Reduced habitat suitability and landscape connectivity in a songbird migratory divide

Aim

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.

研究目标
季节性迁徙是动物界的普遍现象，它将地理上相距遥远的生态系统连接起来。目前已有大量研究记录了该行为的显著变异，而迁徙分界（migratory divides，即不同种群为绕开生态屏障而采用不同迁徙路线的种群间接触区）便是这类变异的典型例证。迁徙分界可能对生态物种形成具有重要意义，因为分界区内的杂交个体将采取中间路线，且已有理论预测这类中间路线在生态上处于劣势——杂交个体将不得不跨越亲本种群所规避的生态屏障。本研究以斯温森鸫（Swainson’s thrush）沿海亚种与内陆亚种间一处特征明确的迁徙分界为研究对象，首次对该理论预测进行了检验。

研究区域
北美洲、中美洲与南美洲

研究时段
2006年—2013年

研究主要类群
鸟类

研究方法
本研究采用生态位模型（ecological niche models）与景观连通性分析，预测迁徙过程中沿海与内陆鸫类的适宜栖息地，并识别最优迁徙路线。同时，我们将基于光感地理定位器（light-level geolocators）追踪得到的鸟类实际迁徙路线，与栖息地适宜性及景观连通性的估算结果进行对比。

研究结果
与迁徙分界的理论预测相符，生态位模型估算显示，亲本种群迁徙范围之间的中间区域，其栖息地适宜性更低。此外，我们的景观连通性模型预测，最优迁徙路线将绕开该中间区域。采取中间路线的鸟类，其中途停歇地的预测栖息地适宜性低于沿分界两侧迁徙的鸟类，且其活动范围与最优路径的重叠度也更低。

主要结论
本研究结果证实了理论预测，即迁徙分界区内杂交个体的中间迁徙路线在生态上劣于亲本种群的路线。这一发现表明，生态差异有助于维持迁徙分界，且该结论可能具有广泛的研究意义——全球范围内众多类群的迁徙分界均可能塑造物种边界的分布与维持机制。