In producing multispecies indicators, all species are weighted equally. Wouldn´t it be better to weigh them differently, for instance to give specialists more weight?
The indicators we have developed weight species equally and at first sight that might seem to be too simple an approach. But the simplicity is one of the reasons why this approach has been adopted. The more complicated the procedure, the less understandable and the more prone to misinterpretation an indicator is. Our focus has been to develop comprehensible biodiversity indices that convey information on bird trends and the simple geometric mean of species population trends has proven effective in that respect.
But one might, for example, weigh species indices by the habitat specialization of the species included, the desirability of these species, species abundance, species body mass, precision of the species trend, degree of endemism, conservation value, phylogenetic uniqueness or diversity, cultural preference, or sensitivity to a particular driver (see Gregory & van Strien, 2010). In each case, the selection of weighting variables is subjective and because weighting might affect the indicator, the resulting index would have a particular meaning, interpretation and use.
We already explored the idea that biodiversity indices might be weighted by species biomass. One can imagine a number of reasons why indices of biomass might deviate from indices of abundance because, for example, different drivers may affect large- and small-bodied species differentially. But we found that the European farmland bird indices for biomass and abundance were highly similar (Voříšek et al., 2010). We would welcome studies to other weightings.