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1.The size of disease epidemics remains difficult to predict, especially
when parasites interact with multiple species. Traits of focal hosts like
susceptibility could directly predict epidemic size, while other traits
including competitive ability might shape it indirectly in communities
with a ‘dilution effect’. 2.In a dilution effect, diluter taxa can reduce
disease by regulating (lowering) the density of focal hosts (i.e., through
competition), or by reducing encounters between focal hosts and parasites.
However, these dilution mechanisms are rarely grounded in focal host
traits, and the relative importance of host regulation vs. encounter
reduction remains understudied. 3.Here, we map focal host traits to
disease—via these dilution mechanisms—in communities with diluters. We
measured two traits (competitive ability and susceptibility) for eight
genotypes of a focal host (Daphnia), tracked the densities of each
genotype in experimental mesocosms (+/- Ceriodaphnia competitor/diluters),
and monitored their infections with a virulent fungal parasite
(Metschnikowia) over 6-8 host generations. We disentangled the impacts of
both traits on the density of infected hosts and partitioned dilution
mechanisms using path models. 4.Higher susceptibility directly fueled
larger epidemics. Simultaneously, weaker competitive ability indirectly
suppressed epidemics by enabling higher densities of diluters. These
higher densities of diluters reduced the density of infected hosts
indirectly via host regulation. In contrast, encounter reduction was much
weaker. 5.Our experiment strengthens the dilution effect paradigm with a
predictable, traits-oriented framework. Similar traits—susceptibility,
competitive ability, and their covariance—could help predict epidemic
severity in a variety of other systems. Partitioning the direct and
indirect effects of diluters could also delineate how they impact disease.
Such trait-based insights could help broadly predict the size of epidemics
in diverse communities.
186 views reported since publication in 2019.