Within a complex life cycle, a parasite may use more than one life-history strategy, exploiting several host species in succession. Most complex life cycles can be considered predator-prey (e.g., acanthocephalans), vector (e.g., malaria), or free-living stage transmitted (e.g., trematode cercariae). For instance, trematodes most likely began as parasites of mollusks and later added vertebrate definitive hosts, while many parasitic nematodes whose adults live in vertebrate guts later added intermediate hosts. There are a variety of ways parasites can add new hosts. Biting arthropods clearly provided a convenient means for blood and tissue parasites to contact new hosts. In addition, there should be selection for parasites to survive the predation of their hosts by parasitizing their host's predator. The latter case provides a new category ofparasite, the trophically transmitted parasite. Such parasites do not kill their intermediate host, but require its death for transmission.
Parasites could tip the balance in competitive interactions between native and introduced species. Invasive species typically bring only a small fraction oftheir parasites to invaded regions, and what they pick up from the native community rarely makes up for the difference. Alternatively, an invader has no coevolved history with the few new parasites it acquires, and these could limit the invasion. Parasites can cause two species to interact indirectly even if these species do not compete for resources. Such apparent competition occurs because one host (the more tolerant or resistant) helps maintain the abundance of a natural enemy that then differentially affects the second species.
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