Saturday, 20 August 2016

Making sense of data for July & August 2016

Further to my last post, I had a look at the same dataset (i.e. 01 July to 13 August) to see how the data for the same period in each of the previous three years compares with 2016.

A total of 126 'species' were recorded over this period between 2013 and 2016:

Number of species
Number of records

Making something of this is not straightforward as the yearly figures are so different. The simplest way of expressing the data is as a proportion of the total records. For the purposes of this quick analasys it will have to suffice although there more complicated statistics might be applied. More detailed investigation will have to wait for the time when we have a much bigger dataset covering many more years.

Top 50 species in 2016

A relatively small proportion of species make up the bulk of the records so when they are organised in rank order, largest to smallest, for 2016, one is well into the tail of occasional records by the time one gets to the 50th most abundant species (Figure 1).

Figure 1. The 50 most frequently recorded hoverflies fro, 01 July to 13 August organised by rank order for 2016 as a % of the total records for the recording period that year. Orange highlights the highest year proportion and blue the lowest.
Taking the top 50 in 2016 as a cut-off point, I then looked at which year was the best for each species, and which year the least productive and then converted each into a tabulation of larval biology to generate Figures 3 & 4. (Note: I lumped Myathropa florea into the wetland guild because they often breed in locations that might strictly be considered to be wet features rather than dead wood habitat).

Figure 2. Best representation in each year according to larval biology

Figure 3. Poorest representation in each year according to larval biology


What do the data tell us?

At first glance, one might say 'not much', but there are subtle and possibly important differences:

  • Wetland and aphidophagous species are the two guilds that dominate the most frequently recorded species list. It seems that in years when wetland species do well, aphidophages fare less well, proportionately. Of course, it may simply be that when wetland species numbers rise, the ranking of other species is diluted and they apparently fare less well. A more comprehensive statistical analysis is probably needed to investigate this, together, perhaps, with a much bigger dataset.
  • 2014 was a particularly good year for species associated with Hymenoptera; either as scavengers and parsaitoids in social wasp and bee nests, or associated with ant attended root aphids (if larval ecology conforms to current thinking).
  • The data used in this analysis cover the later stages of the hoverfly season, and certain guilds are probably under-represented because they emerge earlier in the year. This is particularly the case for saproxylic (dead wood) species. More detailed analysis will be possible when the full 2016 dataset has been assembled.
  • There has been a greater than threefold increase in the volumes of data assembled using photographic recording, and as such the dataset is starting to be sufficiently big to be a powerful tool in its own right. Good coverage of species is achieved, but it seems likely that there is under-representation of some guilds, especially plant feeders within the genus Cheilosia.
Some bigger questions emerge from this rather crude analysis:
  1.  To what extent are differences in the proportions of hoverflies recorded in individual year  a reflection of environmental factors within in that year?
  2. Conversely, to what extent does the weather (or other environmental factors) in preceding years impact upon numbers recorded in any given year?
At the moment neither question can be answered with certainty. Nevertheless, most of the commonest hoverflies are species that have several broods each year. It therefore seems likely that the weather in preceding months rather than years will play a dominant role in governing individual species' abundance. We know that this happens in species such as Rhingia campestris and it seems equally likely that the same will obtain for both wetland and aphidophagous species.

Hoverfly associates of social Hymenoptera normally have a single generation each year. Therefore, good years for adult hoverflies probably reflect the breeding succes of the preceding year rather than the year in question. This raises the important point that disentangling the ecology of one guild of animals may be dependent upon good data for another group of orgamisms. Are there sufficient data to say how well bumblebees, ants and wasps fare each year? It strikes me that some simple system for monitoring absolute numbers of bumblebees and social wasps might provide important data to help to investigate this relationship. That gets me thinking that perhaps we need more join-up between the HRS and BWARS.

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