The dilution effect is a controversial emerging paradigm in disease ecology. It associates decreases in species diversity with increases in disease risk for a focal host species. This pattern arises when ‘diluters’ interfere with disease transmission among more competent focal hosts, and ‘diluters’ are rare members of communities. Thus, decreases in diversity (specifically, losses of rare diluters) can correlate with increases in disease risk. However, the dilution effect is sometimes criticized because diluters’ effects on disease can depend on the definition of “disease risk.” For example, diluters may reduce density of infected focal hosts, but not infection prevalence (proportion of focal hosts infected). More importantly, we often cannot predict when diluters actually will reduce disease. Here, we address both critiques by focusing on modular interactions between a focal host and single diluter. Specifically, we measure key host traits governing their interactions. Then, we use these traits to predict dynamical disease outcomes in mesocosm experiments. Our zooplankton study system features competing focal hosts (Daphnia dentifera) and diluters (Ceriodaphnia), and a fungal parasite (Metschnikowia bicuspidata). Trait assays and mesocosm experiments compared seven focal host genotypes that varied in key traits and disease outcomes.
Results/Conclusions
Variation in two focal host traits predicted different effects of diluters on disease, depending on the “disease risk” metric used. First, an index of competitive ability (growth on low food) among host genotypes predicted relative abundances of focal hosts and diluters in mesocosm experiments. Higher growth meant relatively more focal hosts and fewer diluters. Second, transmission rate of disease among focal hosts (incorporating both exposure and susceptibility to parasites) predicted density and prevalence of focal host infections in mesocosms. Finally, path analysis clarified how competitive ability and transmission rate jointly regulated prevalence and density of infections. Competitive ability and disease transmission were positively correlated among host genotypes. However, infection prevalence was driven exclusively by variation in disease transmission among genotypes (even in the presence of diluters). In contrast, density of infection hosts was driven by both indices (i.e., though competition and transmission rate). Thus, for these focal host genotypes (and the trait space that they occupy), diluters reduced density of infected hosts but not prevalence of infections. Overall, traits of interacting species - e.g.,competitive ability and transmission rate of focal hosts - can help predict different effects of diluters on multiple metrics of disease.