Concomitant predation, which occurs when parasites are consumed and
digested along with their hosts, has previously been suggested as a
profound factor determining food web structure. Few studies have
adressed the impact of concomitant predation in research on
behaviourally parasite-modified prey or in biological control studies.
However, empirical evidence of concomitant predation effects on hosts
infected with multiple parasite taxa is lacking. We investigated the
importance of concomitant predation on digenean trematodes by examining
the degree of snail (Radix balthica, first intermediate host) seasonal
predation by Arctic charr (Salvelinus alpinus) and brown trout (Salmo
trutta) by contrasting infection rates of free-living snails obtained
from a lake vs predated snails retrieved from fish stomachs and
intestines. The fish consumed infected snails nearly at all seasons,
demonstrating that concomitant predation in the model subarctic lake is
common, likely indirectly affecting trematode transmission by reducing
host and parasite populations. The overall trematode prevalence in both
snail groups was season-independent, being however substantially higher
in free-living compared to predated snails. The net effects of
underlying mechanisms related to prey availability, fish feeding
ecology, continuous presence of dominant trematodes and, most
importantly, size of fish and snails drove the strength of predator-prey
interactions and infection patterns in both snail groups. Larger fish
preying upon larger snails, which simultaneously harboured more
infections, may induce a substiantial negative effect of concomitant
predation on snail and parasite population dynamics, with serious
implications for food web structure and ecosystem functioning. This
study contributes to a better understanding of the role of non-host
predators in regulating trematode infection, community structure and
transmisison patterns, biomass transfer and energy flow in food webs.
Our findings also highlight the importance of studying the impact and
extent of concomitant predation in terms of parasite seasonal dynamics
and biological control of infectious diseases.
Concomitant predation on parasites -- removal of trematode-infected snails by predacious fish in a subarctic lake