Potential spread of introduced black rat (Rattus rattus) parasites to endemic deer mice (Peromyscus maniculatus) on the California Channel Islands

Introduced species have the potential to outperform natives in two primary ways: via increased rates of predation and competition, and via the introduction of new parasites against which native species often lack effective immune defences. To assess the extent to which invasive species' parasites spread to native hosts, we compared the composition of helminth parasites found in introduced black rat ( Rattus rattus ) and endemic deer mouse ( Peromyscus maniculatus ) populations on a subset of the California Channel Islands. Results suggest that the whipworm, Trichuris muris , may have spread from introduced black rats to endemic island deer mice and has continued to thrive in one island population where rats were recently eradicated. These results yield two important conservation messages: (1) although the parasites introduced with invasive species may be few, they should not be ignored as they can spread to native species, and (2) introduced parasites have the potential to remain in a system even after their founding host is extirpated. These findings underscore the importance of parasitological surveys in invasive species research and baseline data for ecosystems where exotic species are likely to invade.