East-west splits in emergence

Yesterday I looked at differing emergence times at various latitudes. The alternative is to look at differences in eastern and western Britain.

Unfortunately I do not think there are quite enough data for northern England and Scotland, but there are plenty of data for 2015 for England and Wales south of the Humber-Mersey line. So I have split this area into four - firstly split at the 200 km level (approximately Thames-Severn) and then again at the 200 km mark - in essence using the junction between  ST and SU northwards to the junction between SJ and SK.

The results for 2014 and 2015 are presented below. They are not entirely convincing but I think it is possible to detect slightly earlier emergence in SW England. More analysis is needed!

Figure 1. Phenology of Eristalis pertinax in the east and west Midlands (west includes Wales) in 2015.

Figure 2. Phenology of Eristalis pertinax in south-east and south-wet England in 2015.

Smoothing data

I quite frequently use a sequence of processes to smooth data. In this short sequence I hope to show how the data change as a consequence of the process.

In many cases day-on-day records for individual species are too few or too volatile to be particularly meaningful. This is especially true in early spring and to a lesser extent during weekdays in more productive times of year. The solution is therefore to break data into blocks. In this example, I have used Eristalis pertinax, a very abundant spring species that starts to dominate the data from around the end of March. I have extracted photographic data for the past three years (2013, 2014 and 2015) to provide a bit more context. In each of the three years, the data have gained strength because there are more active recorders and therefore more records from all parts of the country (especially northern England and Scotland.

The data I have used have been split into three zones - based on 200km sections of the OS grid that roughly equate to:

• South of a line between the Thames and the Severn
• The Midlands between the Thames-Severn line and a further line between the Humber and the Mersey.
• North of the Humber-Mersey line.
• Occasionally, I add a further division north of the Solway, but on this occasion I have not.

The first stage is to generate data for each successive week (Figure 1). As can be seen, the data vary hugely from week to week and inevitably the numbers of records differ for each zone and for each year. Over the three successive years it is very clear that the volumes of data have increased dramatically, which makes it a bigger challenge to put the successive years into context.

Figure 1. Weekly records for the years 2013 to 2015 for Eristalis pertinax according to three zones of latitude.

From this, I construct a further table in which individual weeks' data are converted into a percentage of the total records for the zone in that year. Again, the results tend to be a bit volatile, but they now equate to one another because the effects of the massively greater numbers of records from the south are put into context (Figure 2).

Figure 2. Proportion of records of Eristalis pertinax  for each week according to year and zone.

Finally, I run a three-week centred running mean for the data created as percentage of total records. This finally smooths the results because the combination of each three week period is averaged. This removes the idiosyncrasies of big gains and falls in the data and picks out the overall trend - either rising numbers of records or falling numbers of records (Figure 3).

Figure 3. Three-week centred running mean for records of Eristalis pertinax for each zone in the years 2013 to 2015.

The resulting graphs clearly show how the overall phenology pattern differs according to latitude, and how emergence times also vary from year to year. In this example, the indications are that E. pertinax emerged a little earlier in 2014 than in either 2013 or 2015, with the possible exception of the North in 2013. Unfortunately, the dataset for the North in 2013 was rather sparse and therefore I would treat the result with a little caution. Nevertheless, the data do help to show year-on-year regional and latitudinal variation.

The results also show how emergence times are far more protracted in the Midlands and in Southern England, whilst in the northern zone emergence times are far more compressed (hence the higher peaks in 2014 and 2015).

Application

This sort of analysis is only possible for species where there are substantial blocks of data. Nevertheless, it is clear that photographic data can be used for some forms of monitoring and quite remarkable levels of precision in representing species' phenology.

If the numbers of recorders in more northerly regions improves, then it may be possible to break data into four zones in future, thus providing the opportunity to investigate a much wider range of parameters. There may also be scope to split the country further into east and west sub-units of each zone - something I may try in due course.

The benefits of total recording

Many members of the UK Hoverflies Facebook page record everything they see on a daily basis; others are confined to weekends, but are really very active. The combined efforts of this increasing band of recorders are showing what can be done with less well-known groups of invertebrates.

