Disentangling the causality between parasite infections and poor host
conditions in the wild population
Abstract
Although parasites reduce host health, parasite infections also occur as
a consequence of compromised host health. Both causalities could induce
positive feedback, in which infected hosts with poor body conditions may
suffer further infection, but it has rarely been demonstrated in the
wild, possibly due to methodological difficulties. We used a
mark-recapture survey combined with structural equation modelling (SEM)
to examine whether both causalities and positive feedback occurred in
stream salmonid and parasitic copepod systems. We found that parasitic
copepods reduced host conditions and hosts with poor conditions were
likely to be infected, suggesting that positive feedback can occur in
the wild. Importantly, heavily infected hosts with poor body conditions
showed lower apparent survival rates. Our findings provide robust
evidence showing host condition–parasite infection dynamics, offering
novel insights into how positive feedback could strongly undermine the
wild host population via reduction of host survival.