Understanding the ecological impact of insecticides on freshwater ecosystems, including paddies, through biotic interactions is essential for effective ecosystem management. However, predicting these ecological effects of insecticides could become more difficult in the future, as insecticide toxicity may increase under warming conditions. For example, species that are not normally affected by insecticides could be affected; therefore, the indirect effects of pesticides may be more complicated. This study used simple microcosms to predict the pathway and strength of the combined effects of the insecticide fipronil (widely used in Japanese paddy fields) and increased water temperature (approximately 4 °C higher than ambient conditions: IPCC’s worst-case scenario) on paddy ecosystems via predator–prey interactions. We focused on the interactions among dragonfly nymphs (predator), snails (prey), and producers (dissolved oxygen was monitored as an indicator of primary producers consumed by snails) in a paddy ecosystem. Fipronil had a positive indirect effect on the snails through a reduction in dragonflies under ambient water temperature conditions. Under warming treatments, the fipronil also affected dissolved oxygen, which also indirectly benefitted snails. The positive indirect effect of fipronil on snails increased with increasing water temperature and resulted in an extreme increase in the number of individuals. Given these results, in ecological risk assessments of warming scenarios, it is essential not only to obtain toxicity data for various chemicals but also to predict the processes by which chemicals spill over into ecosystems via biotic interactions.

The responses of assemblages and ecosystems to river intermittency are often considered on large spatial scales, such as those associated with the Köppen climate classification. However, river intermittency may naturally occur even in humid regions, and the effects may differ from those where intermittency occurs throughout the area. This study aimed to examine the differences in taxonomic and trait composition of aquatic insects between intermittent rivers in the Seto Inland Sea watersheds and neighboring humid-temperate rivers in Japan using national census data. While the climate in the most part of the Japanese Archipelago is categorized as humid-temperate, and rivers are usually perennial, local climate in the Seto Inland Sea watersheds has low summer precipitation compared with that in neighboring regions, thereby resulting in river intermittency. Consistent with the previous studies in the regions where rivers are intermittent on a large spatial scale, we observed that aquatic insects with multivoltinism and/or aerial respiration notably tolerated river intermittency. In contrast, the aquatic insect fauna in Japanese intermittent rivers represented subsets of those in the humid-temperate rivers, although previous studies such in Mediterranean regions have reported high endemism and species diversity in intermittent rivers. These results highlight the importance of exploring the role of river intermittency on small to medium spatial scales, as well as on large scales, to project effects of ongoing and future climate changes on species diversity.

In floodplains, which are highly modified and less inundated, trees are expanding, thus raising concerns regarding their impact on freshwater organisms. We analyzed the relationship between forest expansion and the change in Odonata (dragonflies and damselflies) assemblage over an extended period of time in the floodplain with ponds of the Kiso River, central Japan. In the 1970s, the ponds were mainly surrounded by grasslands, and approximately 80% of Odonata species were non-forest species. However, the number of forest species increased and that of non-forest species largely decreased with the forest expansion over the past five decades, resulting in almost a similar number of forest and non-forest species in 2021. Whereas the abundance of the non-forest species had been greater than that of the forest species until the 2000s, the difference has been much smaller in the 2010s, and the non-forest species occupied approximately only 30% of the number of individuals in 2021. The forest expansion may have reduced the open-lentic habitats for the non-forest species, which require riparian grasslands for resting, foraging, and reproduction, and may disturb the immigration of non-forest species from the adjacent rice paddy fields and rivers. The development of canopy cover over the waterbodies may have decreased the light and temperature above and within the ponds, which might have caused a decline in species that prefer warm and open-water environments. Maintaining shifting-mosaic patterns of vegetation around the floodplain waterbodies through active management may be effective in conserving floodplain Odonata communities, including both forest and non-forest species.