How sensitive are the indicators to changes in species selection?
Changes in the list of species that contribute to an indicator may affect that indicator. We improved the species classification procedure and finally based it on bio-geographical regions. That has led inevitably to changes in the species lists.
The farmland bird indicator (especially when statistically smoothed) has always shown the same message, i.e. a dramatic decline of farmland birds in Europe, regardless of the species classification used. It appears to be robust and less sensitive to changes in species lists. The index of common forest birds shows a higher sensitivity to changes in species lists, probably linked to the fact that forest habitats are much more diverse and different driving forces operate at a national or regional scale. This requires a further study.
To make the indicators less sensitive to changes in species composition, we include as many species in the indicators as possible. Furthermore, criteria used for species selection and classification for the indicators are chosen so as to prevent too much influence of one or a few species.
See also related question “How did PECBMS classify the species that are characteristic for each habitat type?“