Fig. 3.

Introduced parasites – native/introduced hosts: hypothetical examples of the potential effects of invasive crustaceans on native parasites. Note that only a subsample of non-exclusive scenarios from a number of potential outcomes of non-native parasite introduction is represented here. The hypothetical non-native parasite considered here has a two-host life cycle involving a definitive host predator and an intermediate host prey, transmission from the intermediate host to the definitive host requiring consumption of infected intermediate host prey. The variable sizes of squares, circles and diamonds represent relative intermediate and definitive hosts, and parasite abundances, respectively. During transmission, some parasites are unsuccessful and therefore lost from the system (parasite loss); the thickness of the arrows indicates the relative numbers that are either lost or successfully transmitted. The life cycle at the top left represents the situation in the ecosystem of origin of the parasite, providing a benchmark for comparisons. Prior to the invasion, the hypothetical recipient ecosystem does not contain native parasites for simplification of representation. (A) The parasite is co-introduced with its intermediate host prey. The invasive parasite retains its original, co-introduced hosts and uses native definitive hosts to complete its life cycle. The situation represented here is the simplest one where the native predator exactly replaces the original definitive host of the parasite with no effect on either parasite dynamics or host abundance. However, parasite invasion may in turn negatively affect native predators and change parasite dynamics compared to that observed in the original ecosystem (shown at the top left). (B) The parasite is again co-introduced with its intermediate host prey. The invasive parasite retains its original, co-introduced hosts and uses native definitive hosts to complete its life cycle but also uses the native prey species as an alternative transmission vector. The introduced parasite may negatively influence native host abundance, thus influencing invasion success of its co-introduced host, as shown here. This may in turn lead to greater infection levels in definitive hosts in the recipient ecosystem than in the original ecosystem of the parasite (situation not represented here) (C) The non-native parasite is introduced without its original host (or this host does not survive translocation) but is subsequently included in the recipient food web. The novel parasite may in turn have drastic effects on intermediate and/or native hosts and reach higher infection levels in these novel hosts as represented here. However, a multitude of alternative scenarios are possible with as many outcomes in terms of parasite dynamics.