Thursday, 10 March 2016

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).

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