| Habitat study is a basic research topic in ornithology, which is particularly important for the conservation of endangered bird species. Scaly-sided Merganser is a globally endangered species restricted to eastern Asia, and it is also listed as the first class protected animals in China. However, major gap exist between its population estimate in wintering ground and breeding areas. The wintering habitat requirements of Scaly-sided Merganser are qualitative and vague, even in contradiction, which requires thorough studies and knowledge update.This study focuses on the following questions:1) How to estimate the wintering population of Scaly-sided Merganser in mainland China? 2) What are the factors that influence its wintering habitat selection? 3) The importance of maintaining natural river ecological characters on Scaly-sided Merganser conservation.The study compiled 107 geo-referenced sighting records from the field surveys, literature review, and from other publicly accessible databases. To predict the potential distribution range of the Scaly-sided Merganser, we used maximum entropy implemented in the Maxent package, and then, overlaid the suitable habitat polygons with river polylines, and summed up all intersected river length. Combined field survey and previous study data, we calculated the population density of Scaly-sided Merganser, and estimated its population by projecting the observed population densities to the entire modelled distribution rivers. Secondly, field survey of Scaly-sided Merganser were conducted every second week from November to February in three winters (2010/2011,2011/2012,2012/2013), and acoustic surveys on fish resource were conducted using BioSonics DT-X echosounder in Yuan River, at both upstream and downstream of Lingjintan Dam. Visual Analyzer and Visual Habitat were used for the analysis of echo data of fish and water depth. Then structural equation model was built to study the direct and indirect effects. Furthermore, to study effect of these habitat characters on the distribution of Scaly-sided Merganser by comparing river landscape, water depth, and fish abundance upstream and downstream. Thirdly, in the downstream of the dam, combined with gravel bars landscape analysis in Fragstats 3.3 and data of human disturbance, we then built generalized linear models to explore the effects of gravel bars on the presence of Scaly-sided Merganser. The main findings include:1) On large scale, the most significant environmental variables include annual mean temperature, mean temperature of coldest quarter, minimum temperature of coldest month and precipitation of driest quarter. Suitable conditions for Scaly-sided Merganser are predicted in the middle and lower reaches of the Yangtze River, especially in Jiangxi, Hunan and Hubei Provinces. The predicted suitable habitat embraces 6,984 km of river length. Based on survey results from three consecutive winters and previous studies, we estimated the entire wintering population of Scaly-sided Merganser in mainland China to be 3,561 ± 478 individuals.2) Based on the structural equation model, fish abundance had a direct effect of 0.44 on Scaly-sided Merganser. The total effect of river landscape was 0.4672, and the the total effect of water depth was 0.4552. The model explained 64% of the variations in the duck occurrence and 63% in fish abundance. The average water depth in upstream of the dam is 10.7m higher than that downstream. There are also significant differences in the fish density at each water depth, as well as the occurrence probability of Scaly-sided Merganser between upstream and downstream river section.3) During the whole 24 surveys (2011-2013),514 Scaly-sided Mergansers, belonging to 127 flocks were counted. The number of individual in a flock varied from 1 to 20. All these generalized linear models had considerable prediction power: based on the psedu-R2, the models explained 67% -78% of the variations in the duck occurrence. Among these predictor variables, CONTIG_AM (Area weighted mean Contiguity Index), PARAAM (Area weighted mean perimeter area ratio) and BLD (Number of buildings per km along the river) were the top variables with the high importance to predict the occurrences of the Scaly-sided Merganser based on model Wi. Scaly-sided Merganser tended to congregate at river segments with more gravel patches; with clear preference for larger and contiguous gravel patches; and the number of houses along the river bank (a proxy for anthropogenic disturbance) had significantly negative impacts on the occurrence of the Scaly-sided Merganser.The wintering population estimate in the study is consistent with estimate in its breeding ground, which confirms the Scaly-sided Merganser is still in endangered status. Dam construction has changed the river depth and gravel bars landscape, and influenced the distribution of fish, and therefore causes negative effects on Scaly-sided Merganser distribution. Gravel bars are vital to the Scaly-sided Merganser wintering as it provides shelters from disturbance, as well as sites for feeding and roosting.The study suggests that maintaining the exposure of gravel bars in regulated rivers during the low water period in winter is the key for the conservation of the Scaly-sided Merganser, and should be taken in to consideration at the municipal planning and dam construction. Equally important, within-channel alluvial gravel extraction is one of the important forms of anthropogenically induced morphological change in river channels, and is causing gravel-size sediment starvation, which leads to lowering of riverbed and decrease of gravel bars, therefore sand mining and gravel extraction should be restricted and regulated. We also suggest taking landscape of gravel bars into index base of River Rapid Assessment. Furthermore, illegal fishing should be under strict supervision. |
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