Conventional wisdom among ecologists holds that the more species there are inhabiting an ecosystem, the less vulnerable any one species will be to a threat like a parasite. A new study of tadpoles illustrates how overlapping biological and environmental factors can complicate how we value protecting diverse animal communities. The researchers found that environmental pollutants like road salt influence whether increased biodiversity helps or hinders disease outbreaks in wildlife, which can complicate how we value protecting diverse animal communities.