DOI: 10.5061/dryad.qv9s4mwdm
title: Eiders, nutrients and eagles: Bottom-up and top-down population dynamics in a marine bird_dataset

description: The main objective of this long-term study (1978-2016) was to find the
 underlying factors behind the declining trends of eider Somateria
 mollissima in the Baltic/Wadden Sea. Specifically, we aimed at quantifying
 the bottom-up effect of nutrients, through mussel stocks, on reproduction
 and abundance of eider, and the top-down effects caused by white-tailed
 eagle Haliaeetus albicilla predation. Bottom-up effects increase marine
 primary productivity with subsequent effects on food availability for a
 major mussel predator. Top-down effects may also regulate eider
 populations because during incubation female eiders are vulnerable to
 predation by eagles. Our structural equation modelling explained a large
 percentage of the variance in eider abundance. We conclude that the
 Baltic/Wadden Sea eider population was regulated directly by white-tailed
 sea-eagle predation on incubating females and indirectly by the amount of
 nutrients in seawater affecting both mussel stocks and the breeding
 success of eiders, reflecting density dependence. These findings may
 explain the decreasing trend in the Baltic/Wadden Sea eider population
 during the last decades as an additive effect of top-down and bottom-up
 factors, and likely as an interaction between them.
description: Eider numbers Due to lack of
 long-term estimates of the Baltic/Wadden Sea eider population the Danish
 eider bag during 1960-2003 was used as a proxy for population size. After
 2003, the open season was shortened, and thus not comparable to population
 size. The eider bag increased from 93,000 individuals in 1960 to a maximum
 of 190,300 in 1982, followed by a decrease to 75,200 in 2003. It is
 mandatory for hunters in Denmark to report their bag after each hunting
 season, and in most years, more than 85% of all hunters report the bag.
 Detailed information includes assessment of reliability by Strandgaard and
 Asferg (Strandgaard & Asferg 1980) and by Christensen and Hounisen
 (Christensen & Hounisen 2014). Significant positive correlations
 between the Danish eider bag and counts of eiders in Finland, Sweden and
 at migration sites is given in ESM (S1).
 Breeding success of eider The ratio of
 juveniles to the number of adult females in wing samples reported by
 hunters under the Danish Wings Survey scheme was used as estimates of
 breeding success (Clausager 2004; Christensen 2013). We included wings
 from adult females and juveniles shot during the first two months (1
 October - 30 November) of the open season during 1982-2003, which provided
 an average (SE) of 514 (44) wings per year, all of which were sexed and
 aged (Boyd., Harrison & Allison 1975). Comparison of the breeding
 success index and fledging success (log<sub>10</sub>
 transformed) measured in the large eider colony at Tvärminne, Finland show
 a significant, positive relationship (linear regression: F = 4.88, df =
 1,18, p = 0.041, estimate (SE) = 0.2801 (0.1268), data from Markus Öst for
 1990-2014 (no data available for 1992-1993)).
 Monitoring of white-tailed sea-eagles During
 May to June all suitable sites were monitored to assess occupancy and
 breeding success by a team of experienced volunteers. Due to building of
 more nests in a territory search for alternative nests were done. However,
 logistic constraints restricted it in some territories. A nest was
 considered occupied when it was decorated with fresh twigs. Of the
 occupied nests, those with at least one fully-grown young was considered
 successful. In total 150 territories of white-tailed sea-eagles were
 monitored during 1978-2016 at Åland, Finland. The territories have been
 surveyed for 1-48 years, and 28 territories have been followed for more
 than 20 years. Fertilizer and outlet of
 nitrogen Fertilizers are used by farmers to stimulate
 growth of the crops. However, parts of the fertilizers are not used by
 plants, but are transported via rivers to coastal waters, where they
 stimulate plankton growth. Thus, fertilizer and the amount of outlet is
 two measurements of nitrogen supply to the environment (earth and marine
 water).  An index of fertilizer (1950 = 100) used by Danish farmers during
 1920-1998 is reported by Duus and Zinglersen (Duus & Zinglersen
 2000) and for 1990-2016 by Blicher-Mathiesen et al. (Blicher-Mathiesen
 <i>et al.</i> 2019). The two data sets were calibrated for
 1998 by estimating the means of the two data sets during the overlap
 period (1990-1998, correlation between the two data set was
 r<sup>2</sup> = 0.96), and multiplying the values from
 Blicher-Mathiesen et al. (Blicher-Mathiesen <i>et al.</i>
 2019) by the ratio. The index of fertilizer increased from 44 units in
 1920 to 385 units in 1991 subsequently decreasing to 272 units in 2015.
 The annual outlet of total-N (t) via rivers to coastal waters in Denmark
 was used as an indicator of nutrients in the marine environment at the
 winter grounds as given by Conley et al. (Conley <i>et al.</i>
 2007). Total outlet of total-N increased from 54,415 t in 1960 to 124,311
 in 1982, and decreased afterwards to 46,250 t in 2003.
 Estimation of mussel stocks The total annual
 stocks of blue mussels <i>Mytilus edulis</i> have been
 estimated (metric t) for the intertidal zone in the Danish (1986-2007) and
 Schleswig-Holstein (1988-2015, missing values in 1994-1997) parts of the
 Wadden Sea (Kristensen, Borgstrøm & Laursen 2006; Büttger, Wittes
 & Nehls 2012). There was a positive correlation between the annual
 mussel stocks in the two parts of the Wadden Sea (<i>r</i> =
 0.92). The data suggest that mussel stocks develop synchronously across
 years and in large marine areas (Nehls <i>et al.</i> 2009).
 The mussel stocks increased from 64.100 t in 1986 to 171.800 t in 1994
 after which it decreased to 43.350 t in 2013. Further information on
 estimation of mussel stock is given in ESM (S1). The
 dataset contains the metadata in the second excel sheet, describing each
 field. 
description: The dataset contains the metadata in the second excel sheet,
 describing each field.  Additional information about
 the dataset could be asked to the corresponding author: E-mail: anders.moller@u-psud.fr