Impacts of abundance and habitat area weighting in allocating species trends to habitats

The dynamics of species depend on the management of their habitats. However, in the absence of good habitat monitoring data for many types of species, reliance has been placed on identifying habitats seeing marked changes in biodiversity through combining trends in their associated species into a habitat level metric. Several data sources on species occupancy, abundance within different habitats, and habitat area for two example taxa, bryophytes and lichens, were linked to assess how different methods of allocating existing species’ abundance trends to habitats influenced the habitat statistics. In general, trends through time were similar, but the method of allocation had an impact on the absolute values of the Distribution Index that summarises weighted occupancy. Allowing generalists to contribute equally to specialist species in a habitat gave higher values of habitat level Distribution Index than methods which weighted species according to abundance in that habitat and habitat area. There were also impacts on the analysis of long-term and short-term trend data, with the more complex methods, including abundance within habitats and extent of habitat, detecting more differences between habitats, and, for some habitats, changing positive trends for bryophytes to no significant trend or even negative for sparsely vegetated habitats. If species trend data is to be used for identifying habitats where biodiversity trends are marked, then it is clear that weighting species, such that their total weight across the analysis is the same, is necessary. Developing the precise means to achieve that needs careful thought and the creation of a robust method that works across different species groups, but using unweighted data could lead to erroneous conclusions as they are so dependent on the dynamics of widespread species.