Global change and ecosystem connectivity: How geese link fields of central Europe to eutrophication of Arctic freshwaters
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Migratory connectivity by birds may mutually affect different ecosystems over large distances. Populations of geese overwintering in southern areas while breeding in high-latitude ecosystems have increased strongly over the past decades. The increase is likely due to positive feedbacks caused by climate change at both wintering, stopover sites and breeding grounds, land-use practices at the overwintering grounds and protection from hunting. Here we show how increasing goose populations in temperate regions, and increased breeding success in the Arctic, entail a positive feedback with strong impacts on Arctic freshwater ecosystems in the form of eutrophication. This may again strongly affect community composition and productivity of the ponds, due to increased nutrient loadings or birds serving as vectors for new species.
KeywordsArctic Connectivity Eutrophication Migration
Ecosystems are rarely closed entities, and with few exceptions like islands, lakes, isolated forests and mountain areas, boundaries are often arbitrarily defined. Moreover, even lakes and islands are clearly affected by their surroundings and neighbouring ecosystems. For rivers, the concept of ecosystem connectivity or donor-fed systems originates from the observation that catchment properties affect recipient systems in fundamental ways (Polis et al. 1997; Bartels 2012), which also holds for lakes (Cloern 2007; Soininen et al. 2015). Aquatic ecosystem connectivity often deals with adjacent ecosystems, e.g. where litterfall or dissolved organic matter from catchments may serve as an energy subsidy to aquatic systems (Jansson et al. 2007; Bartels 2012; Soininen et al. 2015).
Ecosystems may also be connected over long distances. Migratory animals often represent the most conspicuous and long-range type of ecosystem connectivity both with regard to nutrients, organic matter, toxicants, propagules, parasites and pathogens, as well as by direct or indirect trophic effects (Bauer and Hoye 2014). For aquatic ecosystems, migrating fish often represents major fluxes of energy and nutrients, e.g. post-spawning carcasses from anadromous salmon-fertilizing rivers or rivers banks (Cederholm et al. 1999). In such cases, there is also a feedback component involved, since litter fall (from land) may promote survival and growth of fish fry. This may also be linked to trophic cascades within the ecosystem, where fertilization may boost autotroph production, propagating up the trophic ladder (cf. Ripple et al. 2001). Also birds may constitute important links between distant ecosystems (Webster et al. 2001; Jefferies et al. 2004a, b), especially in the context of nutrient loads (van Geest et al. 2007; Hahn et al. 2008; Dessborn et al. 2016).
The major transitions or degradation of ecosystems worldwide, combined with climate change and change in population size of many migrating animals may affect ecosystems profoundly (Bauer and Hoye 2014; Doughty et al. 2016). Here we will use goose migration and Arctic freshwater ecosystem impact as an illustration of this interplay between changed climate and management regimes, and how it may affect properties of distant ecosystems.
States of ecosystem connectivity
It should be stressed that there are gradual transitions from (1) to (4), and this is not an exhaustive list of types of connectivity. There has been a dramatic decline of many animal populations worldwide, which may have a huge impact on global rates of nutrient transport (Doughty et al. 2016). Additionally, structural changes in ecosystems with loss of apex predators may have cascading effects down the trophic ladder (Strong and Frank 2010) and may also affect migrating species both positively and negatively. While climate change is likely to impose further constraints on many species and populations, it may however also in cases promote population increase and give some literally far-reaching and unforeseen consequences. Below we will describe the development of the geese breeding in the high-arctic archipelago of Svalbard, a typical example of this scenario, and also point to the severe ecosystem impacts in the high Arctic.
The Svalbard case
Drivers and development of Arctic goose populations
In temperate regions, agricultural schemes have been used as a “green policy” (Madsen et al. 2014) providing agricultural land to grazing geese (Owen 1977; van Eerden 1990; Patterson and Fuchs 2001; Tombre et al. 2013), further increasing the survival of the European goose populations. For the Svalbard barnacle geese, most of the wintering areas in UK are protected agricultural land (Cope et al. 2003), being one of the main reasons for the population’s success (Owen 1977). For the pink-footed geese, the improved climate on the nesting grounds at Svalbard has increased their breeding success significantly over the last decade (Madsen et al. 2007; Jensen et al. 2014). More pink-footed goose pairs are able to find nest sites within the narrow time window, characteristic for the arctic-breeding conditions. A series of seasons with early snowmelt has caused an almost exponential increase for this population over the past decade (Fig. 3, Madsen and Williams 2012; Madsen et al. 2013). Earlier spring development, and thus an extended growing season, is likely to continue over the coming decades as judged from climate scenarios (Førland et al. 2011). Accordingly, this may also expand the distribution of the goose species in Svalbard, as has been predicted for the pink-footed goose population (Jensen 2008; Wisz et al. 2008). Future scenarios for goose population sizes however not only depend on direct effect of warmer climate and extended growing seasons, but also on indirect effects, e.g. polar bears have increased the goose egg-predation rate in Svalbard (Prop et al. 2015), farmland practices in overwintering areas and stopover sites as well as management actions in the form of increased hunting pressure may also reduce the survival rate for geese on the long term (Madsen and Williams 2012).
Regardless of future population scenarios, a large number of geese are at present affecting surface waters at their breeding grounds in Svalbard. They release nutrients in the watersheds and directly in the water bodies, and in Svalbard such ponds are mostly shallow permafrost ponds in coastal areas where the geese breed and graze (van Geest et al. 2007). The goose-mediated effect will add to the direct stimulatory effects of climate change for primary production in arctic lakes and ponds, due to warming of the ponds (Quinlan et al. 2005; Smol and Douglas 2007) as well as climate impacts to the surrounding soil and plant communities, resulting in increased fluxes of terrestrial organic matter and nutrients to the ponds (Luoto et al. 2015; Smol et al. 2005). Geese may also strongly affect community composition and food web structure of the Arctic freshwater by potentially serving as vectors for spreading of invertebrates, plants and microorganisms (cf. Green 2002; Figureola and Green 2002). This implies linkages between freshwater and terrestrial environments, and demonstrates that ecosystem effects at lower latitudes, i.e. increased survival of geese during winter and migration, may have local consequences in arctic ecosystems.
