Community‐level prevalence of a forest pathogen, not individual‐level disease risk, declines with tree diversity

Understanding why diversity sometimes limits disease is essential for managing outbreaks; however, mechanisms underlying this ‘dilution effect’ remain poorly understood. Negative diversity‐disease relationships have previously been detected in plant communities impacted by an emerging forest disease, sudden oak death. We used this focal system to empirically evaluate whether these relationships were driven by dilution mechanisms that reduce transmission risk for individuals or from the fact that disease was averaged across the host community. We integrated laboratory competence measurements with plant community and symptom data from a large forest monitoring network. Richness increased disease risk for bay laurel trees, dismissing possible dilution mechanisms. Nonetheless, richness was negatively associated with community‐level disease prevalence because the disease was aggregated among hosts that vary in disease susceptibility. Aggregating observations (which is surprisingly common in other dilution effect studies) can lead to misinterpretations of dilution mechanisms and bias towards a negative diversity‐disease relationship. We studied natural forests infested by sudden oak death to empirically evaluate whether previously observed negative diversity‐disease relationships were driven by dilution mechanisms that reduce transmission risk for individuals, or from the fact that disease was averaged across the host community. Due to distinct underlying drivers at different hierarchical levels, the effects of diversity on disease risk were positive, negative and neutral depending on how the disease was measured. We show that aggregating observations (which is surprisingly common in other dilution effect studies) can lead to misinterpretations of dilution mechanisms and bias towards a negative diversity‐disease relationship.