5. Ecological role of fungal hyperparasitism

Although a common phenomenon in nature, the real impacts of hyperparasitism on the ecology and evolution of the organisms involved and its cascading effects throughout food webs is understudied. In the broad sense, hyperparasites are analogous to predators, where the secondary hosts (primary parasites) act as herbivores and the primary hosts replace primary producers. Therefore, as predators, hyperparasites are able to shape ecosystem stability through top–down cascades (Parratt and Laine, 2016). Hyperparasitic fungi also influence the dynamics of the interactions between the primary hosts and the primary parasites, increase the complexity of the food webs, and play a significant role in regulating population sizes of either partner (Gleason et al., 2014; Sandhu et al., 2021). By decreasing the fitness of their host, hyperparasites may essentially exert a net positive effect on the fitness of the primary host (Northrup et al., 2021; Sandhu et al ., 2021). However, a convincing conceptual framework is lacking, and tractable model systems to study hyperparasitic interactions in natural populations are scarce (Péter et al., 2022; Parratt and Laine, 2016).
It is hypothesized that zoosporic parasites have a role in the structure and function of aquatic food webs, by lengthening food chains and carbon paths. As their life cycles are shorter, zoosporic hyperparasites also increase and accelerate the energy flow among trophic levels, by producing biomass in the form of zoospores and zoosporangia that enter the food web contributing different types of energy for predators (Gleason et al., 2014).
The range of interactions among hyperparasites, their hosts (i.e., the secondary hosts), and the primary hosts is wide and complex, and sometimes difficult to establish (Gleason et al., 2014; Kiss, 2001). Studies on host specificity in hyperparasitic fungal systems are scarce (but see Barnett and Lilly, 1958; Jeffries and Young, 1978), and those examining all three trophic levels in the same analysis are even rarer. One recent study analyzed the ecological interactions among the three levels of the multitrophic network among bats, bat flies, and microfungi and found that bat flies are much more host specific at the community-level compared to their Laboulbeniales hyperparasitic fungi (de Groot et al., 2020).