Abstract
Identifying drivers of population trends in migratory species is difficult, as they can face many stressors while moving through different areas and environments during the annual cycle. To understand the potential of migrants to adjust to perturbations, it is critical to study the connection of different areas used by different populations during the annual cycle (i.e. migratory connectivity). Using a large-scale tracking data set of 662 individual seabirds from 2 sympatric auk meta-populations (common guillemots Uria aalge and Brünnich’s guillemots U. lomvia) breeding in 12 colonies throughout the Northeast Atlantic, we estimated migratory connectivity in seasonal space use as well as occupied environmental niches. We found strong migratory connectivity, within and between species. This was apparent through a combination of seasonal space use and occupied environmental niches. Brünnich’s guillemot populations grouped into 2 and common guillemot populations into 5 previously undescribed spatiotemporal clusters. Common guillemot populations clustered in accordance with the variable population trends exhibited by the species, while Brünnich’s guillemot populations are declining everywhere where known within the study area. Individuals from different breeding populations in both species were clustered in their space and environmental use, utilising only a fraction of the potential species-wide range. Further, space use varied among seasons, emphasising the variable constraints faced by both species during the different stages of their annual cycle. Our study highlights that considering spatiotemporal dynamics, not only in space but also in occupied environmental niches, improves our understanding of migratory connectivity and thus population vulnerability in the context of global change.
Highlights
Migratory animals face specific challenges in a rapidly changing world, such as loss of habitat, new physical barriers, overexploitation of seasonal food resources and climate change impacts (Wilcove & Wikelski 2008, Robinson et al 2009)
Both species exhibited little inter-population mixing between their identified clusters, with Common guillemot (COGU) even less so than Brünnich’s guillemot (BRGU) (Table S1.4)
We provide evidence of strong migratory connectivity within and between 2 congeneric seabird species at an ocean basin scale, and highlight the importance of considering space use, and its seasonality and occupied environmental niches
Summary
Migratory animals face specific challenges in a rapidly changing world, such as loss of habitat, new physical barriers, overexploitation of seasonal food resources and climate change impacts (Wilcove & Wikelski 2008, Robinson et al 2009). Assessing the response of migratory species or populations to perturbations requires an understanding of migratory connectivity (Taylor & Norris 2010), which is the connection of different areas used by different populations during the annual cycle It is typically described along a continuum from low ( termed weak, or diffuse) to high (or strong) (Webster et al 2002). Strong migratory connectivity (i.e. population-specific non-breeding areas) is necessary for differential population trends of geographically distinct breeding populations to be driven by factors away from the breeding sites (Kramer et al 2018). Another aspect of migratory connectivity is population spread, which refers to the size of the geographic area occupied by a population during different parts of the annual cycle (Finch et al 2017). Populations with smaller geographic spread have a limited variety of migratory movements and destinations and may be more vulnerable to perturbations than those with larger spread (Cresswell 2014, Gilroy et al 2016)
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