Scaling up to understand disease risk: distinct roles of host functional traits in shaping infection risk of avian malaria across different scales.

Understanding the impacts of diversity on pathogen transmission is essential for public health and biological conservation. However, how the outcome and mechanisms of the diversity-disease relationship vary across biological scales in natural systems remains elusive. In addition, although the role of host functional traits has long been established in disease ecology, its integration into the diversity-disease relationship largely falls behind. By examining avian haemosporidians of 1101 birds from 86 species, we investigated how host functional traits and diversity may shape infection risk across individual and community levels. We found that host traits affect individual-level infection risk but fail to scale up the effect to the community level when testing community-weighted means. Moreover, functional divergence reduced community-level infection risk, indicating the dilution effect of functional diversity. Host richness also showed dilution effect at the community level, but not individual level for one parasite genus, suggesting that the dilution mechanism results from the aggregation of non-competent hosts into richer communities. These results demonstrate that the outcome and mechanism of diversity-disease relationship depend on biological scale, and aggregating observations may cause biased evidence and misattributed mechanisms. Overall, our work suppports the integration of trait-based ecology to further understand the diversity-disease relationship across biological scales.