Effective number of communities reveals extent of biodiversity change – a cross-organism analysis in the Wadden Sea

Biodiversity changes in ecosystems worldwide, but assessing the true magnitude of this transformation requires the analysis of multiple dimensions of compositional change. Trends in α-diversity only capture the net difference between gains and losses, while year-on-year temporal turnover in composition does not reflect long-term changes in species identity. We used extensive monitoring data from the Wadden Sea UNESCO World Heritage Site as a case to explore the accumulated change over time through presence-absence and dominance-weighted data. Most time series showed significant temporal trends in α-diversity, but increases or decreases occurred in roughly equal proportions, such that average slopes did not differ from zero. Turnover between subsequent years was higher for fast-growing taxa, but showed only subtle signs of acceleration or deceleration over time. Organism groups did not show an overall decrease in spatial turnover, contrasting concerns of biotic homogenization. The full extent of change became visible only when analysing the accumulated turnover of species composition with time as the effective number of communities (ENC temp ), defined as the overall diversity in a time series divided by the median annual diversity. ENC temp increased with time, such that even when focusing strongly on dominant species, 8% (phytoplankton) to 36% (plants) of time series effectively exchanged community composition at least once (depending on organism group and dominance-weight). Implementing ENC temp in biodiversity assessments provides practitioners and scientists with unique information about the addition or loss of rare species as well as shifts in dominance.