Understanding how and why hosts vary in their ability to transmit infection (host competence) is fundamental to predicting and controlling epidemics. Linking host competence to other ecological probabilities (like extirpation risk) is also key to predicting the response of pathogens to changes in host community composition. The Leaf Economic Spectrum (LES) is a global axis of covariation among plant leaf traits that runs from slow-return to quick-return phenotypes; it has been used to predict community assembly and the outcome of species interactions. We hypothesized that host competence increases along the LES across species and environments. In two separate greenhouse experiments, we used 23 species of plant hosts and a nitrogen supply treatment to generate variation in leaf traits. For each combination of species and nitrogen treatment, we measured leaf traits to quantify host position along the LES. In the first experiment, we then exposed all hosts to a shared, generalist, vector-borne virus to quantify susceptibility. In the second, we fed uninfected vectors on paired, virus-infected hosts to quantify infectiousness, and recorded vector reproduction on these hosts. We used the product of susceptibility, infectiousness, and vector reproduction to estimate host competence, the number of new infected vectors produced per day.
Results/Conclusions
The LES was evident in both experiments: the first principal component (PC1) in a principal components analysis of leaf traits captured 50.4% and 49.3% of the variation in leaf traits across species and nitrogen treatments, and all traits loaded onto PC1 in directions that corresponded to the LES. Also across species and nitrogen treatments, as hosts became more quick-return along the LES, competence increased significantly, as did each of its components: susceptibility, infectiousness, and vector reproduction. While these relationships could result from phylogenetic correlations between leaf traits and the components of competence, tests for phylogenetic signal indicated that all relationships were phylogenetically independent. Our results demonstrate that the LES can be used to predict host competence. They also suggest that physiological traits could provide a link between host community assembly and models of pathogen transmission, and thus improve predictions of pathogen response to changes in host diversity and composition.