Do we understand pollinator abundance and population trends?

Dave Goulson's recent editorial in British Wildlife raises an important point about the issue of pollinator abundance. His analysis does, however, overlook the fact that there are active data collection processes for the most obvious pollinators. Existing datasets compiled by the Hoverfly Recording Scheme and BWARS are regularly used by CEH and various university groups to produce new analyses and in the development of new analytical techniques. So, everybody who posts on the UK Hoverflies Facebook page is contributing to the research. Nevertheless, he is certainly right in saying that there is a lack of data for many families of flies and for some other insect Orders.

In broad terms, we have a very good understanding of what is happening to our hoverfly fauna - somewhere in the order of 40 to 50% is declining and perhaps 15-20% is increasing. We see an expansion in the range and abundance of southern thermophilic species such as Volucella zonaria and V. inanis and Rhingia rostrata. Declines are highly apparent among those species that favour damper conditions. I think we are starting to see the loss of some species from south-east England. For example, the spectacular blue-marked Leucozona glaucia seems to have substantially disappeared from much of south-east England in the past 20 years.

Techniques used in analysis of trends have evolved, and new occupancy models are emerging on a relatively regular basis. They are all dependent upon a continuing stream of data, which makes it imperative that BWARS and the HRS remain active and train new recorders. Perhaps even more critically, we need to develop succession plans to make sure that the process of data assembly and dissemination continues; it is a very labour-intensive  and is starting to become more of a challenge as the numbers of active recorders grow. Nevertheless, this challenge is, in effect, a good news story because it shows that there is positive progress in data assembly.

BUT, are hoverflies really important pollinators?

What are we actually talking about when it comes to pollinators? Politically, the main focus will be on pollinators of commercial crops, many of which flower early in the spring. Thus, a decline in species that fly after critical crop pollination time may be of little commercial interest and therefore equally of little political concern. I am not sure we have really made that distinction yet, but if one was to do so it might help to focus attention on other parts of the insect assemblage such as Anthomyids, Muscids and Calliphorids; or perhaps even Bibionids that occur in vast numbers for very short periods of time?

We do have other proxies for insect abundance. For example, the numbers of insectiverous birds. These too are substantially declining. Why? Well some of the reason lies in general agricultural intensification and loss of wild places in the countryside matrix. Partly it may be associated with the development of vast monocultures and the apparent focus on the same crops in the same fields year after year. A reliance upon pesticides is a likely further factor that diminishes the food supply at critical times of year.

But there are other factors too. For example, there is a developing trend for insects to emerge earlier in the year during a brief warm spell in late March and early April, before a cold snap in late April and May knocks them down. This sort of seasonal change seems to me to be having a profound impact on insect populations. Equally, we quite frequently get short bursts of intense heat in late June and early July, which probably knock out larval stages. I suspect drought and heat stress are a very important factor behind changing insect abundance in south-east England. Warm, damp summers are always looked upon by the public as undesirable, but regular rainfall is probably the single most important factor behind the maintenance of insect populations in Britain. If anything, we are seeing greater stability in insect numbers in northern and western areas - this certainly seems to be the case for hoverflies.

Do we need better monitoring?

Of course the simple answer is yes! But we also need to be clear about the objectives of monitoring and the degree to which we try to link monitoring results to politically sensitive issues rather than to a broad spectrum of ecological factors. Should monitoring focus on species that are potentially important commercially, or should we be looking across a wider spectrum? If so, will the same monitoring systems deliver the results needed for each purpose? Or, can we use proxies - a general pollinator monitoring scheme that assumes that the trends are the same in different families of bees and Diptera?

Looking at the issue of monitoring from a purely practical perspective, I think the current use of datasets compiled by the HRS and BWARS is probably the most viable option in the long-term. Government-funded packages might generate data collected in a more consistent fashion, but they are always going to be vulnerable to cuts. We have already seen this with the long-term data for Atlantic plankton and in the Rothampstead moth trap programme; both of which generate immensely important and instructive messages; unfortunately the messages are not all positive and they don't generate the steady stream of high impact papers that keep research funding flowing. So, voluntary data collection remains the only reliable source of information on trends. That makes it imperative that the core functions of the Biological Records Centre at Walligford are safeguarded.

So, what can you do to help?

There are several aspects to monitoring:

• Range and distribution: what occurs and where does it occur?
• Abundance: in what numbers do individual species occur?
• Variation: how do numbers of species and absolute numbers change regionally and nationally over individual years and in longer-term units?

At the moment, we have a relatively small number of people who regularly record from their garden or wildlife area. There are several very committed members of the UK Hoverflies Facebook group who record on a daily basis. More of this sort of recording is likely to be helpful because it may be possible to take sub-sections of data for analysis if we have more such datasets. Occasional ad-hoc records are also useful because they help to fill in gaps in distribution data. Occupancy models depend upon good general coverage, so even a few common species recorded from a location can make a difference; this is particularly important in places where there are limited numbers of active recorders or that people don't regularly visit.