Résumé
1. Despite growing interest in ecological consequences of parasitism in
food webs, relatively little is known about effects of parasites on
long-term population dynamics of non-host species or about whether such
effects are density- or trait- mediated. 2. We studied a tri-trophic food
chain comprised of: (i) a bacterial basal resource (Serratia fonticola),
(ii) an intermediate consumer (Paramecium caudatum), (iii) a top predator
(Didinium nasutum), and (iv) a parasite of the intermediate consumer
(Holospora undulata). A fully-factorial experimental manipulation of
predator and parasite presence/absence was combined with analyses of
population dynamics, modelling, and analyses of host (Paramecium)
morphology and behavior. 3. Predation and parasitism each reduced the
abundance of the intermediate consumer (Paramecium), and parasitism
indirectly reduced the abundance of the basal resource (Serratia).
However, in combination, predation and parasitism had non-additive effects
on the abundance of the intermediate consumer, as well as on that of the
basal resource. In both cases, the negative effect of parasitism seemed to
be effaced by predation. 4. Infection of the intermediate consumer reduced
predator abundance. Modelling and additional experimentation revealed that
this was most likely due to parasite reduction of intermediate host
abundance (a density-mediated effect), as opposed to changes in predator
functional or numerical response. 5. Parasitism altered morphological and
behavioural traits, by reducing host cell length and increasing the
swimming speed of cells with moderate parasite loads. Additional tests
showed no significant difference in Didinium feeding rate on infected and
uninfected hosts, suggesting that the combination of these modifications
does not affect host vulnerability to predation. However, estimated rates
of encounter with Serratia based on these modifications were higher for
infected Paramecium than for uninfected Paramecium. 6. A mixture of
density-mediated and trait-mediated indirect effects of parasitism on
non-host species creates rich and complex possibilities for effects of
parasites in food webs that should be included in assessments of possible
impacts of parasite eradication or introduction.