Jake R. Walsh; Stephen R. Carpenter; M. Jake Vander Zanden

　　Despite growing recognition of the importance of ecosystem services and the economic and ecological harm caused by invasive species, linkages between invasions, changes in ecosystem functioning, and in turn, provisioning of ecosystem services remain poorly documented and poorly understood. We evaluate the economic impacts of an invasion that cascaded through a food web to cause substantial declines in water clarity, a valued ecosystem service. The predatory zooplankton, the spiny water flea (Bythotrephes longimanus), invaded the Laurentian Great Lakes in the 1980s and has subsequently undergone secondary spread to inland lakes, including Lake Mendota (Wisconsin), in 2009. In Lake Mendota, Bythotrephes has reached unparalleled densities compared with in other lakes, decreasing biomass of the grazer Daphnia pulicaria and causing a decline in water clarity of nearly 1 m. Time series modeling revealed that the loss in water clarity, valued at US$140 million (US$640 per household), could be reversed by a 71% reduction in phosphorus loading. A phosphorus reduction of this magnitude is estimated to cost between US$86.5 million and US$163 million (US$430–US$810 per household). Estimates of the economic effects of Great Lakes invasive species may increase considerably if cases of secondary invasions into inland lakes, such as Lake Mendota, are included. Furthermore, such extreme cases of economic damages call for increased investment in the prevention and control of invasive species to better maximize the economic benefits of such programs. Our results highlight the need to more fully incorporate ecosystem services into our analysis of invasive species impacts, management, and public policy. （来源：PNAS, 2016, doi: 10.1073/pnas.1600366113）

　　　美国威斯康星州的Mendota湖的一种入侵物种已经减少了水透明度大约1米，其损失相当于所谓的“生态系统服务”损失1.4亿美元。生态系统服务包括人类从自然环境中得到的收益，但是对这类服务的损失以具体财务术语的评估仍然具有挑战性。研究人员评估了Mendota湖由于长柱尾突蚤造成的水透明度下降的经济影响，长柱尾突蚤是一种非原产的捕食性浮游动物物种，在20世纪80年代入侵了美国的大湖地区。作者首先通过更新一个2001年的分析，评估了目前水透明度的损失大约是1.4亿美元，即每户640美元，那个2001年的分析估计了当地居民为1米的水透明度付款的意愿。之后，作者使用一个数学模型模拟了随着时间推移水清晰度的变化，他们确定了减少来自农场径流的磷可能逆转这种破坏，其成本是8650万到1.63亿美元。这组作者说，这些发现证明了入侵物种造成的经济影响。