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The occurrence of Protostrongylus stilesi in a population of introduced muskoxen, Ovibos moschatus wardi, on the Arctic Coastal Plain, Yukon Territory (YT) and Northwest Territories (NT), Canada, is consistent with a contemporary colonization event from Dall’s sheep, Ovis dalli dalli, which indicates that host specificity may be ecologically based and contextual for this parasite. Colonization of muskoxen by P. stilesi may be a predictable event in zones of sympatry with Dall’s sheep; exposure to infection may coincide with occupation of winter ranges of Dall’s sheep by muskoxen during the summer season. Disruption of contemporary ecological isolating barriers can result from anthropogenically or climatologically driven habitat perturbation, and result from management practices that influence the distribution of ungulate hosts. Thus, if zones of contact become more extensive or the temporal limits on allopatry are relaxed, we may observe increasing instances of host switching by parasites or pathogens at the interface of newly emerging ecotones. Impacts to northern systems linked to climatologically and anthropogenically driven global change and the effects of management must be tracked within the context of biodiversity survey and inventory and archival collections, as foundations for monitoring ecosystem-level perturbations. A developing interface for muskoxen, wild sheep, and parasites along the Mackenzie River ecotone represents a natural model or field laboratory to examine these processes. Additonally, lungworms, Protostrongylus spp., had not been reported in muskoxen, and a new geographic record for this nematode was established in Dall’s sheep from the northern Richardson Mountains, NT.