Symbiont virulence is a poor predictor of impacts on host population dynamics

Symbionts are classified as parasites or mutualists based on their impacts on host individuals, but timescale, ecological context, and level of biological organization can all influence symbiosis outcomes, and hence whether they are deemed harmful or beneficial. We designed experiments exposing the key freshwater grazer Daphnia dentifera to two symbionts varying in virulence, to test how well individual-level effects translate into population-level patterns. We found that a seemingly mildly virulent microsporidian strongly reduced host population size, with consequences for primary production, while a yeast that was highly virulent to individuals had no detectable negative effects on host population size. Our experiments show that studies that fail to consider the impacts of symbionts at multiple scales risk focusing mitigation and control measures on the wrong symbionts.