Spread of an introduced parasite across the Hawaiian archipelago independent of its introduced host

Summary Co‐introductions of non‐native parasites with non‐native hosts can be a major driver of disease emergence in native species, but the conditions that promote the establishment and spread of non‐native parasites remain poorly understood. Here, we characterise the infection of a native host species by a non‐native parasite relative to the distribution and density of the original non‐native host species and a suite of organismal and environmental factors that have been associated with parasitism, but not commonly considered within a single system. We examined the native Hawaiian goby Awaous stamineus across 23 catchments on five islands for infection by the non‐native nematode parasite Camallanus cotti . We used model selection to test whether parasite infection was associated with the genetic diversity, size and population density of native hosts, the distribution and density of non‐native hosts, land use and water quality. We found that the distribution of non‐native C. cotti parasites has become decoupled from the non‐native hosts that were primary vectors of introduction to the Hawaiian Islands. Although no single intrinsic or extrinsic factor was identified that best explains parasitism of A. stamineus by C. cotti , native host size, population density and water quality were consistently identified as influencing parasite intensity and prevalence. The introduction of non‐native species can indirectly influence native species through infection of co‐introduced parasites. Here, we show that the effects of ‘enemy addition’ can extend beyond the range of non‐native hosts through the independent spread of non‐native parasites. This suggests that control of non‐native hosts is not sufficient to halt the spread of introduced parasites. Designing importation regulations to prevent host–parasite co‐introductions can promote native species conservation, even in remote areas that may not seem susceptible to human influence.