to place the current situation into a historical context. Yes, the honeypot sites remain, and yes the charismatic species are still regularly seen (although data available to the HRS suggests that this is not the case because we rarely get records from The Forest). But, does this really mean that The Forest is actually being managed in a way that maintains its biological diversity? From a dipterological perspective, I suspect that most emphasis is placed on charismatic woodland species, whilst those animals that are associated with open flushed systems are largely overlooked or ignored (with the honourable exception of Steve Falk’s interest in The Forest).

In theory one might effect change by lifting grazing pressure; after all, if the soils are undamaged (in terms of herbicide, pesticide and nutrient application) then there is a far better chance of restoring the broad range of animals that might previously have occurred. It would be very informative to see trial exclosures from some of the heavily grazed flush systems. Will this ever happen? I doubt that this is likely or perhaps even possible given limitations on the ability of managers to exclude grazing animals.

It seems to me that there is an urgent need to draw together all of the historic data on the invertebrate fauna of The Forest in order to establish what is really known about species that are more cryptic or do not attract the attention of those general natural historians that visit. I understand that this has been done for the Coleoptera but apparently not for the Diptera. It is a huge task as there must be many records from the late 19th and early 20th Centuries lodged as museum specimens.

If it is unrewarding to visit anywhere but known honeypot sites, then there is little chance that the grazed lawns will receive any attention and the myth of The Forest will be maintained by reports from the historic ‘hot spots’. For me, that is not attractive – I favour visits to less well known areas in order to improve overall knowledge of our fauna. So, return visits to The Forest are not frightfully appealing. That will doubtless be the case for many other Dipterists, so the problem of data weaknesses will be compounded in years to come.

A reflection on field meetings

So, as another field season draws to a close it is interesting to reflect on the successes. We have held three field meetings (Norfolk in May, Nottingham in July and the Isle of Purbeck and the New Forest in October). The numbers attending the Norfolk meeting were very encouraging, with nearly 30 Dipterists attending for all or part of the weekend. Nottingham attracted far fewer participants than previous summer field meetings but was successful because we managed to visit a range of sites in an area that has not been heavily worked for some of the less popular families. And, this autumn? Well of course it was a success – we had a similar-sized group to previous years (8) and gathered important data for a range of sites. Perhaps what is even more important is our having maintained the tradition of autumn field meetings and extended the run to more than 40 consecutive years.

Dipterists Forum members at Studland NNR. L-R: Chris Spilling, Peter Chandler, Malcolm Smart, Andrew Halstead, Alan Stubbs and Tony Davis

Dipterists Forum members at Pondhead Inclosure. L-R: Slan Stubbs (photographing an uncooperative spider), Tony Davis, Malcolm Smart, Andrew Halstead, Keith Alexander and Peter Chandler.

 This was the last year that I will run a summer field meeting. I have been running them since 2005 and have run out of steam. We have nobody that is willing to take the job on, although there will be a meeting in 2016 thanks to a combined effort by Howard Bentley and Amanda Morgan. I shall probably continue to organise spring and autumn meetings but I am finding it increasingly difficult to maintain impetus. As yet I have not fixed on venues for these meetings, although it is likely that one will be in Scotland in late August/early September. The spring meeting may well be in Somerset as I think I can find suitable accommodation there. Unfortunately, as accommodation costs spiral, it is increasingly difficult to find affordable venues and as one of those with very limited income I am acutely aware of the cost implications for members who wish to participate!

1 comment:

  1. Hi Roger,
    Apologies for not managing to join you this week. By the time the week arrived, other things had loomed.

    I agree with your comments about the Forest. When we moved here two years ago I got my collecting permissions and then descended on the Forest in high anticipation. After a couple of almost fruitless visits I largely stopped going there for Diptera.

    I don't see much chance of reducing grazing pressure when there are so many Commoners wanting to increase grazing! Even if stock numbers could be reduced the deer would probably take up any slack. The ecological powers-that-be staunchly defend the current level to the point of praise - I agree they're taking a narrow botanical view.

    Malcolm Storey