Virulent disease epidemics can increase host density by depressing foraging of hosts

Abstract

<jats:title>Abstract</jats:title><jats:p>All else equal, parasites that harm host fitness should depress densities of their hosts. However, parasites that alter host traits may increase host density via indirect ecological interactions. Here, we show how depression of infected host foraging rate can produce such a hydra effect. Using a foraging assay, we quantified reduced foraging rates of a zooplankton host infected with a virulent fungal parasite. We then parameterized a dynamical model of hosts, parasites, and resources with this foraging function, showing how foraging depression can create a hydra effect. Mathematically, the hydra arose when increased resource productivity exceeded any increase in resource consumption per host. Therefore, the foraging-mediated hydra effect more likely emerged (1) for hosts which strongly control logistic-like resources and (2) during larger epidemics of moderately virulent parasites. We then analyzed epidemics from 13 fungal epidemics in nature. We found evidence for a foraging-mediated hydra effect: large outbreaks depressed foraging rate and correlated with increased densities of both algae and hosts. Therefore, depression of foraging rate of infected hosts can produce higher host densities even during epidemics of parasites that increase host mortality. Such hydras might prevent collapse of host populations but also could produce higher densities of infected hosts.</jats:p>

Parasite-driven cascades or hydra effects: susceptibility and foraging depression shape parasite-host-resource interactions

“…Previously, others have shown how parasites can increase biomass (Ohlberger et al 2011;Preston & Sauer 2020) or both body mass and survival for a specific life stage of hosts (Washburn & Mercer 1991). Total host density also increased in large fungal epidemics in this host in lake populations (Penczykowski et al 2020). Here, in a controlled experiment, parasites increased total host density.…”

“…This trait-mediated effect of parasites and may be strong (large α) or absent (α = 0). We assume equal foraging rate and foraging depression for susceptible and infected classes as a first, simplest approach (see Penczykowski et al 2020 the proportion of hosts infected per encounter. Exposure and susceptibility jointly determine the transmission rate (often denoted β) from the environment to susceptible hosts, uf0e -αZ (third term, eq.…”

Genotypic variation in an ecologically important parasite is associated with host species, lake and spore size

“…Metschnikowia bicuspidata is a widespread parasite of Daphnia (Green, 1974;Ebert, 2005) with substantial impacts on the ecology (Duffy, 2007;Penczykowski et al, 2020) and evolution (Duffy and Sivars-Becker, 2007;Duffy et al, , 2012 of its hosts. As reviewed in the introduction, prior studies failed to detect phenotypic or genetic variation in Daphnia hosts, even though outbreaks are large, common, and occur in multiple hosts on multiple continents.…”