Predation risk magnifies fungicide-driven suppression of keystone shredder feeding and behaviour, slowing freshwater leaf-litter degradation

Chemical pollution is a growing global threat, and freshwater habitats, which harbor disproportionate biodiversity and provide key ecosystem services, now face overlapping chemical and biological stressors whose combined effects can accelerate biodiversity loss. Fungicides, heavily used in forestry and agriculture, often leach to freshwater environments, but their impacts remain understudied, especially sub-lethal impacts such as changes in prey behaviour or detritivore-mediated decomposition. We examined the interactive effects of a widely used triazole fungicide (Tebuconazole) and a common biotic stressor (dragonfly predators) on a keystone detritivore, the freshwater isopod Asellus aquaticus, using a 2x2 factorial microcosm experiment in the laboratory. We reared isopods in groups in a combination of two fungicide (0 vs 50 μg/L Tebuconazole) and two predator (presence or absence of a caged predator) treatments and assessed changes in performance (behavioural activity, food consumption) after 21 days, followed by a survival assay in presence of a free-ranging predator. We found that Tebuconazole alone reduced feeding by 89 % and predator cues by 44 % relative to benign (clean water-no predator present) conditions, while both stressors together reduced feeding by 91 %, indicating that chemical stress overrode effects of predators. Isopods reared in the absence of a caged predator reduced their behavioural activity by 35 % when exposed to predator cues, whereas fungicide-exposed conspecifics showed no behavioural change, indicating impaired threat detection. Prior experience with predators nearly doubled survival against a free-ranging predator (45 % for predator-naive isopods vs 80 % for isopods reared with a caged predator), but this advantage vanished in the fungicide treatments (62 % survival). Collectively, our results demonstrate that environmentally relevant pesticide concentrations can change prey behaviour, increase predation risk, and potentially destabilize trophic interactions that regulate nutrient cycling. These findings underscore the need for freshwater risk assessments that capture how multiple interacting stressors affect organisms and the ecosystem functions they support.