What I particularly like is the fact that there is no discrimination between records of common or rare species. All records get logged, and as a result we are starting to build a more realistic picture of the relative abundance of hoverflies across the country and at particular times of year. This approach is essential if we are to truly understand what is happening to wildlife.

Which dates matter?

It is all very well recording first and last dates, but what happens in the intervening days/weeks/months is really crucial. First/last dates are rather serendipitous whereas the median date for all records is far more reliable and meaningful. That means that we need all data and an army of diligent recorders. I suspect that data is pretty robust within moth recording where lots of people make daily counts. The same probably holds for butterflies and maybe dragonflies. But, can we say the same for other invertebrate groups? Probably not.

So, it would be nice to think that the advent of the UK Hoverflies Facebook group heralds a major forward step for hoverflies. Feedback I have received from some specialists is not always positive – the question 'why bother with common species' has been posed, and it has been argued that 'this does nothing to advance our knowledge of Britain's fauna'. It remains to be seen whether they are right, but I would like to think there is developing evidence to the contrary.

What can be done with the data?

Two small analyses that I undertook today illustrate the benefits of mobilising an army of recorders with varying levels of experience. To my mind, enthusiasm matters as much as experience. People with enthusiasm will go out when conditions are less than perfect; conversely, those of us who have yaers of experience will look out of the window, note that there are clouds and it is slightly windy, and settle back to the computer. microscope or whatever we were doing.

The data largely show this. Hitherto, hoverflies have been the preserve of a relatively small body of recorders. For forty years, about 20 people supplied 50% of the data assembled by the Hoverfly Recording Sscheme (HRS), very few of whom would have made much effort until the spring had fully arrived. The dynamics have changed, and although we still have a nucleus of about 25-30 highly productive specialist recorders (above 500 records a year), we have many more whose combined efforts are really helping to develop a valuable picture (several of whom now contribute many hundreds, or in one case thousands, of records). The new recruits start from a very different perspective and want to get out.To illustrate this, I offer two brief analyses relating to patterns of spring emergence.

Composition of the early spring fauna

If one starts with the question 'how does spring 2016 compare with previous years?' We can break the data into the numbers of records of individual species, but this does not take account of variations in the numbers of recorders. So, presenting data as a proportion of all records received for a given timescale is one option.

Figure 1. Records of hoverfly species in March 2014 to 2016 presented as the proportion of all photographic records covering the period 01-25 March.

Figure 1 illustrates this quite nicely: it shows how, in the period 1-25 March, relative composition of records changes from 2014 to 2016. This figure is based solely on photographic records, which is really useful because relatively few entomologists are stirring from their winter hibernation. The photographic team, however, is raring to go and every time there is a bit of sunshine they are out. I seriously doubt one could manage to secure this sort of picture from data prior to 2013, although it would be possible later in the year when insect activity increases and traditional entomologists stir from their torpor!

What is very clear is that the composition of the fauna in different years can vary considerably amongst the more abundant species. As might be expected about a quarter of the total species list makes up the bulk of the records, but within this there are two obvious trends. In 2014 and 2016, Eristalis pertinax is far more prevalent in the data than in 2015. There is an obvious reason when you look at the composition of 2015 data: In 2015, Episyrphus balteatus, Eristalis tenax and Meliscaeva auricollis make up far more of the dataset. These three species are typical of winter months and E. balteatus and E. tenax predominantly over-winter. Thus it seems that 2015 was a later year than either 2014 or 2016 where Eristalis pertinax was much more abundant. E. pertinax, in contrast does not generally over-winter and thus the dominance of this species in the dataset tends to suggest that spring has arrived a little earlier.

And at species level?

We have known for a long while that species' phenology changes from year to year, and that in some emergence times have advanced considerably. These changes are nicely illustrated by Epistrophe eligans, which is highly responsive to spring temperatures; its spring emergence is now several weeks earlier than it was 40 years ago. We can now pick this up because it is easily photographed and recorded. In the past, many recorders might have logged the first record but all too often I have been told 'I'll give first and last dates but I cannot be bothered with the rest'. Now we have the whole lot! And it beautifully illustrates differences in both season and latitude.

