Trait-based mechanisms for friendly competition and the dilution effect

Background/Question/Methods

Emergent and often surprising dynamics can arise from interactions between hosts, parasites, and other species. For instance, a focal host benefits from a dilution effect when the presence of alternative hosts reduces disease transmission, either because diluters absorb ‘wasted bites’ of a vector, or because diluters actively remove infective propagules of the parasite from the environment. But how do epidemiological and ecological dynamics change when focal and diluting hosts engage in exploitative competition?  Indeed, species that contact the same parasites often do compete with each other for space or resources. Thus, ‘friendly competition’, wherein a focal host benefits from dilution but suffers from heterospecific competition, may apply to many systems. We explored the range of outcomes of friendly competition by developing a fully dynamic model. We parameterized the model around an algae-zooplankton (Daphnia)-fungus (Metschnikowia) system known to exhibit friendly competition, by measuring relevant traits (e.g. feeding rate, susceptibility, etc.) of three focal host genotypes and a heterospecific friendly competitor. Then, we inoculated mesocosms with each focal host genotype (and friendly competitors and fungus) and tracked the densities of susceptible and infected classes of the host, the friendly competitor, and resources through time. Finally, we compared model simulations with experimental observations.  

Results/Conclusions

The model predicted and our experiment confirmed that community dynamics responded sensitively to changes in focal host traits. When the focal host had low susceptibility (i.e. was epidemiologically similar to the diluter), no dilution effect was detected, but competition strongly reduced focal host density. When the focal host had high susceptibility, both the effects of dilution and competition were detected. When the focal host had high susceptibility but also high feeding rate and spore yield, we again failed to detect a dilution effect. However, this time the failure arose because density of the diluter was reduced and a high production of spores overwhelmed its capacity for dilution. Furthermore, this scenario led to spillover (disease in the potential diluter). This range of outcomes emphasizes the strength of a trait-based approach to community ecology of disease using a combination of modeling and experiments. Moreover, all of these outcomes of friendly competition must be considered when considering potential benefits of the dilution effect for disease control.