Ongoing Movement of the Hermit Warbler X Townsend's Warbler Hybrid Zone

Abstract

BackgroundMovements of hybrid zones – areas of overlap and interbreeding between species – are difficult to document empirically. This is true because moving hybrid zones are expected to be rare, and because movement may proceed too slowly to be measured directly. Townsend's warblers (Dendroica townsendi) hybridize with hermit warblers (D. occidentalis) where their ranges overlap in Washington and Oregon. Previous morphological, behavioral, and genetic studies of this hybrid zone suggest that it has been steadily moving into the geographical range of hermit warblers, with the more aggressive Townsend's warblers replacing hermit warblers along ∼2000 km of the Pacific coast of Canada and Alaska. Ongoing movement of the zone, however, has yet to be empirically demonstrated.Methodology/Principal FindingsWe compared recently sampled hybrid zone specimens to those collected 10–20 years earlier, to test directly the long-standing hypothesis of hybrid zone movement between these species. Newly sampled specimens were more Townsend's-like than historical specimens, consistent with ongoing movement of the zone into the geographical range of hermit warblers.Conclusions/SignificanceWhile movement of a hybrid zone may be explained by several possible mechanisms, in this case a wealth of existing evidence suggests that movement is being driven by the competitive displacement of hermit warblers by Townsend's warblers. That no ecological differences have been found between these species, and that replacement of hermit warblers by Townsend's warblers is proceeding downward in latitude and elevation – opposite the directions of range shifts predicted by recent climate change – further support that this movement is not being driven by alternative environmental factors. If the mechanism of competitive displacement is correct, whether this process will ultimately lead to the extinction of hermit warblers will depend on the continued maintenance of the dramatic competitive asymmetry observed between the species.

Phenotypic divergence during speciation is inversely associated with differences in seasonal migration

“…R. Soc. B 282: 20151921 large section of southern British Columbia and Alberta, Canada [59]; the Townsend's and hermit warblers, in which a hybrid zone is spreading rapidly southward and may lead to the eventual extinction of hermit warblers as a plumage form while preserving much of its genomic variation in the resulting Townsend's warblers [60]; and the bluewinged and golden-winged warblers, in which a hybrid zone is moving northward [61].…”

Causes and Consequences of Behavioral Interference between Species

“…As well as limiting ranges, behavioral interference has been implicated in some ongoing species replacements [4,7,15,48,49]. Aggressive interference in conjunction with habitat changes appears to account for the cyclical replacement of mountain bluebirds (Sialia currucoides) by western bluebirds (Sialia mexicana) in the northwestern USA [15].…”

“…Mismatches of fundamental versus realized altitudinal distributions provide indirect evidence of behavioral interference, and greater mismatch in one species suggests that aggressive interactions are asymmetrical [47]. These scenarios raise the prospect under climate change of subordinate high-elevation species being squeezed into extinction if dominant low-elevation species shift upslope, or of dominant highelevation species preventing the upward movement of subordinate species, which are then left occupying a physiologically suboptimal habitat [17,19].As well as limiting ranges, behavioral interference has been implicated in some ongoing species replacements [4,7,15,48,49]. Aggressive interference in conjunction with habitat changes appears to account for the cyclical replacement of mountain bluebirds (Sialia currucoides) by western bluebirds (Sialia mexicana) in the northwestern USA [15].…”

An Integrated Framework for Hybrid Zone Models

“…Asymmetry in either dispersal or selection results in moving (Buggs 2007) or temporally ephemeral (Grant 1993;Grant and Grant 2002;Pfennig 2007) zones. Zone movement in space and time varies from temporary introgression resulting in increased genetic diversity of one or more of the parental taxa (Grant and Grant 1994;Hedrick 2013) to introgression of separate traits in a zone of secondary contact (Brumfield et al 2001;Parsons et al 1993;Hedrick 2013) to shifts that entail replacement of one species (Krosby and Rohwer 2010). A more common taxon may swamp a rarer taxon (Patten and Campbell 2000;Perry et al 2001) or even a previously widespread taxon (Vallender et al 2007).…”