Multidimensional assessment of biodiversity change reveals substantial transformation across taxonomic groups

Biodiversity is changing in ecosystems worldwide, but assessing the true magnitude of this transformation requires the analysis of multiple dimensions of compositional change. Assessments often focus on single indicator types or metrics, but here we show that there is very limited correlation between different dimensions of transformation. Our example are the extensive monitoring data from the Wadden Sea UNESCO world heritage site (26560 unique samples distributed across 37 monitoring programs with 455 stations total). We simultaneously analyze trends in population sizes, annual (alpha) diversity, temporal turnover in composition (between subsequent years and with increasing temporal distance), and temporal changes in spatial (beta) diversity across different Hill numbers. Time series with increasing and decreasing alpha diversity were roughly equally abundant, with notable exceptions for plants and zoobenthos (more negative trends) as well as phytoplankton (more positive trends). Turnover in species composition between subsequent years was high, but showed only subtle signs of acceleration or deceleration over time. However, with larger temporal distance, dissimilarity increased to the extent that at least 15% of the time series effectively replaced the community composition at least once. Almost all organism groups showed increased spatial beta-diversity over time, thus, we found no signs of biotic homogenization between locations. In summary, the biodiversity of the Wadden Sea has been transformed substantially, but with little synchrony between organism groups in terms of phases of rapid and slow changes. The dimensionality of these shifts becomes only evident when simultaneously addressing multiple facets of biodiversity ranging from local alpha diversity to the cumulative turnover in time and space. None of these metrics alone would be able to indicate the amount of change, as each captures a different facet. To convert these issues into meaningful actions, we need to even extend beyond the quantitative assessment of biodiversity change and its functional consequences by including human values and emotions as an integral part of biodiversity reporting.