PS 6-75 - Spatial scaling of the dilution effect for pathogen infection and herbivory in restored Missouri glades

Abstract: Ongoing anthropogenic biodiversity loss and threat of disease spillover to novel hosts underscore the importance of understanding patterns of disease prevalence across host community diversity gradients. Empirical evidence suggests that relationships between host diversity and disease are scale-dependent. For example, meta-analyses have documented a positive relationship between biodiversity and human disease burden across countries, but a negative diversity-disease relationship (i.e., "dilution effect") for many host taxa at smaller spatial scales (median replicate size: 1.5 km²). However, the dilution effect remains understudied at much smaller spatial scales where key mechanisms driving plant-pathogen and plant-herbivore interactions should operate. Specifically, mechanisms of encounter reduction and host regulation are likely to operate at scales of less than 1 m² in plant communities. Moreover, the spatial scale at which dilution effects are most likely to be detected may vary between different types of pathogens and herbivores, due to differences in transmission and host specificity. Here, we aim to quantify the effect of spatial scale on the sign and magnitude of relationships between plant diversity and prevalence of foliar infection and herbivory damage. We examine these relationships for two types of pathogens (rust and leaf spot) and three types of herbivory (chewing, mining, and sucking-rasping) through an observational field study performed in three Missouri glade restoration sites.

In each site (n=3), we measured plant species diversity and relative abundance in ten 1 m² quadrats. In each quadrat, we assessed plants for presence/absence of disease symptoms and herbivory damage, and additionally measured disease severity in a subset of quadrats. Plant species varied in prevalence of each damage type, suggesting species-specificity of pathogens and herbivores. The three glades had similar overall species richness, but different community composition and prevalence of each damage type. Across all 30 quadrats, no significant trends were detected between plant species richness and community-level disease or herbivory. When the analysis is narrowed to two common focal species found in each site, a weak negative relationship is observed between species richness and rust prevalence, while weak positive relationships are observed between species richness and herbivory. Further analyses will be performed by analyzing data pooled across multiple quadrats to assess how diversity-damage relationships change with spatial scale, and by testing relationships between species richness and disease severity.