Figures 2 and 3 present the data for Epistrophe eligans in 2014 and 2015 for three zones: south (to the 200km division across the UK, midlands (200-400km) and north – above 400km i.e. pretty well above the line between the Humber and the Mersey. What a difference it shows. Emergence times are separated both between the years and across the latitudes. To do this, I have separated the data into weekly blocks from 1st March onwards. The data were then turned into a weekly percentage of total records, after which they were turned into a centred three week running mean. This smoothing process helps to refine the picture and it is pretty powerful.

Figure 2. Phenology of Epistrophe eligans in 2014 based on photographic records.

Figure 3. Phenology of Epistrophe eligans in 2015 based on photographic records.

These graphs suggest that spring in 2015 came a little earlier than in 2014. It remains to be seen how 2016 fares, but the records of E. eligans to date point to a situation more akin to 2015 than to 2014, albeit potentially marred by bad weather forecast for the coming week.

And the moral of the story?

Commoner species matter! One gets too few records of rarer species to develop strong pictures of change; but at least in some commoner species one can detect change very nicely. That does not mean there is no place for recording rarer species or taxonomically difficult species – it is essential, but it may only ever be the preserve of a small sub-section of recorders.

And, for that splendid band of photographic recorders? I hope these brief examples show just how important your contributions are. This level of activity starts to elevate the hoverflies from the margins to the mainstream of insect recording.

In defence of non-native planted trees

In my younger days I spent many weeks/months/years clearing Sycamore from local sites. At the time the mantra was that Sycamore was an undesirable and invasive alien. Many of the allegations against sycamore have some foundation; it is undoubtedly capable of taking over sites where it can readily gain a foothold. Such sites are often secondary woodland and decaying urban open space. My local site, Mitcham Common, had a significant issue with Sycamore: many of the youngest woodlands were being over-run by young trees. So, we cleared the mature trees (I spent a lot of time gaining valuable chainsaw skills on these trees) and had our teams pulling the seedlings. Today, sycamore is nothing like the problem it once was. But, were we right to take such drastic action?

Some years ago, Ted Green published a think piece in British Wildlife in which he described Sycamore as 'Northern Plane'. At the time, I thought 'I wonder if Ted has struck a more sensible note?' Certainly, if one travels north, Sycamore is a very important component of our landscape, and does not appear to be the problem that I had encountered further south. Indeed, I think we could almost say it is a fundamental part of the ecosystem on northern England and Scotland. For example, I recall one evening in Perthshire, around 9 pm, when we came across a huge Sycamore in flower that was literally buzzing. Closer inspection revealed that this tree was the focus of huge numbers of bumblebees; hence the buzzing sound. Obviously it was important in a local context and I wonder whether it plays a bigger role than we think as a nectar source of bumblebees (across the British Isles)?

After many years investigating Sycamore I have reluctantly concluded that it actually plays quite an important role in the woodland ecosystem too. True, it is fast-growing, shades out other trees and perhaps vernal flush species, and does not fit neatly into the perceived wisdom about woodland ecology. Yet, its wood and bark has many of the important characteristics demanded by epiphytic lichens, and the rot processes lead to excellent rot holes for Diptera and to very nice sap runs that support many of the species that native Elm once did. In parkland situations, old Sycamores can be immensely important habitat for saproxylic Diptera.

Ancient Sycamore at Burghley Park. The rot hole is of exceptional size and is indicative of the tree's value for Saproxylic invertebrates.

Not only is Sycamore good for saproxylics, it is also a fantastic tree to monitor when looking for hoverflies. If I visit a woodland and the ride lacks flowers I am more than happy to monitor the sunlit leaves of Sycamore, especially in the early spring. These leaves are ideal for leaf baskers and are a standard place for finding Brachyopa adults. The flowers are also great for hoverflies and for solitary bees. Meanwhile, the leaves are often infested with aphids that are favoured by a wide range of hoverfly larvae. Unfortunately, the numbers of Lepidoptera utilising Sycamore are low, and the biomass they provide for birds is correspondingly low. Nevertheless, in a controlled situation Sycamore is not the threat it is perceived to be.

