The “dilution effect” concept in disease ecology offers the intriguing possibility that clever manipulation of less competent hosts could reduce disease prevalence in populations of more competent hosts.  The basic concept is straightforward: host species vary in suitability (competence) for parasites, and disease transmission decreases when there are more incompetent hosts interacting with vectors or removing free-living stages of a parasite.  However, host species also often interact with each other in other ecological ways, e.g., as competitors for resources.  The net result of these simultaneous, multiple interactions (disease dilution and resource competition) is challenging to predict.  Nonetheless, we see the signature of both roles operating concurrently in a planktonic host-parasite system. 

Results/Conclusions

We document pronounced spatio-temporal variation in the size of epidemics of a virulent fungus (Metschnikowia bicuspidata) in midwestern USA lakes populations of a dominant crustacean grazer (Daphnia dentifera).  We show that some of this variation is captured by changes in structure of Daphnia assemblages.  Lake-years with smaller epidemics were characterized by assemblages dominated by less suitable hosts (“diluters”, D. pulicaria and D. retrocurva, each of whose compentence was determined in lab experiments and field surveys) at the start of epidemics.  Furthermore, within a season, less suitable hosts increased as epidemics declined.  These observations are consistent with a dilution effect.  However, more detailed time series analysis (using multivariate autoregressive models) of three intensively sampled epidemics show the signature of a likely interaction between dilution and resource competition between these Daphnia species.  The net outcome of this interaction likely promoted termination of these fungal outbreaks.  Should this outcome always arise in “friendly competition” systems where diluting hosts compete with more competent hosts?  To address this question, we created a dynamical eco-epidemiological model, built around the natural history of this system.  The model mixes resource competition and the dilution effect and illustrates how disease termination is just one of several possibilities arising from friendly competition.