Why Chinese wintering geese hesitate to exploit farmland

Abstract

Many Northern Hemisphere goose populations are rapidly increasing, the hypothesized result of fitness consequences from shifts to improved winter feeding opportunities on agricultural land. Support for this hypothesis comes from the contrasting declining wintering numbers in China, where geese remain ‘prisoners’ of degrading natural habitats because human disturbance and persecution keeps them from potential farmland feeding areas. Compared with the unfavourable conservation status of many bird species associated with agricultural landscapes (Donald et al. 2001, Peterjohn 2003), Northern Hemisphere wild goose populations wintering in Korea and Japan, North America and Europe are increasing dramatically, some exponentially (Jia et al. 2016, Fox & Madsen 2017). Following over-exploitation and habitat loss up until the middle of the last century, the subsequent implementation of conservation measures, including hunting regulation and provision of protected areas, has undoubtedly contributed to increases in goose numbers. However, mounting evidence suggests that recent shifts from subsisting on natural wetlands to farmland landscapes outside of the breeding season has greatly improved wild goose food intake rates as well as vastly extended their wintering habitat carrying capacity (Abraham et al. 2005, Fox & Abraham 2017, Fox et al. 2017). This has resulted in an ability to meet daily energetic and nutritional requirements in far shorter times on agricultural subsidies, and in all likelihood to attain enhanced body condition (Fox & Abraham 2017). This, in turn, has probably contributed to improved survival and in one population at least seems to have elevated reproductive success (Fox et al. 2005). In stark contrast, goose populations breeding in East Asia and overwintering in China show mostly declining trends (Jia et al. 2016). Telemetry studies showed that 50 wintering individuals of three declining species (Greater White-fronted Goose Anser albifrons and the globally threatened Swan Goose Anser cygnoides and Lesser White-fronted Goose Anser erythropus) were almost entirely confined within natural wetland habitats in China (Yu et al. 2017). In contrast, 17 individuals from two species showing stable trends (Tundra Bean Goose Anser serrirostris and Greylag Goose Anser anser) left wetlands to exploit farmland on 17 and 10% of occasions, respectively (Yu et al. 2017). Although many wintering European and North American geese use wetlands as safe night-time roosts, the majority now glean their food primarily from artificial agricultural landscapes by day (Fox et al. 2005, Fox & Madsen 2017). As ‘prisoners’ of their natural habitats, Chinese-wintering geese have suffered declines as a direct response to reductions in wetland area and degradation of the associated food supply in the globally important Yangtze River floodplain (YRF) wetlands, where the vast majority of wintering geese in China now occur. Swan Geese, for example, grub in shallow mud for highly nutritious below-ground overwintering tubers of the submerged macrophyte Vallisneria, but environmental change has reduced the range and extent of this plant, reflected in the reduced winter distribution and abundance of the species (Fox et al. 2011). Greater and Lesser White-fronted Geese are specialist grazers of ephemeral recessional grasslands and sedge beds, respectively, and both plant communities have disappeared from many wetlands because of hydrological change, denying both goose species vital food resources (Zhao et al. 2012, Wang et al. 2013, Aharon-Rotman et al. 2017). Loss of key autumn feeding habitat for Lesser White-fronted Geese in one studied year had adverse consequences for the accumulation of body stores that persisted right through winter to their spring departure (Wang et al. 2013). Such effects may have particular consequences for all of these goose populations, which are now known to migrate across the Far East Asia taiga forest zone on spring migration to northern breeding areas without refuelling (Wang et al. 2018). Habitat loss and degradation in the YRF have increasingly concentrated these specialist geese at fewer sites in winter, with the majority of formerly widespread Swan Geese, and Greater and Lesser White-fronted Geese now concentrated at just one to two major sites each within the YRF (Yu et al. 2017). These factors explain the unfavourable conservation status of Chinese wintering geese compared with the same and similar goose species wintering in adjacent Korea and Japan, Europe and North America, where geese have become generalist foragers, feeding on agricultural habitats, effectively freeing them from any potential winter