Longer durability of host-parasite interaction increases host density

Comparing cases of parasitism and predation that lead to victim death, parasites need more time to complete victim exploitation. This longer “interaction durability” delays energy transfer from host to parasite. During exploitation, parasite virulence differentiates the infected from the susceptible host dynamics. However, how this parasite characteristic influences the dynamics of their host and nonhost (insusceptible) species in the same community is largely unknown. Here, we use mathematical modelling to investigate the influence, exemplifying an experimental plankton community. In this community, nonhost zooplankton feeds on edible nonhost phytoplankton (resource competitor of the host) and parasite propagules released from infected inedible phytoplankton (“mycoloop” pathway). To assess the effects of parasite-host durability, we contrast parasite-host implementations as Lotka-Volterra predator-prey interaction (immediate energy transfer) with susceptible-infected (SI) host-parasite interactions. For the latter, parasite energy intake depends on infected host density but not susceptible hosts directly (delayed transfer). We further consider the difference between susceptible and infected host dynamics modulated by parasite virulence via its effect on host nutrient uptake. To assess the within-community effects, subcommunities are also investigated, excluding/including the parasite without/with the mycoloop. Our results show that, besides host elimination, longer interaction durability of the host-parasite interaction delays parasite attacks on susceptible hosts, allowing them to increase further (a hydra effect), independent of parasite virulence level. This effect observed in the isolated host-parasite systems is preserved in larger communities with negative consequences for the nonhost species, independent of the mycoloop. These theoretical results are supported by empirical observations within and beyond plankton realms. Our study reveals distinctive influences of parasites on community shot-term dynamics, which stem from the longer interaction durability.