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

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 foraging rate of infected hosts 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 that 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-mediate...在其他条件不变的前提下，损害宿主适合度的寄生虫理应会降低宿主种群密度。然而，能够改变宿主性状的寄生虫却可能通过间接生态相互作用提升宿主种群密度。本研究揭示了受感染宿主的觅食速率抑制如何引发此类九头蛇效应（Hydra Effect）。我们借助觅食测定法（foraging assay），量化了感染强毒力真菌寄生虫的浮游动物（zooplankton）宿主的觅食速率降幅。随后基于该觅食函数，我们对宿主、寄生虫与资源的动力学模型进行了参数化拟合，以此阐明觅食速率抑制可催生九头蛇效应。从数学机理上看，当资源生产力的提升幅度超过单宿主资源消耗量的增幅时，便会产生该效应。因此，由觅食行为介导的九头蛇效应更易在两类情境下出现：(1) 可强力调控类逻辑斯蒂资源的宿主种群；(2) 中等毒力寄生虫引发的大规模流行期间。随后我们分析了自然界中13起真菌寄生虫流行事件，并找到了支持觅食介导……的相关证据。