Reduced habitat suitability and landscape connectivity in a songbird migratory divide

AbstractAim Seasonal migration is a common phenomenon in animals and connects geographically distant ecosystems. Considerable variation has been documented in this behaviour and migratory divides (contact zones between populations that use different routes to navigate around ecological barriers) are an example of this. Migratory divides could have important implications for ecological speciation as hybrids in divides take intermediate routes and it has been predicted that these routes will be ecologically inferior as they bring hybrids over barriers that parental forms avoid. We provide the first test of this prediction here using a well-characterized divide between coastal and inland subspecies of Swainson’s thrushes. Location North, Central and South America Time period 2006-2013 Major taxa studied Birds Methods We used ecological niche models and analyses of landscape connectivity to predict suitable habitat for coastal and inland thrushes on migration and identify optimal migratory routes. We compared estimates of habitat suitability and landscape connectivity along the actual migratory routes of birds tracked with light-level geolocators. Results Consistent with predictions for migratory divides, niche models estimated lower habitat suitability in the intermediate range between the migratory ranges of parental forms. Furthermore, our models of landscape connectivity predicted optimal routes that circumvent this intermediate area. Birds taking intermediate routes used stopover sites of lower predicted habitat suitability than birds migrating on either side of the divide and overlapped less with optimal paths. Main conclusions Our results support the prediction that intermediate routes of hybrids in migratory divides are ecologically inferior to those of parental forms. This finding suggests ecological differences are helping to maintain divides and could have broad implications, with divides shaping the distribution and maintenance of species boundaries globally and across many taxonomic groups.
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摘要
目的
季节性迁徙是动物界的常见现象，连接着地理上相距遥远的生态系统。该行为存在显著变异且已有大量记录，迁徙分界（migratory divides）——即采用不同路线绕过生态屏障的种群间接触区——便是其中一例。迁徙分界对生态物种形成（ecological speciation）具有重要意义：分界处的杂交种会选择中间路线，据预测，这些路线在生态上更为不利，因为它们会使杂交种穿越亲本种群所避开的屏障。本研究利用斯温森鸫（Swainson’s thrushes）沿海与内陆亚种间特征明确的迁徙分界，首次验证了这一预测。

地点
北美、中美及南美

时间范围
2006-2013年

研究类群
鸟类

方法
我们采用生态位模型（niche models）与景观连通性（landscape connectivity）分析，预测沿海与内陆鸫类迁徙途中的适宜栖息地，并识别最优迁徙路线；同时对比了通过光级地理定位器（light-level geolocators）追踪的鸟类实际迁徙路线的栖息地适宜性与景观连通性估计值。

结果
与迁徙分界的预测一致，生态位模型显示亲本种群迁徙范围之间的中间区域栖息地适宜性更低；此外，景观连通性模型预测的最优路线会避开该中间区域。选择中间路线的鸟类，其停歇地的预测栖息地适宜性低于迁徙分界两侧路线的鸟类，且与最优路径的重叠度更低。

主要结论
研究结果支持预测：迁徙分界处杂交种的中间路线在生态上劣于亲本种群的路线。这表明生态差异有助于维持迁徙分界，并对全球物种边界的分布与维持具有广泛意义。

使用说明
README文件包含数据集中变量与缩写的解释，以及数据来源的信息。