Responses of rice paddy dragonflies to fertilisation in a mesocosm experiment

Abstract

Freshwater biodiversity has been decreasing globally, with wetland habitats facing significant loss due to climate change and changes in land use. In Switzerland, over 90 % of low-elevation wetland habitats have been lost since 1850, mainly due to land transformation for agriculture. Recently, farmers started cultivating paddy rice in Switzerland to meet the increasing food demand and to support wetland biodiversity, particularly dragonflies. However, rice is often produced at high fertiliser levels, raising concerns about potentially harmful effects of high nutrient levels on dragonflies developing in rice paddies. Here, we assessed the impacts of nitrogen fertilisers on dragonflies from a conservation perspective. We exposed three dragonfly species to mineral and organic fertilisers at different concentrations in experimental mesocosms mimicking rice paddies. The effects of fertilisation on survival as well as development time and size at emergence were quantified. In total, 77 % of all dragonfly larvae survived until emergence, and survival was not significantly affected by either fertiliser type at concentrations that are used in Swiss rice paddies (ranging from 25 kg N/ha to 150 kg N/ha). Also, there were no negative impacts of any fertiliser type and concentration on emergence time and body size, which albeit differed significantly among the three dragonfly species. The study thus provides valuable and encouraging insights for conservation and management strategies aimed at promoting wetland biodiversity, particularly the diverse and vulnerable dragonflies, through paddy rice cultivation.