The links between global climatic cycles and the diversification and migration of Arctic shorebirds = La relación entre los ciclos climáticos y la diversificación y migración de las aves limícolas árticas

Abstract

In this thesis we studied how the changes in the climate determine the distribution, diversity and conservation of a very representative group of the Arctic avifauna, the shorebirds. In the first chapter we explored the changes in the ranges of the Arctic shorebirds during the Pleistocene glacial cycles, and how that shaped the current diversity of subspecies. We combined species distribution models, fossil evidence and estimation on the distribution of the tundra to compare the distribution of the breeding and wintering ranges between the present (representing an interglacial period) and the last glacial maximum (LGM) ca. 21,000 years ago. We found that the species with described subspecies predominantly experienced fragmentation of their breeding ranges during glacial cycles, especially during the glacial periods. On the other hand, most of the monotypic species maintained continuous breeding ranges during glacial and interglacial periods. This supports that the Pleistocene glacial cycles provided a mechanism for allopatric speciation that originated the current pattern of intraspecific diversity in these species. Additionally, we recovered that, despite changes in the breeding ranges, there was an overall lack of overlap with the wintering ranges and the long-distance migration between them remain uninterrupted during glacial periods. This migration likely contributed to originate and maintain the diversity between populations over multiple glacial cycles. While the first chapter provided a spatial mechanism of diversification during the glacial cycles, this needs to be confirmed by genetic data to support that the timing of this diversification falls within the Pleistocene. To better estimate divergence time within species, we performed the most comprehensive calibration of the mitochondrial molecular clock in birds to date. We included full mitochondrial genomes from 621 bird species and 25 reliable fossil calibrations and estimated the substitution rates for each of the mitochondrial genes in every lineage of the phylogenetic tree. We applied the obtained rates to molecular clock and coalescent hypothesis testing techniques in multiple Arctic shorebird species. We found that overall the diversity within the species originated during the Pleistocene, and especially during the last 900,000 years, coinciding with a period of longer and more intense glacial periods. Species with similar patterns of past and present distribution showed parallelism in their patterns of genetic diversification. Since the climate has played such a key role in the origin of the current diversity of these species, in the third chapter we explored the potential effects of the current climate change in their future distribution and conservation. We performed species distribution models to forecast their breeding ranges over three different climate change scenarios over the current century. The results show that the species' ranges will displace northwards due to the warming climate, and many species will experience severe range reduction as they are already at the northernmost part of the continents. This is especially critical in the Palearctic, while in the Nearctic the Arctic Archipelago could provide shelter for the species to expand their breeding ranges. We also estimated the potential changes in the extent of the Arctic and its ecosystems since the last interglacial and until the end of the century. Our results indicate that although the species have previously experienced extreme changes in the Arctic conditions, the current rate of change greatly exceeds those from recent events. This could lead to even more drastic range reductions and population declines than predicted and threaten all the diversity that originated during the Pleistocene. As a whole, this thesis provides an integrative perspective on the effects of the climate origin and conservation of the diversity within Arctic species. This work sheds light on the detailed spatial and temporal mechanisms that promoted the current diversity across a large group of Arctic birds, and the implications that the current climate change will have in the persistence of the Arctic biodiversity.

En esta tesis se estudia el papel de los cambios en el clima en la diversificación, distribución y conservación de las aves limícolas que crían en el Ártico. En primer lugar, se exploró el efecto de los ciclos glaciares del Pleistoceno (desde hace 2,6 millones de años) en la distribución y migración de estas aves, y sus implicaciones en el origen de la diversidad actual de subespecies. Para ellos combinamos modelos de distribución de especies y datos fósiles, comparando los rangos potenciales de cría e invernada del presente y del último máximo glacial (hace 21.000 años aproximadamente). Los resultados muestran que las especies con subespecies experimentaron mayoritariamente fragmentación del rango de cría, especialmente en el periodo glacial, mientras que las monotípicas mantuvieron rangos de cría continuos. Nuestros resultados también sugieren que la migración entre cría e invernada se mantuvo ininterrumpida incluso durante periodos glaciares, y pudo haber reforzado el proceso de diversificación entre poblaciones. Para confirmar el origen pleistocénico de la diversificación, recurrimos a análisis basados en datos genéticos. Realizamos una calibración del reloj molecular de los genes mitocondriales para todas las aves, y aplicamos las tasas obtenidas en las limícolas para analizar la diversificación en varias especies. Los resultados apoyan que diversificación intraespecífica de las especies estudiadas tuvo lugar durante el Pleistoceno, y en especial en los últimos 900.000 años, coincidiendo con un aumento de intensidad y duración de los periodos glaciares.

Finalmente, evaluamos los posibles cambios en los territorios de cría de las limícolas árticas en relación al cambio climático. Observamos un desplazamiento hacia el norte de las distribuciones de cría, que en el Paleártico implica una pérdida significativa de los rangos de muchas especies que ya se encuentran distribuidas en el límite septentrional del continente. En el Neártico en cambio el archipiélago Ártico podría proporcionar nuevos territorios y paliar la perdida de rango. No obstante, el ritmo actual del cambio climático supera todos los precedentes que han experimentado estas especies en su historia evolutiva reciente, y supone un gran riesgo para la conservación de la diversidad originada en el Ártico durante el Pleistoceno.