Abstract
1. Spatio-temporal variation in population dynamics of migratory populations is shaped by exposure to different environments during the annual cycle. Hence, exposure to similar environments should translate into synchrony in vital rates. Despite a wide-ranging breeding population, the Baltic/Wadden Sea flyway population of eiders (Somateria m. mollissima) shares wintering grounds in the southern Baltic Sea, inner Danish waters and the Wadden Sea; different colonies within this flyway population are therefore likely to exhibit some degree of synchrony in vital rates. 2. Here we used capture-recapture-recovery data to investigate the impact of hunting, winter climate (the North-Atlantic Oscillation Index), winter temperature, nitrogen runoff, autumn-winter body condition of blue mussels Mytilus spp., natural predation and epidemic disease (avian cholera) on annual survival of adult females in ten study colonies distributed between the Netherlands and Finland. Moreover, we tested how the degree of similarity in spatial winter distributions affected the degree of similarity in annual survival among colonies. 3. None of the covariates universally affected female survival. While the quality of blue mussels in the wintering area explained almost 40 % of the variation in survival of eiders breeding on Christianso in the south-western Baltic Sea, incidence of epidemic disease explained > 60 % in two affected colonies. Furthermore, the spatial winter distribution did not appreciably influence annual survival rates in these ten colonies. 4. The lack of universal effects of spatial winter distribution and winter conditions on survival suggests that local breeding conditions may be more important, and hence prime targets for conservation efforts. Better monitoring of e.g. food quality, predation pressure and epidemic disease at the time of breeding could be the key to better understand the population dynamics in this endangered flyway population.
Highlights
Spatio-temporal variation in vital rates of migratory populations is driven by spatial heterogeneity and temporal fluctuations in the environment (Harris et al, 2005; Grosbois et al, 2009; Frederiksen et al, 2014)
We showed that female survival in this flyway population was driven by global as well as local-scale year-to-year variation in the environment, and that variation in survival was not related to the use of a specific wintering location
We found that effects varied considerably even between closely located colonies. This finding suggests that there may be no universal driver of variation in female survival operating in the largely shared wintering area, suggesting a higher importance of environmental conditions at the breeding grounds
Summary
Spatio-temporal variation in vital rates of migratory populations is driven by spatial heterogeneity and temporal fluctuations in the environment (Harris et al, 2005; Grosbois et al, 2009; Frederiksen et al, 2014). Joint analysis of long-term demographic datasets from multiple sites constitutes an obvious potential for examining the importance of different environmental drivers in different sub-populations (Frederiksen et al, 2005). Effective and sustainable management of migratory populations should apply empirical data to evaluate relevant natural and anthropogenic drivers causing population dynamics to change in space and time. Thorough evaluations of how potential drivers affect vital rates and population dynamics require both longterm demographic data as well as time series of temporally matching and ecologically relevant covariate data (Frederiksen et al, 2014). Single-site studies of demographic rates might fail to represent the larger population at a scale appropriate to management (Öst et al, 2016)
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