For the case with geese and ponds in Svalbard, the patterns in connectivity between temperate and arctic regions have shifted from cases (2) to (4) over the last decades (cf. Fig. 1). Goose numbers were previously controlled both by restricted breeding areas, short breeding seasons, winter mortality, but as the conditions have improved, survival and breeding rates have increased, so will the impacts on arctic ponds in the form of fertilization from larger goose populations also increase.
Goose-promoted eutrophication in the arctic
The surveyed ponds in recent years were distributed over three main areas: Isfjorden, Kongsfjorden and Northern localities (Fig. 4). They displayed a wide span in nutrient concentrations, ranging from <1 (detection limit) up to 60 µg Phosphorus l−1 at the Isfjorden sites, from 5 to almost 80 μg P l−1 in the Ny-Ålesund area in Kongsfjorden, while the northernmost sites spanned from <1 to almost 150 µg P l−1. During surveys in 2003–2014, geese exerted a variable impact on the ponds at all these sites, and comparisons between bird impacted and non-impacted ponds gave strong evidence of a eutrophication mediated by birds (van Geest et al. 2007: Alfsnes et al. 2016).
From this previous survey, average P was 4.3 µg P l−1, and the maximum was 18 µg P l−1. Hence, the average concentrations of P from this area have increased fourfold over 50 years compared to the samples in 1962, and a close association between high levels of nutrients and visual signs of goose activity (droppings, feathers) has been reported from the area (van Geest et al. 2007).
Also for the Kongsfjorden area and the Northern localities, the most eutrophied sites had the most prominent signs of geese (or other birds) in terms of droppings and feathers at the shores (van Geest et al. 2007; Alfsnes et al. 2016). Hence, it is likely that birds, and in most cases geese, were the primary source of nutrients to these localities. Moreover, the increasing numbers of geese have also influenced properties of the water bodies by providing organic carbon via droppings, which changes the vegetation cover, which again changes the runoff. In a study by van Geest et al. (2007), it was also demonstrated that molar N:P-ratio of fresh droppings on the ground from barnacle geese was in the range of 6–9. In ponds where N was analysed, it appeared to be closely correlated with P, indicating that P mostly had a biotic origin (i.e. not related to inorganic clay particles).
Increased nutrient concentrations may not necessarily result in a higher standing stock of phytoplankton, since a large fraction of primary producers in these systems are benthic algae (Rautio and Vincent 2006). Moreover, the fact that zooplankton grazers in all these fishless systems constitute the top trophic levels implies a strong grazing pressure and low autotroph biomass in the open waters, since nutrients are channelled into zooplankton (Van Geest et al. 2007; Van der Wal and Hessen 2009). Hence, the fertilization may indirectly affect not only productivity, but also shifts in the relative abundance of species in the community by promoting more nutrient-demanding species of both autotrophs and heterotrophs.
Another consequence of increased bird migration is the potential of transporting zooplankton resting eggs, or stages, via gut content or feathers on geese. It may promote the establishment of invertebrate invaders and infectious diseases (bacteria, fungi, unicellular parasites) both between Svalbard localities and potentially also from mainland Europe to the Arctic. While we still have insufficient data to actually link species shifts and new species to birds or climate changes (or a combination of both), this will be an important task to address in the future. There are, however, already at present pronounced differences in both clonal and species composition of Daphnia in ponds with different nutrient status, which may be related to the impact of geese (Van Geest et al. 2007; Alfsnes et al. 2016). If larger parts of the high Arctic become a pre-breeding area for geese (Hubner 2006), and there will be an expansion of the breeding distribution (Wisz et al. 2008), this will increase the probability both for bird-induced dispersal of zooplankton species and community shifts due to eutrophication.
Collectively, our synthesis demonstrates how changes in climate and land use in terrestrial ecosystems in Central Europe may have far-reaching consequences for “pristine” and completely different ecosystems thousands of kilometres further north. The improved conditions in the high Arctic (from a goose perspective), partly related to climate change and extended growth season, serve as a feedback affecting the overwintering habitats in terms of more geese. The development and impacts reported in the present study have some similarities to those reported on the North American continent, where increasing numbers of snow geese (Anser caerulescens caerulescens) have resulted from improved overwintering conditions. This cause a set of impacts on the salt marches at the arctic La Pérouse Bay (Canada) due to intensified grazing, grubbing and nutrient cycling (Jefferies et al. 2004a, b), demonstrating a shift from case (2/3) to case (4) as described in Fig. 1. At the Svalbard sites, there are signs of grubbing and grazing by geese on the tundra (Van der Wal et al. 2007; Speed 2009), but even more striking are the impacts on the freshwater ecosystems in the form of nutrient enrichment. This demonstrates the often unforeseen and complex effects of climate and land use on ecosystems due to ecosystem connectivity, highlighting the need for integrated and international ecosystem management.
This paper has benefited from data gathered from several projects, notably FRAGILE-project (Fragility of Arctic Goose Habitat: Impacts of Land Use, Conservation and Elevated Temperature) and DWARF (Declining Size—a general response to Climate Warming in Arctic Fauna?) funded by the Norwegian Research Council (NFR). We are indebted to our colleagues in these projects. We are also most grateful to Håvar Skaugrud for use of data from his master thesis “Effects of increased temperature and bird migrations on zooplankton communities in the high Arctic”.
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