Thinking more broadly, Sycamore coppice has many strengths: it is fast-growing and produces good biomass; the timber is relatively light and is readily transported; and the root system develops like other coppice stools and generates excellent decaying wood. There is therefore something of a case for thinking about Sycamore as a possible way of facilitating rapid carbon capture and use in sustainable fuel provision.

Let us not get too carried away by its strengths. Sycamore can be very invasive, its leaves support a very limited invertebrate fauna, and it does produce an awful lot of shade and leaf litter.

Thinking in a broader context, I have the good fortune to live in Stamford and to have the wonderful Burghley Park on my doorstep. I visit the park almost daily and spend a lot of time gazing at the wonderful hybrid Limes. These magnificent giants are, again, hardly native; but they have fantastic saproxylic features. The older examples are substantially hollow and support big Ganoderma fruiting bodies. Some also have fantastic sap runs, so beloved by a wide range of Diptera and Coleoptera; and, again, they can be great for aphids and species that are predacious upon aphids. In some ways they rank higher than Sycamore in the pantheon of most favoured trees by saproxylic ecologists (I recall the immense outcry amongst entomologists when it was proposed to fell the ancient Limes of Queen Anne's Drive at Windsor Great Park).

Keith Alexander recording the saproxylic features of an ancient Lime at Burghley Park in March 2016

Close-up details of sap run and young fruiting bodies of Ganoderma on the same ancient Lime

We then  move to Horse Chestnut, a southern European species that is so favoured as a parkland species. Dense foliage and tough leaves give the impression that this is not a species to be loved by the ecologist. BUT, like Sycamore, it develops fantastic rot holes and sap runs. Its decay processes lead to excellent subterranean decaying timber, and it grows quickly; thereby creating new habitat for some species where other longer-lived trees may fail to deliver in time.

And, finally, what about Sweet Chestnut? Now this is one that I have never been fond of. But, I am a Dipterist. Were I to be a Coleopterist, I might think differently, as Sweet Chestnut supports many of the important heart rot fungi of Oak, and as such it also harbours many of the noteworthy beetles that like dry red heart rot.

Thus, I hear the calls - why are you an apologist for the 'Foreign Invader'? Well, I am not! But, in today's World I think we have to start to think in broader terms. We have already lost the Elms that supported many important Diptera. Thank goodness for Sycamore and Horse Chestnut that have maintained the necessary habitat. Various Oak diseases threaten our iconic ancient trees and thus the beetles and other invertebrates that utilise decaying timber. Thank goodness for Sweet Chestnut!

And then there is the stately hybrid lime! A magnificent feature tree, fast growing and capable of supporting a wide range of saproxylic invertebrates.

Now, I am not advocating replanting the landscape with non-natives; but I think in the current climate where new pathogens threaten out native trees such as Ash, we have got to look seriously at the possible alternatives that will allow our native fauna to survive!

Some reflections on collecting techniques

From time to time the question arises as to the merits of particular trapping techniques. Different people hold differing views. Everything depends upon what you want to achieve and how deep ones pockets are both in terms of time and capacity to get material identified. I sometimes come across projects in which a Malaise trap has been employed without real thought being given the the scale of the undertaking, so perhaps a few notes may be helpful for those readers who are thinking about invertebrate surveys.

If one wants to develop a full list of species from a site,a range of trapping techniques may be necessary. It may also help to employ a number of different specialists to undertake active sampling: each insect Order requires detailed knowledge of how to find them that comes from specialisation. The techniques used by Dipterists differ from Coleopterists for example - Dipterists generally swish gently through vegetation using a light net, whereas Coleopterists use much heavier equipment and tend to thrash the vegetation.  Engaging large numbers of specialists can be hugely expensive and is probably not justifiable in any but the most extreme circumstances.

