Data from: Rapid evolution rescues hosts from competition and disease but—despite a dilution effect—increases the density of infected hosts

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.