Parasitism effects on coexistence and stability within simple trophic modules

Abstract

Parasites are important components of food webs. Although their direct effects on hosts are well-studied, indirect impacts on trophic networks, thus on non-host species, remain unclear. In this study, we investigate the consequences of parasitism on coexistence and stability within a simple trophic module: one predator consuming two prey species in competition. We test how such effects depend on the infected species (prey or predator). We account for two effects of parasitism: the virulence effect (parasites affect the infected species intrinsic growth rate through direct effects on fecundity or mortality) and the interaction effect (increased vulnerability of infected prey or increased food intake of infected predators). Results show that coexistence is favored when effects have intermediate intensity. We link this result to modifications of direct and apparent competitions among prey species. Given a prey infection, accounting for susceptible-infected population structure highlights that coexistence may also be reduced due to predator-parasite competition. Parasites affect stability by modulating energy transfer from prey to predator. Predator infection therefore has a stabilizing effect due to increased energy fluxes and/or predator mortality. Our results suggest that parasites potentially increase species coexistence. Precise predictions however require an assessment of various parasite effects. We discuss the implications of our results for the functioning of trophic networks.

Parasites make hosts more profitable but less available to predators

“…To conclude, we encourage further studies at a larger ecological scale, considering that prey infection has repercussions on predators (Flick et al 2016) and other potential prey (Decaestecker et al 2015, Prosnier et al 2018), potentially leading to the modification of trophic links. As shown in many food web studies, it is crucial to understand the implications of parasites on community composition, stability, and functioning (McCann 2000, Kondoh 2003, Frainer et al 2018).…”

Parasite‐mediated changes in host traits alter food web dynamics

“…In theoretical community studies, earlier attempts to incorporate interaction modifications by parasites have been limited to simple three‐ or four‐species trophic networks. These studies have shown that changes in predator attack rate or in prey susceptibility to predation can destabilise the dynamics of predator–prey communities (Fenton and Rands 2006) and alter coexistence outcomes among competing species (Hatcher et al 2014, Prosnier et al 2018). However, to improve our understanding of the community‐wide consequences of such parasite effects requires incorporating them into more complex and realistic food‐web models.…”

Top-down and bottom-up effects and relationships with local environmental factors in the water frog–helminth systems in Latvia

“…However, there are no fieldwork studies linking helminth infections with the frog population size estimates in nature and it is not clear how much they actually affect population welfare and sustainability (the presence of adverse effects can be suggested from a study showing that S. robusta may cause local population declines in European newts 18 ). In turn, parasites depend on their hosts and a severe effect on their host populations would not fit an optimal virulence strategy 19 , parasites may potentially increase species coexistence 20 , but their community richness could be important driver for biodiversity and ecosystem productivity 21 .…”