Targeted surveys are far more likely to be required. Knowing something about the target organisms is essential before one employs a particular technique or combination of techniques. Understanding something about the ecology of a site is also really important: there is perhaps less point in employing a Dipterist in oak-dominated wood pasture, whereas specialist dipterological survey is essential on base-rich flushes. In other words, think about outcomes rather than simply collect data and then wonder what you can do with it!

If quantitative data are required, then sampling that must be repeatable by others. In the 1980s the then Nature Conservancy Council had three survey teams (Welsh Peatlands, East Anglian Fens and Dungeness) who all used a combination of pitfall and water traps. I ran the Dungeness survey, which was highly illuminating! If nothing else, it highlighted the huge commitment of time that was needed to set and service traps; and the even larger time commitment needed to sort the sample and get it identified.

Pitfall/Water trap combinations

The effort required to set out and harvest the samples involved two to three days (by two people) on each occasion for about 40 sample locations.

There was a huge volume of specimens, and although we sorted to family for many Orders, it proved impossible to get all of the samples identified. At the time, we used a combination of paid and voluntary effort. I doubt many would volunteer to do such identification today!

Water traps were very effective in sampling aculeate Hymenoptera and I am convinced they are really useful for sampling spider-hunting wasps. I am less convinced of their efficacy for hoverflies: one generally gets a sample of a few common species, but not much else. I also identified the hoverflies from the water traps run in East Anglia; these were rather more interesting and did yield incredible numbers of Neoascia on some occasions. They were also quite useful for Sciomyzidae (as were some pitfall traps).

Pitfall traps sometimes generated huge catches of a single species - many hundreds of isopods, anything up to several hundreds of the ground beetle Calathus fuscipes and large numbers of ants if placed close to a colony. The overall species list for a set of traps was often substantial but it had very few dominant species and a long tail of occasional records that made multivariate analysis unconvincing. Cruder analysis showed the relative frequency of some interesting animals across the sites, but making a match to the NVC communities proved to be almost impossible. The best splits occurred where there was a big difference in environmental parameters; whereas different lichen heath communities showed very little difference in the assemblage.

Perhaps with more analytical time we might have made something of the data but this was not available to us. It illustrates the need to build in a lot of time for analysis!

What about the Malaise trap?

Correctly sited, Malaise traps can generate phenomenal numbers of specimens across a wide range of taxa. But, the critical issue is getting the orientation right. It is important to remember that Malaise traps will intercept tourist species, so one must treat species lists based on Malaise traps with some caution. Not everything will be resident on the site.

Obviously, a single Malaise trap is not sufficient to generate data that can be used for multivariate analysis. Many traps are needed, and this means huge effort and vast volumes of by-catch. This would generally rule out Malaise traps as a practical way of sampling for detailed ecological studies. Nevertheless, a single Malaise trap run over a number of years may help to generate valuable data for trend analysis. This was very much the case for Jenny Owen's study of her Leicestershire garden.

The real challenge is what to do with trapped material? There are relatively few specialists who will take huge volumes of specimens for free. So, anyone thinking of running a Malaise trap needs to think very carefully about how the sample will be identified. If an animal has died then the least one owes it is to get a name and to create a useful record!

In general I am not a great fan of Malaise traps unless they are used for a specific purpose and there is the capacity to deal with the catch. Using them to live-catch may be more helpful where it is possible to regularly monitor the collecting head and to release live unwanted specimens.

So what are the other alternatives?

The main point to consider is that one needs to think out carefully precisely what is wanted:

•  an inventory of species for a site?
•  sufficient data to undertake multivariate analysis?
•  long-term monitoring?
• autecology of a single species or suite of species associated with a highly defined habitat?

Sweep net sampling is often used (e.g. Natural England site condition monitoring), with the quantitative sample generated by using a given number of sweeps. For one-off studies, this may be enough to generate useful and repeatable data. Unfortunately,  no two net operators will generate the same sample and so year-on-year data will not be comparable if different entomologists are used to sample the site.

