Alternative states of a simple predator-prey system induced by competition between small edible and large inedible algae and fungal parasitism (APPS)

在食草动物存在的情况下，浮游植物群落往往转向大型不可食用藻类的优势地位，限制了放牧链上的能量流动。这可以被寄生真菌(例如，壶菌)抵消，寄生真菌可以感染大型藻类，并通过游动孢子使它们可供浮游动物(例如微型和大型浮游动物)觅食。这种所谓的“Mycolop”对食物网络的互动产生了深远的影响。在浮游动物、食草动物和真菌之间缺乏摄食联系的情况下，寄生真菌能够减少大型藻类的数量，从而有利于食用绿藻。然而，在真菌与浮游动物之间存在强大的摄食联系的情况下，浮游动物甚至可能通过降低感染率来培育不可食用的藻类。寄主种群的性状变异可能会导致转移到更具抗性的基因类型，减少对不可食用藻类的直接自上而下控制，并削弱食草动物-真菌之间的联系。我们的目标是将性状变异、浮游动物放牧和真菌寄生联系起来，以研究在真菌寄生或放牧以及两者存在的情况下，宿主-寄生虫/捕食者-猎物之间的相互作用以及可食用和不可食用浮游植物之间的竞争。我们假设，真菌寄生有可能将我们的模型系统转变为不同的替代状态，可食用或不可食用的浮游植物占主导地位，受真菌-浮游动物联系的强度调节。我们想从实验上测试真菌寄生的影响和霉菌对食物网动态的影响。我们预计，大型不可食用藻类的正反馈环的强度将取决于捕食者-真菌联系的强度和捕食者的身份。我们预计，与非选择性食草动物(爬行动物)相比，选择性食草动物(桡足类)的这种联系最强。对相应食物网模型的理论研究将深入了解不同的驱动因素和相互作用强度如何影响小型可食用藻类和大型不可食用藻类之间的竞争。结合起来，这将能够确定真菌寄生如何调节食物链动态及其对浮游植物多样性和功能的影响。

In the presence of grazers, phytoplankton communities often shift to dominance of large inedible algae, limiting energy flow up the grazing chain. This can be counteracted by parasitic fungi (e.g. chytrids), which can infect large algae and make them available to zooplankton grazing (e.g. micro- and macrozooplankton) via zoospores. This so-called “Mycoloop” has profound consequences for food web interactions. In the absence of a feeding link between zooplankton grazers and fungi, parasitic fungi are able to reduce large algae, thereby favouring edible green algae. However, in the presence of a strong feeding link from fungi to zooplankton may even foster inedible algae through decreased infection rates. Trait variability in the host population may enable shifts to more resistant genotypes, decreasing the direct top-down control on inedible algae and weakening the grazer-fungi link. We aim to link trait variability, zooplankton grazing and fungal parasitism to investigate host-parasite/predator-prey interactions and competition of edible vs. inedible phytoplankton in the presence of either fungal parasitism or grazing and in combination. We hypothesize that fungal parasitism has the potential to shift our model system into different alternative states with dominance of either edible or inedible phytoplankton, modulated by the strength of the fungi-zooplankton link. We want to experimentally test the effect of fungal parasitism and the strength of the mycoloop on food web dynamics. We expect the strength of the positive feedback loop on large inedible algae to be dependent on the strength of the grazer-fungi link and on grazer identity. We expect the link to be strongest for selective grazers (copepods) compared to unselective grazers (daphnids). Theoretical investigations of a corresponding food web model will give insights on how different drivers and interaction strength affect competition between small well edible and large inedible algae. In combination, this will enable to determine how fungal parasitism modulates food web dynamics and its implications for phytoplankton diversity and functionality.