Indexing butterfly abundance whilst accounting for missing counts and variability in seasonal pattern

1. Volunteer-based ‘citizen science’ schemes now play a valuable role in deriving biodiversity indicators, both

method for analysing such data based on counts of invertebrate species characterised by highly variable numbers

makesmore efficient use of the data than previous analyses, whilst accounting for missing values. Firstly, generalised

The estimated daily values were then normalised to estimate a seasonal pattern that is the same across sites

to estimate annual changes in abundance accounting for the varying seasonality.

3. The method was tested and compared against the current approach and a simple linear interpolation using

study demonstrated accurate estimation of linear time trends and improved power for detecting trends compared with

predicted trends over time, but confidence intervals were generally narrower for the two-stage model.

5. In addition to creating more robust trend estimates, the new method allows all volunteer records to contribute

average, the current model only enables data from 60% of 10 km2 grid squares with monitored sites to be

monitored area. As many invertebrate species exhibit similar patterns of emergence or voltinism, our two-stage

method could be applied to other taxa.