Poyang Lake has been an important wintering site for Common Cranes. In this paper we monitored the dynamics of population size and spatial distribution of Common Cranes wintering in Poyang Lake during 1998-2011 and analyzed the potential determinants of its population dynamics. This was combined with the monitoring record of the annual maximum number of Common Cranes wintering in the Poyang Lake National Nature Reserve during 1984-2011. We illustrated the changing trend of this Common Crane population over a long time scale, and analyzed the impact of climate variation and water level change on the population size dynamics.
The results showed that the Common Crane population wintering in Poyang Lake significantly increased from 1998 to 2011, with an average population size of 2335±651 and a maximum of 7640 in the winter of 2011. The exponential curve fit well with the population growth trend (R²=0.808, F=46.234, df=12, P=0.000). The wintering Common Cranes clustered in the Poyang lakes, with the lakes of Dahuchi, Dalianzihu, Sanhu, Hanchihu, Qihu, Zhuhu, Nanhu (gongqing), and Dachahu being the hotspots used by them. On average, (74.9±5.6)% of Common Cranes wintered outside nature reserves.
The spatial distribution of the common crane was significantly positively correlated with the areas of mudflat habitat, and was negatively correlated with areas of agricultural land, human density, village areas, and areas of net primary productivity (NPP) in August and November. The mudflat area was an important factor determining the spatial distribution of Common Cranes, which accounted for 15% of the spatial change in occurrence frequency, and when combined with the NPP factor in November, accounted for 24.6% of the spatial change in the average number of Common Cranes. The results mean that there are still factors which may affect the spatial distribution of Common Crane in Poyang lakes which were not assessed, such as competition, the structure of the microhabitat, and human interference, which maybe contribute to a more comprehensive understanding of the impact mechanisms of crane population dynamics wintering in the Poyang Lake.
In the Poyang Lake National Nature Reserve, the average annual maximum number of Common Cranes was 276±51 (n=28), which showed a significant linear increase (R²=0.176, F=5.537, df=27, P=0.026), but there was a drastic annual fluctuation. The results of Pearson correlation analysis indicated that the annual maximum population number of Common Cranes wintering in the Poyang Lake National Nature Reserve was significantly positively correlated with the average air temperature in October (r=0.480, P=0.010, n=28) and the average minimum air temperature in October (r=0.480, P=0.010, n=28), and significantly negatively correlated with the average maximum wind speed in October (r=-0.450, P=0.016, n=28).
As for the water level of Poyang Lake, no significant correlation was found between the annual maximum population size of Common Cranes in the Poyang Lake National Nature Reserve and the annual minimum water level (r=-0.259, P=0.192, n=27), or the annual maximum water level (r=-0.373, P=0.051, n=28). There was also no significant correlation between the population size and the monthly average water level in the wintering period. Although the water level of Poyang Lake in winter was tested and found to significantly change the structural landscape of Poyang Lake wetland, influencing the availability of habitat and food, the water level of those lakes in the nature reserve was largely determined by the artificial aquaculture discharge time and velocity of the lake, which eliminated the influence of water level in Poyang Lake.