Tuesday, 14 November 2017

Interpreting data: Portevinia maculata

One of the big benefits of a developing network of new and enthusiastic recorders is that it is possible to cover new ground and look for species that are relatively easy to identify and find.

Portevinia maculata is one such species. Its larvae live in the bulbs of Allium ursinum (Ramsons or Wild Garlic) where they exist from late May through to early April. Adults emerge after a brief spell as a puparium and males (see photo) can be very abundant sitting on the Ramsons leaves for a short spell between late April and early June. We can therefore make use of this species biology to test the reliability of records and can also see how recording efforts have changes as people have become more interested in hoverflies.

Portevinia maculata - male (photo by John Bridges)
 The statistics tell us some interesting stories:

Figure 1. Trend in frequency of records

Figure 1 clearly shows how levels of recording remains substantially constant until around 2011 or 2012, after which they have changed dramatically. This cannot be a result of the plant spreading, as it is largely associated with older woodlands and does not move quickly. Nor is it likely that the insect has moved dramatically. It was always relatively easy to find and when looked for in new locations was usually located if sufficient plants were present and repeat visits made to coincide precisely with what is a very short emergence period. The map, Figure 2, shows the current situation.

Figure 2. Current distribution of Portevinia maculata (to 2017)
And then comes the phenology histogram (Figure 3). Most of the records sit tightly between early April and the middle of June. Experience with the UK hoverflies Facebook group suggests that the earliest dates are indeed around the end of the first week in April and that by the second week in June it has disappeared, even from Scotland. So, we must start to question outlying records in March and from mid-June onwards. We can probably discount the March records because the Ramsons won't have properly emerged and the insect is likely still to be a larva or a very early puparium. Beyond the middle of June it is highly unlikely that adults will be found (possibly the odd female into early July) and the larvae are first or second instar, so absolutely tiny and unlikely to have been found. So records within these timeframes can, with substantial confidence, be logged as erroneous.

Figure 3. Phenology histogram for Portevinia maculata
This is a nice example of the sort of problems one has to consider with all datasets, but is helped because the animal and its associated plant have very well-defined life cycles that make it relatively easy to interpret the data. Where we have less biological information it is much harder to make definitive statements and to question records.

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