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

Meade Krosby,Sievert Rohwer,Brock Fenton

doi:10.1371/journal.pone.0014164

Meade Krosby, Sievert Rohwer + Show 1 more

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https://doi.org/10.1371/journal.pone.0014164

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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.

Highlights

Empirical documentation of moving hybrid zones – areas of overlap and interbreeding between species – is relatively rare [1]
Hybrid zone movement due to dramatic competitive asymmetries may lead to rapid extinction of the competitively inferior species [1], [3], so that direct observation is precluded by the ephemeral nature of such events
Recent studies of moving hybrid zones have demonstrated their utility for studying such competitive replacements in action: competitive interactions can be measured directly at hybrid zones, while the extent of historical replacement can sometimes be revealed by genetic signatures of hybrid zone movement

Summary

Introduction

Empirical documentation of moving hybrid zones – areas of overlap and interbreeding between species – is relatively rare [1] This is likely because hybrid zones are most often stable over time [2], and because movement, when it does occur, may proceed too slowly to be measured directly [2]. Experimental field studies have revealed significant differences in competitive ability: Townsend’s males have higher testosterone levels [7] and are much more aggressive than hermit males [8]; they are more successful at attracting mates and maintaining territories [9] This body of information suggests that the massive geographic replacement of hermit warblers by Townsend’s that is likely to have occurred along the northern Pacific coast has been driven by the competitive superiority of male Townsend’s warblers. If the hybrid zone has moved into the hermit geographical range (i.e., if Townsend’s warblers are continuing to replace hermit warblers where they overlap), the new samples should be more Townsend’s-like

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