Wilson Scott, Anderson Eric M, Wilson Amy S G, Bertram Douglas F, Arcese Peter
Environment Canada, Canadian Wildlife Service, Saskatoon, Saskatchewan, Canada.
PLoS One. 2013 Jun 19;8(6):e65408. doi: 10.1371/journal.pone.0065408. Print 2013.
Marine waterbirds have shown variable trends in abundance over the past four decades with some species displaying steep declines along the Pacific coast from British Columbia through California. One of the most dramatic changes has been that of western grebes (Aechmophorus occidentalis) in the Salish Sea. This region was a former core of the species wintering distribution but they have become increasingly rare prompting calls for conservation action. A more thorough understanding of this situation requires the analysis of trends at broader geographic scales as well as a consideration of mechanisms that might have led to a change in abundance. We used hierarchical modeling with a Bayesian framework applied to 36 years of Audubon Christmas Bird Count data to assess continent-wide and regional population trends in western and Clark's grebes (A. clarkii) from 1975 to 2010. Our results show that the North American wintering population of Aechmophorus grebes decreased by ∼52% after 1975, but also that western grebes displayed strongly opposing regional patterns. Abundance decreased by about 95% over 36 years in the Salish Sea but increased by over 300% along coastal California. As a result, the mean centre of the species distribution shifted south by an estimated 895 km between 1980 and 2010. Mechanisms underlying this shift require further study but we hypothesize that it may be related to a change in the abundance and availability of their forage fish prey base. Since the mid-1980s, the Pacific sardine stock off the California coast increased from a few thousand metric tonnes to over two million. At the same time both the abundance and availability of Pacific herring declined in the Salish Sea. Studies are needed to examine this hypothesis further and additional consideration should be directed at other changes in the marine environment that may have contributed to a range shift.
在过去的四十年里，海鸟的数量呈现出不同的变化趋势，一些物种在从英属哥伦比亚到加利福尼亚的太平洋沿岸急剧减少。最显著的变化之一发生在萨利希海的西部䴙䴘（Aechmophorus occidentalis）身上。该地区曾是该物种越冬分布的核心区域，但如今它们越来越稀少，这促使人们呼吁采取保护行动。要更全面地了解这种情况，需要在更广泛的地理尺度上分析趋势，并考虑可能导致数量变化的机制。我们使用贝叶斯框架下的层次模型，对36年的奥杜邦圣诞鸟类统计数据进行分析，以评估1975年至2010年北美西部和克拉克氏䴙䴘（A. clarkii）在整个大陆和区域的种群趋势。我们的结果表明，1975年后，北美越冬的䴙䴘属种群数量减少了约52%，但西部䴙䴘在不同区域呈现出强烈的相反趋势。在萨利希海，其数量在36年内减少了约95%，而在加利福尼亚沿海则增加了超过300%。因此，该物种分布的平均中心在1980年至2010年间向南移动了约895公里。这种变化背后的机制需要进一步研究，但我们推测这可能与它们的食饵鱼类猎物数量和可获得性的变化有关。自20世纪80年代中期以来，加利福尼亚沿海的太平洋沙丁鱼种群数量从几千公吨增加到了超过200万公吨。与此同时，萨利希海的太平洋鲱鱼数量和可获得性都有所下降。需要进一步研究来检验这一假设，并且应该更多地考虑海洋环境中的其他变化，这些变化可能导致了分布范围的改变。
