When can multiple parasites with complex life cycles coexist under conflicts of host manipulation?

Parasites are diverse and play crucial roles in ecosystems, yet mechanisms underlying their diversity remain poorly understood. Many parasites have complex life cycles, manipulating their hosts to facilitate transition through inter-host trophic interactions. When multiple parasites share a common intermediate host but transition to different definitive hosts, they face two conflicts in host manipulation strategies. First, host manipulation may increase predation by non-host predators, leading to "dead-ends." Second, interactions among parasites may complicate host manipulation in a co-infected intermediate host. To explore how host-manipulating parasites compete and coexist under these conflicts, we developed a mathematical model describing population dynamics of two parasites, one intermediate host (prey), and two definitive hosts (predators). The competitive exclusion principle predicts that the two definitive hosts (and thus their parasites) cannot coexist due to competition for the intermediate host. However, our results show that host-manipulating parasites can alter this outcome. We identified three conditions that promote parasite coexistence: (i) the parasite infecting the competitively inferior predator is more prone to dead-ends as a result of adopting a target-generic host manipulation strategy, (ii) co-infected intermediate hosts are manipulated so that predation by the competitively superior predator decreases, whereas predation by the competitively inferior predator increases, and (iii) host-parasite community dynamics exhibit limited fluctuations. Future studies are encouraged to evaluate the likelihood that these conditions are met. Furthermore, we found that alternative states can emerge across a broad parameter space, suggesting potential regime shifts in their community composition. These findings provide novel insights into the mechanisms underlying parasite diversity and its fragility, laying the groundwork for future research on more complex scenarios.