2018/11/02 by Anthony D. Fox, Anthony David Fox, RaSmus Due Nielsen +2 · 1 citation
Earth and Planetary Sciences · Environmental Science · #Arctic and Antarctic ice dynamics #Avian ecology and behavior #Species Distribution and Climate Change
paper · pdf · doi:10.1111/ibi.12675
Mid-winter sea surface temperatures in the Barents and Kara Seas have shown amplified warming since the mid-2000s, rapidly removing winter sea-ice from very extensive areas along the northern Russian Arctic coast, associated with reductions in the extent of winter sea-ice in the White and Baltic Seas, as well as ice cover in coastal and inland freshwaters throughout the region. Recent northern European studies have shown substantial north and east shifts in the wintering distributions of in particular diving duck species compared with their former distributions in response to climate warming. The very sudden availability of open water in the Russian Arctic offers novel overwintering habitat especially to sea ducks in the marine environment, but also to other species using currently unsurveyed brackish and freshwater areas far to the north and east of their former wintering range. These major climate change effects constitute a particular challenge to our abilities to monitor adequately the distribution and abundance of many of our common ducks, especially the sea ducks, quarry species and other waterbirds of conservation importance, but also potentially many other terrestrial bird populations. There is a proud tradition of waterbird monitoring by citizen scientists in Europe stretching back to the 1940s (e.g. Eltringham & Atkinson-Willes 1961) thanks to concerns raised about habitat loss and overexploitation by hunting of many duck and goose species early last century (e.g. Berry 1939). As a result, and potentially unique among biodiversity monitoring in northwest Europe, we have continuous annual winter counts of common species from some sites extending over 70 years. Such data have been, and continue to be, of outstanding importance when combined internationally with the results of ringing-recovery data to establish the geographical distribution and subsequently monitor the abundance of common waterbirds within discrete flyways (e.g. Boyd 1961, Atkinson-Willes 1976, Boere & Stroud 2006, Guillemain et al. 2017). Such monitoring data have formed the fundamental basis for site safeguard programmes (such as designation of Ramsar sites under the 1% criterion and Special Protection Areas under the European Union Birds Directive), which have made a vital contribution to protecting these populations and maintaining their generally favourable conservation status. Such population monitoring gives early warning of adverse population change (e.g. Žydelis et al. 2006), the role of macro-environment factors acting on population regulation (e.g. Nolet et al. 2013) and feedback monitoring in the case of adaptive management programmes (e.g. Madsen et al. 2017), and is therefore central and vital to our ability to manage these populations effectively. However, we live in times of rapid and profound transformations, not least because of the consequences of global climate change, and our monitoring systems need to adapt to respond to such change. Dramatic northeasterly shifts in the wintering centre of gravity of Tufted Duck Aythya fuligula, Common Goldeneye Bucephala clangula, Goosander Mergus merganser and Smew Mergellus albellus have occurred in recent years as a result of manifold increases in overwintering numbers of wintering waterbirds in Poland, Sweden and Finland (Lehikoinen et al. 2013, Pavon-Jordan et al. 2015, Marchowski et al. 2017). Such partial winter short-stopping (sensu Elmberg et al. 2014) has correlated with increases in average winter temperatures in recipient winter quarters maintaining open shallow water conditions in the Baltic Sea previously denied to these birds in winter by total ice cover. They have also resulted in the contraction of winter ranges at the milder (generally southwest) margins of their former winter distributions (Lehikoinen et al. 2013). For many other species, especially the dabbling ducks, which depend on shallow waters and show specialist feeding niches that require higher midwinter temperatures, geographical shifts in winter range edges in relation to the January 0 °C isotherm are also manifest. However, there has yet to be evidence of the same major shifts in the centres of gravity of their wintering distributions (e.g. Dalby 2013, Dalby et al. 2013, Fox et al. 2016). In the meantime, the rate of change in the extent of winter sea-ice cover to the north of Europe in the last 15–20 years has continued apace. Research shows that the Arctic has warmed disproportionally in recent decades, leading to dramatic reductions in the general extent of sea-ice (Carmack et al. 2015). These effects are far from uniform and some of the greatest temperature increases have occurred in the northern Barents Sea (Screen & Simmonds 2010). Recent studies show that this effect is due to increased oceanic heat content, not simply atmospheric conditions (Lind et al. 2018), because recent declines in sea-ice import to the area have caused a major loss of annual freshwater input. This process has seemingly weakened ocean stratification, and enhanced water column mixing, elevating the rate of the vertical flux of heat and salt, which in turn inhibits