Active searching may also be helpful. Coleopterists often target a range of habitats such as under objects or within decaying wood. Some such systems are rather destructive and are perhaps not to be recommended in sensitive locations. In the case of hoverflies we might seek out larvae such as those of leaf and stem-miners, or perhaps those found in rot holes.

For hoverflies, I generally favour active searching and sweeping. My form of active searching is to make detailed observations of suitable sunning and feeding stations as well as investigating possible breeding sites. I might use a Malaise trap under specific circumstances but would generally avoid water traps (they are my favoured system for sampling Pompilidae though).

The crucial point about all of the options is to design the project in consultation with an experienced entomologist who has some understanding of the logistics. Invertebrate recording is complex, time-consuming and expensive, and relies upon a very small nucleus of experienced entomologists. A few hours of such a person's time at the start of a project can save an awful lot of money (and save the lives of many insects).

In homage to the recording 'patch'

This last weekend I was asked which type of recording I favoured? A patch or wide-ranging recording? Bearing in mind a recording scheme is often judged by geographic coverage, it may come as a surprise to some that I thoroughly endorsed the 'patch' approach. Why might this be? Well, the big advantage of the 'patch' and regular recording, is that one starts to build up a year-on-year picture of a the local environment, with sufficient data to start to pick up trends. That is what we have lacked and what is needed to help to build the contextual picture.

Some context

Before the days of cheap transport, field naturalists tended to have a 'patch' and took an interest in many aspects of the natural history of their chosen site. Unfortunately, data collection systems were confined to card indexes and notebooks; many of which have probably been lost. The lack of systems to capture data mean that on the whole we have only a sparse record of what occurred prior to the 1960s. For invertebrates, museum collections are a great source of information, but inevitably they tend to hold examples of rarer species: there is not the space to retain all specimens of commoner species, recorders also limited the extent of their collections. Doubtless there were those who placed a lot of emphasis on seeing rarer species, but the 'patch'  was an important part of their biological recording activity.

Cheap transport has allowed many naturalists to travel much further afield in pursuit of their chosen interest. In Entomology it is commonplace for field meetings to be organised at the richest sites or places where rarities might be found. I was no different to others in the early 1980s when one would turn up at a given locality for X or Y and find that others had arrived with the same intentions. Our collections were very similar and this took mothing into the realms of stamp collecting; hardly an ideal foundation for biological recording . I suppose that in a way we were the forerunners of today's moth twitchers who visit other moth traps to see the poor little soul that has flown off course, ended up in a moth trap and then in a pot in a fridge. What does this contribute to our knowledge of species' biology and conservation status? Precious little!

Thank goodness there was the Rothampstead Insect Survey, which established a network of locations where all moths were recorded. Although greatly trimmed in its coverage, this dataset is perhaps THE most important for invertebrate conservation. Its findings convey a sad story of decline in Britain's moth fauna. Efforts by the National Moth Recording Scheme (NMRS) to assemble the data from individual moth traps compliment the Rothampstead survey, but will always lack the benefits of the consistent approach that the Rothampstead survey has established. Some NMRS  records will be from mobile traps that are run in more interesting places, but many will be people's gardens - the equivalent of the 'patch'.

Data for butterfly transects are also pretty robust and it is possible to say quite a lot about what has happened to them over the past 40 years. Lots of organisations and individuals run butterfly transects and these provide a hugely important resource that helps to chart the fortunes of Britain's butterflies.

Long-term monitoring of other taxa fares less well. There are comparatively fewer enthusiasts, and although there are recording schemes, the volumes of data for more challenging taxa are relatively sparse. This is arguably a serious gap in our knowledge because we cannot automatically extrapolate from the fortunes of moths and butterflies. The Lepidoptera occupy a relatively narrow niche: the vast majority are phytophages, whereas other Orders occupy much wider ranges of niches. Guilds of saprophages, mycophages, filter-feeders, parasites and predators are represented across the insect World beyond the Lepidoptera, and although we do have data they are nowhere near as  robust. Nevertheless, it is still possible to detect trends; many of which mirror those for the Lepidoptera - perhaps 40-50% declining, 30-40% stable and 10-20% increasing.

