Erschienen in: Journal of Ornithology ; 154 (2013), 3. - S. 837-847 https://dx.doi.org/10.1007/s10336-013-0950-7 Trends and population dynamics of a Velvet Scoter ( Melanitta fusca ) population: influence of density dependence and winter climate Go¨ran Hartman • Andrea Ko¨lzsch • Karl Larsson • Marcus Nordberg • Jacob Ho¨glund Abstract As many seaduck populations around the world and post-breeding climate 18.3 % of the variance. That have been reported to be in decline, there is an increasing breeding success is density dependent in a delayed manner demand for knowledge about intrinsic and extrinsic factors is in accordance with the apprehension that Velvet Scoters determining population dynamics of these species. In this breed at the age of 2 years. We conclude that density study, we analyzed long-term dynamics of the summer dependence and winter conditions must be taken into population of Velvet Scoters ( Melanitta fusca) breeding in consideration when discussing population changes in sea- the A˚ land archipelago in the Baltic Sea; in particular, we ducks in general and the Velvet Scoter in particular. examined the influence of winter weather and density dependence on population change. The studied population Keywords PBLR Á NAO Á TRIM Á Seaducks Á Baltic Sea exhibited substantial fluctuations but only a weak negative trend during the total period of 58 years (1949–2007), and no significant trend at all during the latter three decades of Zusammenfassung the study (1977–2007). We tested for density dependence and incorporated the winter North Atlantic Oscillation Trend und Populationsdynamik einer Samtentenpopu- index into the model to test for effects of winter conditions. lation (Melanitta fusca ): Einfluss von Dichteabha¨ngigkeit Our final model explained 56.3 % of the variance of pop- und Winterklima ulation growth of the studied population. Delayed density dependence explained 29.7 %, pre-breeding climate 8.3 %, Weltweit gehen zahlreiche Populationen von Meeresenten im Bestand zuru¨ck. Deshalb beno¨tigen wir mehr Einsicht, wie intrinsische und extrinsische Faktoren die Population- Communicated by F. Bairlein. sentwicklung dieser Arten bestimmen. In dieser Studie G. Hartman ( &) Á K. Larsson wurden Langzeittrends einer Brutpopulation von Samten- Department of Ecology, Swedish University of Agricultural ten (Melanitta fusca) untersucht, die auf den A ˚ landinseln in Sciences, Box 7044, 750 07 Uppsala, Sweden der Ostsee bru¨ten. Wir haben insbesondere die Einflu¨sse e-mail: [email protected] von Winterwetter und Dichteabha¨ngigkeit auf die Popula- A. Ko¨lzsch tionsentwicklung untersucht. Es waren betra¨chtliche Fluk- Project Group Movement Ecology, and Department of Animal tuationen, aber nur ein schwacher negativer Trend in der Ecology, Netherlands Institute of Ecology (NIOO-KNAW), gesamten Zeitspanne von 58 Jahren (1949–2007) und kein Droevendaalsesteeg 10, 6708 PB Wageningen, The Netherlands Trend in den letzten drei Jahrzehnten (1977–2007) der M. Nordberg Studie zu beobachten. Mit einem Populationsmodell haben Na¨ringsavdelningen, A˚ lands landskapsregering, PB 1060, wir auf Dichteabha¨ngigkeit getestet und untersucht, ob der 22111 Mariehamn, A˚ land Winter-NAO-Index im Modell als Indikator fu¨r Winter- wetter einen Einfluss auf die Populationsentwicklung der J. Ho¨glund Department of Population Biology, Uppsala University, Samtenten hat. Das beste Modell erkla¨rt 56.3 % der Vari- Norbyva¨gen 18D, 752 36 Uppsala, Sweden anz im Wachstum der untersuchten Population. Verzo¨gerte Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-11f70jkg1nm8y6 838 Dichteabha¨ngigkeit erkla¨rt 29.7 %, das Klima vor der It has been shown that winter weather conditions may Brutzeit 8.3 % und Klima nach der Brut 18.3 % der Var- influence waterfowl population dynamics (Blums et al. ianz. Das Ergebnis, dass Bruterfolg verzo¨gert dic- 2002; Lehikoinen et al. 2006; Jo´nsson et al. 2009). Adverse hteabha¨ngig war, steht in U ¨ bereinstimmung mit der winter weather may entail increased energy costs and Hypothese, dass Samtenten im Alter von zwei Jahren be- render difficulties in finding food. This may lead to star- ginnen zu bru¨ten. Dichteabha¨ngigkeit und Winterwetter vation and subsequent increase in mortality (Blums et al. sind also bei der Betrachtung der Populationsentwicklung 2002) or poor pre-breeding body condition. Endogenous von Meeresenten im Allgemeinen und Samtenten im reserves deposited during winter affects breeding perfor- Speziellen zu beru¨cksichtigen. mance in waterfowl (Meijer and Drent 1999; Parker and Holm 1990) and poor pre-breeding body condition may result in, e.g., decreased fledgling success (Lehikoinen Introduction et al. 2006) or non-breeding (Coulson 2010). In general, the Velvet Scoter is a seaduck breeding During recent decades, numbers of breeding pairs of Velvet mainly by inland waters of the wooded tundra and taiga Scoters (Melanitta fusca) have declined in several parts of zones of northwestern Eurasia (Dementev and Gladkov the Baltic Sea (Andersson et al. 1978; Hario et al. 1986; 1952; Cramp and Simmons 1977). A substantial part of the Berndt and Hario 1997 ; Rintala and Tiainen 2004; Ro¨nka¨ Eurasian population overwinters in the southern parts of et al. 2005; Skov et al. 2011) and is classified as vulnerable the Baltic Sea (Pomeranian Bay, coastal areas of Lithuania according to the IUCN Red list. An exception to this trend is and Latvia, Gulf of Riga). A survey has estimated the the population in the Swedish part of the Botnian bay, Baltic winter population at 373,000 birds (Skov et al. which increased substantially during the 1980s and 1990s 2011). However, Velvet Scoters also breed in the Baltic (Svensson et al. 1999). Several different hypotheses con- Sea. This Baltic breeding population increased from the cerning extrinsic as well as intrinsic factors explaining or 1920s to the late 1950s (Merikallio 1958; Grenquist 1965). contributing to the decline in numbers have been put for- During the first decades of the twentieth century, the ward: poor fledgling rates (Berndt and Hario 1997), eutro- occurrence of Velvet Scoter was confined to the inner phication (Ro¨nka¨ et al. 2005), predation (Hario et al. 1986; zones of the archipelagos, but during the increase the Nordstro¨m et al. 2002), and increased duckling mortality breeding range expanded to also include the outermost due to boating (Mikola et al. 1994). However, long-term islets and skerries (Grenquist 1951, 1952). This expansion surveys of the Velvet Scoter population on the A ˚ land is considered to be due to colonization along regular archipelago (Nordberg 2002) exhibited substantial fluctua- migration routes (Koskimies 1955), and stands in contrast tions in the number of breeding birds over time. This might to the recent decrease of the species. indicate that other factors than the ones mentioned above Because of the availability of population counts over influence Velvet Scoter population dynamics in this area. many decades, the breeding population of Velvet Scoters of The effect of population density on vital rates of many the A˚ land archipelago in the Baltic is very well suited to species is an acknowledged fundamental in population study the interplay of different factors on population den- ecology (Newton 1998). In recent years, several studies of sity. The aim of this study is to analyze the long-term time series of population counts concluded that density- dynamics of this population, concerning trends and possi- dependent processes influence population dynamics of ble influences of winter weather conditions and density waterfowl (Almaraz and Amat 2004; Viljugrein et al. 2005; dependence on population fluctuations. Seavy et al. 2007; Murray et al. 2010). However, in this type of analysis, it is difficult to assess which factors cause the observed density dependence. In some studies, additional Methods information on breeding success was included in the anal- ysis (Elmberg et al. 2003; Nummi and Saari 2003; Hario Study area and Rintala 2006 ). Examples of factors that may cause density dependence are food limitation, nest-site limitation, The A˚ land islands constitute an autonomous province of predation, and disease (Newton 1998). Experimental stud- Finland, situated between Sweden and Finland at the ies with Mallards (Anas platyrhynchos) have demonstrated entrance of the Gulf of Bothnia in the Baltic Sea. They are density-dependent nest predation (Gunnarsson and Elmberg separated from Sweden in the west by 40 km of open water 2008; Elmberg et al. 2009 ), and duckling survival (Gun- and are more or less continuous with the Finnish archi- narsson et al. 2006 ). There is also experimental evidence for pelago in the east. A ˚ land consists of a main island, nearly the impact of food availability on survival of Mallard 300 smaller islands, roughly 6,000 skerries, and ducklings (Gunnarsson et al. 2004). 11,990 km 2 of brackish water. The islands’ landmass 839 occupies a total area of 1,527 km 2 of which the main island and van Strien 2005 ). Therefore, we analyzed our data with constitutes 1,010 km 2. About 58 % of the land area is the freeware program TRIM (v.3.53; Pannekoek and van covered by forest and 13 % by farmland. Mean annual Strien 2005 ). It uses loglinear models that make specific temperature is 5–6 °C, and mean annual precipitation is assumptions about the structure of the counts to obtain better 500–600 mm. trend estimates. The program interpolates missing counts Seaduck males have traditionally been hunted during the from existing counts and can therefore be viewed as an spring on the A ˚ land islands. In the period 1977–1994, the advanced v2 test (Pannekoek and van Strien 2005 ). annual bag was estimated at 20–25,000 males. From 1995, Two of the 15 areas provided data for longer time hunting became strictly regulated and the annual bag periods, 1949–1975 and 1958–2007, respectively. The decreased to 1,500–3,000 males (A ˚ lands landskapsstyrelse, remaining 13 sites provided no data before 1993 and unpublished data).