Mapping fish community biodiversity for European marine policy requirements
Predictive maps of biodiversity patterns are pivotal to marine conservation and marine spatial planning alike, yet mapping of biodiversity indicators at the community-level is neither straightforward nor well-tested empirically. Two principle approaches exist. A direct approach involves calculation of indices for each sample, followed by interpolation to estimate values at unsampled locations. An indirect approach first interpolates individual species distributions and then determines indices based on the stacked distribution maps. We compared the appropriateness of both approaches to provide management-relevant information by mapping the distribution of demersal fish biodiversity in the German North Sea Exclusive Economic Zone using species richness, Hill’s N1 and a novel traits-based community sensitivity to fishing index (CSI). To substitute zero-inflated species with up to 95% zeros in the sample data, we applied each species’ mean abundance value as a flat surface. Spatial patterns between indicators varied, but certain hot- and cold-spots were revealed, which, under current legislation, might suggest that the present level of biodiversity protection is insufficient. Despite both approaches generating similar main patterns, the direct approach predicted a narrower range of index values and only depicted the most dominant patterns. Contrary to that the indirect approach better reproduced the variability in the data, along with additional information on species distributions and a theoretical advantage pertaining to sampling issues. Although the choice over the mapping approach is context dependent, for our study area featuring a community with relatively few species, we consider the indirect approach to provide the more reliable information for implementing marine environmental legislation.