food limitation on population size. However, there remains the question: why have Chinese geese failed to liberate themselves from the shackles of wetland habitats to exploit the riches of the modern farmland landscape as other Northern Hemisphere geese elsewhere? Many YRF wetlands are intimately associated with adjacent areas of intensive rice and wheat cultivation, the exploitation of which seemed to have fuelled increases in abundance among the same goose species wintering in adjacent South Korea and Japan. It is difficult to convey to western readers just how intensively disruptive human activity can be towards wild geese in China, even on nature reserves and protected areas. In eastern China, perpetual human presence indulging in multiple activities on farmland adjacent to and within YRF wetlands regularly scare and displace birds in ways unimaginable in the west. China supports 11% of the agricultural area of the world, but the average farm size was 0.6 ha and falling in 2010 (http://www.fao.org/docrep/019/i3729e/i3729e.pdf). This compares with 84 ha in the UK (140 times), 63 ha in Denmark (105 times) and 22 ha in the Netherlands (37 times; http://ec.europa.eu/eurostat/statistics-explained/index.php), where very high densities of geese in the non-breeding season are almost totally confined to agricultural land. Human agricultural activity on the land is also far more intense in China than in North America and Europe. Tractor density per 100 km2 of arable land in the Netherlands is 32 times that in China (see: http://data.worldbank.org/indicator/AG.LND.TRAC.ZS), reflecting the far more labour-intensive presence of workers in Chinese fields. Low rural incomes continue to place great pressures on the local economy, for instance, to raise large numbers of free-ranging poultry, which exploit spilled grain in stubble fields, in extremely high densities, also risking rapid avian disease transmission to wild waterbirds (Cappelle et al. 2014, Choi et al. 2016). Human herders tend large numbers of domestic ducks and geese in rice and cereal stubbles to glean waste grain residues, compounding food depletion for wild geese with human presence as a deterrent to feeding geese. Around Poyang Lake, the most outstanding wintering waterbird reserve in the YRF, 10 million people live and raise more than 14 million poultry (Choi et al. 2016). Such intensive free-range poultry production is not widely practised in Japan or South Korea and remains extremely rare on such scales in North America and Europe (http://www.fao.org/faostat/en/#search/Geese%20and%20guinea%20fowls; http://www.fao.org/faostat/en/#search/Ducks). Intensive animal herding (of goats and domestic buffalo) on grasslands in and around wetlands adds further human disturbance in potential wild goose feeding areas, although this may facilitate goose grazing in some areas. Finally, Chinese wintering geese are very heavily exploited through illegal shooting (despite bans on hunting with shotguns), netting, trapping and poisoning, collectively thought to kill at least 80 000–120 000 waterbirds of about 40 species every year (MaMing et al. 2012). These circumstances suggest that human pressure on farmland adjacent to YRF wetlands ‘imprisons’ wild geese within Chinese wetlands, which explains their current unfavourable conservation status, due to the declining quality of such habitats (Jia et al. 2018). The two goose species showing relatively stable population trends, Greylag and Tundra Bean Geese, were the two species that fed most on agricultural land outside of wetlands, often at night (but even then for < 20% of the time; Yu et al. 2017). These data also support the hypothesis that the shift to winter farmland feeding shown by geese in North America, Europe and elsewhere in Asia has supported their spectacular increases by enhancing fitness and removing limits on carrying capacity imposed by their natural habitats (Fox et al. 2005, Fox & Madsen 2017). Confining Chinese geese to wetlands avoids emerging conflicts witnessed elsewhere in the Northern Hemisphere between burgeoning goose populations and agricultural profitability, air flight safety, biodiversity conservation and ecosystem services (Bradbeer et al. 2017, Buij et al. 2017, Fox & Madsen 2017) as well as impacts on arctic systems (Jefferies et al. 2004, Mariash et al. 2018). However, it also underlines the urgent need for improved knowledge (especially better monitoring of goose abundance, distribution and demography, as well as research into foraging ecology, habitat use and causes of mortality) and better enforcement of existing site safeguard and protection policies and legislation to ensure the survival of Chinese wintering goose populations for future generations.