The dilution effect in a freshwater mutualism: impacts of introduced
host species on native symbionts
The dilution effect was originally proposed to describe the negative
effect of increased host diversity on parasite abundance; with greater
host diversity, parasite levels per host are predicted to be lower due
to a higher probability of dispersing parasites encountering
non-competent hosts. Dilution effects could also occur in many
mutualisms if dispersing symbionts encounter hosts that vary in their
competency. The introduction of non-native hosts can change community
competency of a local group of host species. Crayfish introductions are
occurring world-wide and these introductions are likely disrupting
native crayfish-symbiont systems. Branchiobdellidan symbionts declined
on native Cambarus crayfish occurring in the presence and absence
of non-native Faxonius crayfish in the New River, USA. We
performed an experiment investigating the effect of host density (1 vs 2
native hosts) and host diversity (1 native host and 1 introduced host)
on branchiobdellidan abundance. The introduced F. cristavarius is
a non-competent host for these worms. Six C. ingens were stocked
on a C. chasmodactylus in each treatment and worm numbers were
followed over 34 days. Worm numbers decreased over time on C.
chasmodactylus alone and in the treatment in which a C.
chasmodactylus was paired with an F. cristavarius. Worm numbers
remained highest in the 2 C. chasmodactylus treatment .
There was no significant effect of host diversity on worm reproduction.
Crayfish invasions may have negative effects on mutualistic symbionts
depending on the competence of introduced hosts. Loss of native
symbionts is one of the potential hidden, negative effects of invasions
on native freshwater diversity.