For hoverflies we now have a run of about 35 years data in which more than 20,000 records have been submitted each year. This dataset is hugely important because hoverflies occupy a very broad range of niches (apart from parasitism) and can be used to convey important stories about the state of the wider countryside. The dataset is one of the bigger invertebrate datasets and it is possible to extract trends from the data for some but not all species (most notably lacking are the rarest species). The results are not pretty, paralleling those for many other taxa with about 40% of species declining significantly. Unfortunately, we don't have the long runs of data for individual sites like those of the butterfly or moth monitoring schemes.

How do we make something of the data?

There are various analytical techniques that can be used to examine the degree to which data are fully representative of a given area. Several work in similar ways, comparing the presence of given species against a basket of commoner species to determine the intensity of recording, and thus the likelihood that a species' absence is a function of recorder effort rather than being an actual absence. The system develops a 'matrix of confusion' which is somewhat Rumsfeldian in its outputs:

• Locations where the species is known to occur and the model predicts that it will occur;
• Locations where the species has not been recorded and the model predicts that it will not be found; 
• Locations where the species is known to occur but it is not predicted by the model; and
• Locations where the species has not been found but the model predicts its occurrence.

 

The 'matrix of confusion is used to test the reliability of predictive models and is highly dependent upon the availability of records for both commoner and rarer species. A measure of recorder effort can be determined from the proportion of commoner species that might be expected to occur that have actually  been recorded in a given square and in surrounding squares.

Where species X is present in a given square and say only 40% of the commoner species that are likely to be constants in full species lists are present, this represents a very positive result. If it is absent, there remains the possibility that it will occur, assuming the location has the appropriate environmental parameters. Conversely if species X is absent from a square where all 50 of the commonest species have been recorded, it may be inferred that the level of recording has probably been sufficient to generate a reasonably comprehensive picture of the fauna of that square.

If only rarer species are recorded, the analytical outcome is likely to be a little distorted. Conversely, if species X is challenging to identify, it is highly likely to be under-represented in data that is dominated by records of easily identified species. The more comprehensive the dataset, the better the outcome.

Making something of trends on ad-hoc data is always going to be more difficult than an analysis of data collected using a constant system of survey such as butterfly transects, water traps, pitfall traps or a Malaise trap. But, regular repeat recording from a given 'patch' can result in remarkably valuable data. For example, Alan Stubbs has undertaken regular monitoring of his Peterborough garden. In the 25 years or so that he has done this, it has shown that overall species richness has dropped quite substantially. Stuart Ball uses Alan's data and the data generated by Jenny Owen's Malaise trap in Leicester as the only really detailed data that help to provide a picture of environmental change for hoverflies. Jenny stopped running her trap some while ago (and died recently), so that data run has come to an end. Alan is still monitoring his garden, but of course it is just one dataset. What we need is more people who are willing to regularly record from their garden or a favoured patch.

Signs of progress

In the last year, several members of the UK Hoverflies Facebook page have been very active, regularly recording from their 'patch'. This is a welcome boost to the data and if it continues it will start to form the foundations for a pretty decent long-term monitoring project. It parallels the general concept of the garden hoverfly monitoring scheme that we have been trying to develop. Obviously, some people will drop out in time, but others may be recruited. The development of a big dataset from several constant effort sites should provide a really good foundation for the future.

So, for those who like to have a 'patch' do please make the effort to regularly record the hovers. Those who favour more wide-ranging activities - yes please, and don't forget the full species list. The critical issue is the full list of what is seen and can be reliably identified. Hopefully we can develop a system like Birdtrack, which is a really neat way of using 'ad hoc' data to demonstrate aspects of species' biology and to investigate trends at a national or regional level. And, if you do want to twitch special places, do please make sure the records reach the scheme - it is amazing how few records we see for New Forest specialities such as Caliprobola speciosa!