sea-ice formation even more, resulting in rapid amplification that has caused dramatic increases in salinity and sea surface temperatures since 2000 (Lind et al. 2018). The result of this regional loss of sea-ice has been even more geographically widespread, enhancing the prevalence of ice-free shallow waters throughout the winter in the White, Barents and Kara Seas (see Fig. 1). As well as creating ice-free marine areas, the exacerbated warming of this large area in winter has maintained areas of associated freshwaters ice-free in these regions, which collectively now offer vast areas of shallow waters as potential wintering quarters for waterbirds, particularly sea ducks in marine areas, that formerly wintered far to the southwest in western Europe. These novel winter quarters for waterbirds have recently become rapidly available, especially in the 2010s, but were completely frozen and therefore inaccessible before ~ 2003 (see Table S1, but also compare with the graphs of change in sea temperatures in Lind et al. 2018). In the Baltic, the presence of ice-free waters throughout the winter potentially enables large numbers of species such as Tufted Duck, Common Goldeneye and Goosander to secrete themselves among the innumerable islands and shallow inshore areas and Long-tailed Ducks to winter in offshore areas around the coasts of Sweden, Finland and Estonia as never before. Although offshore surveys in Estonian and Finnish waters have found modest numbers of Long-tailed Ducks in very recent years, substantial areas remain unsurveyed. In both cases, the recent availability of extensive areas of near-coast and offshore Baltic waters free from winter sea-ice present real logistical challenges to effective surveying of waterbird distribution and abundance in these ‘new’ potential wintering areas. Photoperiod has a marked effect on diel activity patterns of waterbirds (Jorde & Owen 1988) so the ability of wintering waterbirds to colonize novel habitats further north may be hindered by their inability to forage in darkness or to achieve sufficient foraging during short daylight hours. However, many sea ducks and other coastal duck species already habitually winter in areas subject to 24 h of darkness or short day length. These include Greenland Mallard Anas platyrhynchos conboschas, Long-tailed Ducks Clangula hyemalis and King Eiders Somateria spectabilis and piscivorous Red-breasted Merganser Mergus serrator and Great Cormorant Phalacrocorax carbo (which occur as far north as Kanaatsiak at 68°17′N in Greenland; Helms 1926, Salomonsen 1950, Vibe 1950). Common Eider Somateria mollissima also winter in total darkness in large ice-free coastal areas but also in sea-ice leads on the east and west coasts of Greenland north to Qaanaaq (77°27′N) and Wollaston Foreland (74°16′N; Schaanning 1933, Vibe 1950). Many of these species are known to feed during short days or even in complete darkness (e.g. Great Cormorant; Johansen et al. 2001, Grémillet et al. 2005) or, in the case of Long-tailed Duck, Common, King and Steller's Eider Polysticta stelleri, adapt to feeding in darkness (Systad et al. 2000, Systad & Bustnes 2001). Many diving ducks wintering in the temperate region habitually feed in darkness, often actively selecting to feed at night (McNeil et al. 1992), including species such as Tufted Duck, Greater Scaup Aythya marila and Common Pochard Aythya ferina (Nilsson 1970), Common Goldeneye (Duncan & Marquiss 1993) and scoters (Lewis et al. 2005). There is thus every reason to suspect that short winter day length is unlikely to restrict the winter distributions of such species following the removal of former sea-ice cover that formerly denied them accessibility to food supplies where such light conditions prevail. Sites of international importance mapped in Scott and Rose (1996) show that Red-breasted Merganser, Goosander, Greater Scaup, Steller's Eider, Common Scoter Melanitta nigra, Velvet Scoter Melanitta fusca, Common Goldeneye, Mallard and Eurasian Teal Anas crecca all stage along Kanin Peninsula coasts in excess of 10% of their flyway populations outside of the winter period. This implies that a rich and suitable food source exploited during other seasons in this area could now be available in winter, should sea-ice conditions permit access to these. A specific example of such winter partial short-stopping relates to the prolonged declines in European wintering numbers of Steller's Eider, which began to raise serious concerns about the conservation status of species in the Western Palaearctic (Žydelis et al. 2006). However, as a result of the concern, targeted winter helicopter surveys found that these birds had shifted their wintering distributions from Norwegian and Baltic Sea resorts further north and east into Russian waters, which now support 85% of the flyway population, compared with less than 50% in the 1990s (Aarvak et al. 2013). Throughout Russian Arctic areas, we lack observers to report the presence/absence of waterbirds in these waters, especially during 24 h of darkness, so we have no understanding of how wintering waterbirds have potentially adapted to the availability of open water in these areas. We hypothesize that the birds most likely to shift into novel ice-free but winter dark areas (from former areas to the south and west) are the sea ducks, especially Long-tailed Ducks, Red-breasted Mergansers and the