Virulent parasites can depress the densities of their hosts. Taxa that reduce disease via dilution effects might alleviate this burden. However, ‘diluter’ taxa
can also depress host densities through competition for shared resources.
The combination of disease and interspecific competition could even drive
hosts extinct. Then again, genetically variable host populations can evolve in
response to both competitors and parasites. Can rapid evolution rescue host
density from the harm caused by these ecological enemies? How might such
evolution influence dilution effects or the size of epidemics? In a mesocosm
experiment with planktonic hosts, we illustrate the joint harm of competition
and disease: hosts with constrained evolutionary ability (limited phenotypic
variation) suffered greatly from both. However, populations starting with
broader phenotypic variation evolved stronger competitive ability during
epidemics. In turn, enhanced competitive ability—driven especially by parasites—rescued host densities from the negative impacts of competition,
disease, and especially their combination. Interspecific competitors reduced
disease (supporting dilution effects) even when hosts rapidly evolved.
However, this evolutionary response also elicited a potential problem. Populations that evolved enhanced competitive ability and maintained robust
total densities also supported higher densities of infections. Thus, rapid
evolution rescued host densities but also unleashed larger epidemics.