Parasitism dramatically alters the ecosystem services provided by freshwater mussels

Parasites can indirectly affect ecosystem function by altering host
phenotype, but the trait-mediated impacts of parasitism at an ecosystem
level remain poorly characterised. However, understanding the influence of
parasites is central to understanding the ecosystem services provided by
host species, especially in an era of global environmental change. We
examined the effect of native (the trematode Rhipidocotyle campanula) and
invasive (the bitterling fish Rhodeus amarus) parasites, and their
interaction, on the clearance rates of unionid mussels, a dominant
ecosystem-engineering group that modifies freshwater ecosystems worldwide.
We used a combination of field experiments, laboratory experiments, and
ecological simulations to demonstrate the phenotypic impact of parasites
on the functional response of two mussel species across an environmental
gradient (suspended particle concentration), and extended this with host
and parasite community data to demonstrate the consequences for a
real-world ecosystem, the Old West River in Cambridgeshire, England. Both
parasites altered the clearance rates of their hosts but in contrasting
fashion: while R. campanula increased host clearance rates relative to
uninfected conspecifics under all conditions, R. amarus suppressed
clearance rates at high suspended particle concentrations (eutrophic
conditions) but elevated them otherwise. The parasites displayed different
infection patterns in the two host species, and the invasive R. amarus
rarely co-infects mussels with R. campanula. Given their disparate
effects, the parasites’ distributions reversed the relative filtration
capacity of the two host species under high vs. low concentrations of
suspended particles, demonstrating how differences between the infection
patterns of native and invasive parasites, as well as their individual
effects, need to be considered. Overall, the proportion of daily river
discharge filtered by the combined mussel community changed by up to 96%
in the presence of parasites. By incorporating multiple host species and
multiple parasite species, we provide ecologically relevant evidence for
the trait-mediated effects of parasites on ecosystem processes. Our study
demonstrates that parasitism can significantly alter wider ecosystem
processes by changing the phenotype of their host. Future work on
ecosystem function should take parasitism into account, and consider both
trait-mediated and density-mediated effects.