eiders, for which we have existing evidence of foraging at high latitudes and during short day length. Ironically, it is the Long-tailed Duck (–65%), Velvet Scoter (–65%), Common Eider (–51%), Common Scoter (–47%), Red-breasted Merganser (–42%) and Greater Scaup (–26%) which are showing the greatest declines in European wintering numbers in the Baltic (according to Skov et al. 2011). We are of course aware that not all sea duck experts accept the magnitude of these decreases in abundance, or that they truly reflect reductions in total population size. Furthermore, we are aware of the complex interaction of other factors thought to contribute, for example, to the Common Eider declines. These include predation of breeding females by White-tailed Eagles Haliaeetus albicilla and American mink Neovison vison (Ekroos et al. 2012, Öst et al. 2016), avian cholera Pasteurella multocida outbreaks (Christensen et al. 1997, Tjørnløv et al. 2013), declines in food supply (Waldeck & Larsson 2013), toxic algal blooms (Larsson et al. 2014) and thiamine deficiency (Balk et al. 2009, Mörner et al. 2017). Other sea duck species are also suffering the effects of additional multiple pressures from offshore wind farms, sand and gravel extraction, fisheries, increased shipping and recreational use, pollution, etc. (Skov et al. 2011). We therefore fully accept that these long-distance migratory waterbird populations are currently under considerable environmental stress. Nevertheless, we suggest that the availability of suitable feeding conditions far to the north and east, formally denied them by sea-ice in the last decade or more, could partially explain recent declines in European wintering numbers of some sea duck species. This is especially the case because Long-tailed Ducks and Common Scoter are known from telemetry studies to pass through the Kara and Barents Sea coasts en route to Baltic wintering areas from their Russian breeding areas (FEBI 2013). Sea ducks are the species that we have struggled most to monitor effectively in recent years, because their offshore distributions do not lend themselves to objective assessment through counts from land. It is therefore ironic that, following successful coordination of complex and expensive professionally based aerial surveys throughout the Baltic in 2016 and with plans to repeat these simultaneously with similar coverage of North Sea coasts in 2020, we should encounter potential leakage of ‘our’ wintering sea ducks north and east into Russian Arctic waters hitherto entirely ice-covered in winter. Due to their dramatic aggregations along relatively restricted coastal waters, the sea ducks are perhaps showing the most obvious changes in abundance, which could be explained by major shifts in wintering distributions. However, milder mid-winter temperatures and more extreme weather conditions because of warmer conditions in the Barents and Kara Seas also maintain extensive areas of freshwaters ice-free throughout northeastern Europe, as for example the large Russian freshwater lakes Lake Lagoda and Lake Onega. These patterns will also increasingly affect other species, ultimately affecting the mid-winter distributions of dabbling ducks, swans and geese. Indeed, such changes in the extent of sea-ice in the Barents and Kara Seas will probably have extensive effects on the terrestrial ecosystems of the entire region. Such major shifts in wintering waterbird distribution present us with increasing challenges to our ability to monitor the distribution and abundance of many common waterbird species. A very large proportion of the European wintering waterbird wealth has always returned to Russia in the breeding season to reproduce, where large numbers of birds disperse over vast areas at low densities and create considerable challenges to our abilities to monitor changes in their annual abundance outside of the mid-winter period. Evidence suggests that numbers of some waterbird species returning to western Europe are declining, linked to falling reproductive success and female sex ratios (e.g. Lehikoinen et al. 2008, Fox et al. 2016, Brides et al. 2017, Fox & Christensen 2018). For this reason, it is becoming increasingly important to be able to differentiate between shifts in distribution and genuine declines in population size. Nowhere is this more important than for the sea ducks (and indeed other seabirds that typically winter offshore), which can immediately benefit from access to open ice-free shallow coastal marine waters (see Fig. 2 to appreciate their extent), now a regular feature of the Russian Arctic coast in mid-winter. More open sea areas will also permit greater shipping traffic to operate along Russian Arctic coasts, with elevated risks from oiling incidents and other potential catastrophes that could affect seabirds outside of the breeding season in these waters, further underlining the need for waterbird monitoring in such areas. As stated above, sea ducks have always required international coordination to monitor effectively, but never more so than now, when we suspect major shifts in distribution may be happening undetected. However, it is far from clear to us how we might survey these areas to census sea ducks at times of year with limited light conditions (although even in mid-February Murmansk experiences almost 7.5 h of daylight; https://www.timeanddate.com/sun/russia/murmansk). The use of