Predator diversity impacts herbivore abundance and distribution: consequences for a vector-borne plant pathogen

Traditionally, studies of vector-borne disease dynamics focus on the tripartite interactions between hosts, pathogens, and their vectors, while they often neglect the role of organisms in higher trophic levels, such as predators. However, predators may indirectly influence disease spread in a host population by consuming disease vectors and reducing their abundance, or by inducing vector movement. In a large-scale replicated field experiment, we evaluated the impact of predator diversity on the abundance and spatial distribution of the Bird-cherry oat aphid (Rhopalosiphum padi), a vector of barley yellow dwarf disease in wheat. We manipulated the diversity of five commonly occurring predators and compared the effectiveness of single predator species to that of diverse assemblages with four predator species. We found that a diverse group of predators consumed more aphids, and therefore reduced the abundance of vectors, more so than a single predator species. We also found that vector movement differed with predator diversity treatment such that aphids were more evenly distributed across wheat plots in the presence of a diverse group of predators, but remained clumped in the presence of a single predator species. Taken together, these results provide potentially conflicting implications of predator diversity for vector-borne disease dynamics. On the one hand, predator diversity may reduce disease prevalence locally by reducing abundances of disease vectors, but on the other hand predator diversity may increase disease prevalence on a larger scale by stimulating the dispersal of disease vectors.