Species specific responses to stressors hamper Trichoptera recovery.

Worldwide, aquatic biodiversity is severely threatened as a result of anthropogenic pressures such as pollution, habitat destruction and climate change. Widescale legislation resulted in reduced nutrient- and pesticide loads, and restoration measures allowed modest recovery of freshwater biodiversity. However, from 2010 onwards, recovery in the otherwise unrestored aquatic habitats stagnated. The aim of the present study was therefore to reveal long-term trends in aquatic biodiversity in an anthropogenic landscape and to explain the observed patterns. To this end, over 40 years of biomonitoring data of the indicative taxa group Trichoptera (caddisflies), with an exceptionally high spatial and temporal resolution, was employed. Periods of recovery, stagnation, and decline were delineated using linear and non-linear modelling approaches. Subsequently, species were grouped based on abundance patterns over time and this grouping was used to ascertain species-specific responses to anthropogenic stressors using a trait-based approach. Richness and abundance of all Trichoptera jointly, as well as of the five most abundant and the remaining 136 species, significantly increased from 1980 to significant breakpoints from 2010 onwards, after which these metrics, except the abundances of the 5 most abundant, declined significantly. Trend-based species groupings were not significantly explained by biological traits or ecological preferences. However, Trichoptera species increasing in abundance were less sensitive to climate change and poor water quality, or concerned sensitive species which benefited from restoration measures. Species with stable or declining abundances showed higher sensitivity to climate change. The Trichoptera declining in abundance indicated that conditions in non-protected or restored habitats did not improve due to climate change on top of the other anthropogenic pressures. These observations reinforce the need for increased efforts to improve the only moderately restored water- and habitat quality in anthropogenic landscapes to halt further aquatic ecosystem degradation and to turn biodiversity losses again into recoveries.

Four decades of region- and species-specific trends in lowland stream Ephemeroptera abundance.

Lowland stream ecosystems are under threat from climate change, industrialization, urbanization, and intensive agriculture. Since the 1980s, improvements in water quality have led to an increase in lowland stream biodiversity. Despite restoration efforts, however, further recovery is often hampered by the presence of region-specific (combinations of) stressors, and species-specific stressor responses. Identification of these stressors may not be achieved by the analysis of abundance data over large areas for entire communities or species assemblages. Therefore, our study introduces an alternative in-depth approach, selecting Ephemeroptera as a model organism group and analyzing 41 years of species abundance data across distinct geographical regions. Our findings revealed that 15 Ephemeroptera species had already disappeared before 1985, emphasizing the importance of evaluating an extended historical period when analyzing biodiversity trends. While biodiversity was generally characterized by an initial recovery that stagnated over time, the analysis of the past 41 years of Ephemeroptera abundance data revealed strong differences in species' abundance trends between periods, regions, and species. Certain species were likely to have benefitted from local restoration measures in specific geographical regions, while others may have declined due to the presence of region-specific stressors. Our approach underscores the importance of studying the development of region- and species-specific stream biodiversity trends over time to aid the selection of the appropriate restoration measures to recover lowland stream biodiversity.