Freshwater bodies are now facing a novel threat with increasing frequency of extreme drought events. Droughts may act as an environmental filter favoring freshwater macroinvertebrates with adapted life-history traits such as resistance forms and diapause. The resilience to drought can either be due to aerial recolonization via adults’ dispersion or to the hatching of resting stages. Agriculture represents an additional environmental filter playing a potentially important role in driving assembly rules of freshwater communities by modifying the surrounding landscape and riparian vegetation. Specifically, by modifying both the quality and quantity of resources falling into small and shallow waterbodies and by altering the accessibility of ponds for certain species, such environmental filters can cause shifts in functional diversity.
In this study, we aimed at understanding the combined effects of extreme droughts and agriculture on functional diversity. We sampled 42 fishless kettle ponds located in a moraine landscape of north-eastern Germany characterized by intensive farming. Macroinvertebrate communities were sampled in June (before drought) and October 2017 (after drought). Functional traits were weighted by abundance, and we tested how drought and agriculture (land use and riparian vegetation) modified functional community shifts.
Results/Conclusions
Ponds were located in beech forests, pasturelands or cultivated fields (corn, wheat, rape) and displayed a wide range of canopy cover (from 0% to 95%). Overall, 37% of the ponds dried out and then rewetted between the two sampling campaigns.
We found that macroinvertebrate diversity and abundance decreased from June to October. Our results also showed that the range of functional traits displayed by macroinvertebrate communities became smaller after drought, even more so in ponds without riparian vegetation. Furthermore, there was a functional diversity shift from communities dominated by piercer-sucker predators to communities with grazers as the main functional group, involving substantial effects on food-web structure.
Our results highlight the role of terrestrial surroundings and riparian vegetation in structuring the functional change of freshwater macroinvertebrate communities and will help identify priorities in conservation and restoration efforts of freshwater systems. Already suffering from dramatic damages imposed by increasing human disturbances, freshwater systems in agricultural landscapes challenge the scientific community to find a compromise between human needs and nature conservation in a drying world.