Recovery or reorganisation? Long-term increases in riverine taxonomic and functional diversity are confounded by compositional dynamics

Abstract

When evaluating biological recovery, shifts in species identity such as non-native status, specialisation, and functional traits should be examined alongside changes in species richness. Lithuania, an ecologically overlooked region, serves as a crucial reservoir for boreal and glacial relict macroinvertebrates. We investigate how and why Lithuanian riverine communities have changed during the intensive period of global environmental changes from 2010 to 2020. Using 41 time-series from Lithuanian rivers, we identified changes in taxonomic and functional diversity. Akin to biological recovery, we observed an overall increase in both taxonomic and functional diversity, likely due to improved water quality, evidenced by increased dissolved oxygen levels and reductions in toxic nitrogen and phosphorus products. However, increases in the richness and abundance of insect and indicator taxa were superseded by increases in non-insect taxa, including crustaceans, molluscs, and annelids. These compositional changes were attributed to climatic changes, notably rising temperatures and reduced discharge. Biodiversity responses to environmental changes were stronger in larger rivers, which are economically important and represent sentinels for further upstream changes. While improvements in water quality benefit various stakeholders and should continue, prioritising mitigation of global environmental change should be the top priority for global cooperation and management schemes.