high-resolution digital orthophotos will be limited under the given light conditions in these areas with short day length. However, it would be possible to exploit thermal infra-red remote sensing approaches in mid-winter and/or for Russian colleagues to attempt to undertake line transect aerial surveys of these coasts during the latter part of the winter, when daylight hours are sufficient to cover ice-free areas before sea ducks wintering further south and west have returned in spring. These challenges will also increasingly affect our ability to monitor freshwater diving ducks in the future, because these ducks are potentially also very likely to winter in increasingly greater numbers in areas closer to their breeding areas. The new wintering quarters freed from winter ice cover are increasingly likely to lie outside of our existing count networks, particularly in Russia, but also throughout Fennoscandia and the Baltic region. For this reason, as well as reviewing our options for effective winter census techniques and coverage, we should perhaps also consider other methods for generating independent estimates of annual changes in population abundance. Most obvious of these would be to implement a continent-wide series of aerial survey transects from Scandinavia across Russia to sample objectively annual breeding waterbird abundance, as has been done for decades on a continental scale in North America (U.S. Fish & Wildlife Service 2017). The political, logistical and financial challenges to achieving such a goal appear insurmountable, but as a potential optimal solution, we should at least consider the massive of such Recent has shown that population using and population and data across a range of and sample no between a of from complete through to with complete These suggest that could be using data or with where population estimates are from the same source as and data removing the need for independent of population estimates would always be Such would create a for the programmes to such data for the of for populations that are likely to be most to increasingly count monitoring due to range This far greater than is currently the case in targeted and programmes for specific populations. This would data to support our ability to of reproductive and as well as annual population size. Many of the species are quarry throughout of their from North America shows that the use of of marked birds by combined with about and associated with both of considerable about changes in annual flyway abundance, reproductive and (see for example et al. 2009, It may be to such to sea ducks, which remain to and in large to survey at throughout the annual and to sex and and which are increasingly from hunting in European These species in particular continue to considerable challenges to the effective monitoring of their distribution and abundance. There is therefore no that the and wintering distributions of ducks in the Western Palaearctic in the years will be from by Atkinson-Willes and the other before We are not in the of decades of counts to monitor our on the their is to many among these are the need to maintain to show the effects of populations and shifts in as well as to continue to monitor site for to the conservation status of species within sites and other areas. However, it is clear that climate change not our waterbird populations but also increasingly challenges our ability to monitor them effectively in the For this reason, we need to the and of before We need to the challenge of major shifts in distributions of our common waterbird populations with about how to monitor the distribution and abundance of these birds to effect now, and in the future, in the of environmental change. our has been on duck species, and especially sea ducks, the very recent experiences of the rapid rate of change in conditions of the Barents and Kara Seas are also likely to have major consequences for the entire In the this other sea marine and so other effects are also likely to affect sea ducks in We have no of how the of food in winter will affect species, which have previously the same feeding for during to the loss of winter ice that formerly access to such food at other times of We might that there will be similar effects in freshwater systems as However, the same is also of the entire terrestrial of northern Europe, which will increasingly be by the presence of ice-free areas of open sea to the north of the western part of the European The consequences of changes in winter ice are also likely to be in increasing continental shifts in the wintering distribution of many other migratory birds species not associated with areas of open Such changes similar challenges to our monitoring systems for terrestrial species to differentiate between genuine declines in population and shifts in wintering distribution into areas currently outside of our monitoring especially in the to the avian of bird populations in relation to biodiversity 2010). We to the contribution made by innumerable waterbird have the counts over many decades now that us to monitor our waterbird populations. We are especially to our in the around the Baltic in recent years that have made such a contribution to our of the recent regional status of waterbirds and sea ducks in particular and which to for to use data and to and for to The is not for the or of by the than should be to the for the