Fig. 1 in Impacts of crustacean invasions on parasite dynamics in aquatic ecosystems: A plea for parasite-focused studies

Fig. 1. Hypothetical examples of enemy release (A), dilution effect (B), parasite spillback (C) and spillover (D) following introduction of a non-native host in a recipient ecosystem, illustrating the fundamental differences among the different processes. The theoretical recipient ecosystem is here composed of a native host infected by a parasite with a simple life cycle and direct transmission, invaded by a congeneric non-native host infected with a co-introduced parasite with a similar life cycle, to simplify representation. The variable sizes of squares and diamonds represent relative host and parasite abundances, respectively. The thickness of the arrows represents transmission dynamics of the parasite and account for parasite loss during transmission. Enemy release (A) happens when the introduced species benefits from a reduction, or total loss as represented here, in parasitism as a result of invasion. This may in turn have drastic effects on invasion success and both native and invasive host abundances. Dilution effect (B) results from the failure of native parasites to use invasive hosts for successful reproduction and transmission. Native parasites may be unable to infect or be killed (as represented here) by the invasive host. Dilution may in turn decrease parasite transmission among native hosts and negatively affect parasite population dynamics. Parasite spillback (C) happens when invasive hosts acquire a native parasite that is already present in the native host population. Infected invasive hosts can then act as reservoirs of native parasites, potentially increasing infection levels in native hosts as represented here. Increased infection levels in the native host may in turn reduce native host abundance, compared to pre-invasion levels (not represented here). Parasite spillover (D) follows the co-introduction of non-native parasites with their invasive hosts and infection of native hosts by the introduced parasite. Infection of the native host can be maintained by the invasive host, which acts as a reservoir of infection, self-sustained if the parasite can reproduce in its novel host, or both as represented here. Infection of the native host by the introduced parasite can in turn influence host abundances, compared to pre-invasion levels. Note that in scenario D, the native host may or may not possess native parasites.