Phillips Donald L, Newsome Seth D, Gregg Jillian W
US Environmental Protection Agency, Office of Research & Development, Western Ecology Division, National Health & Environmental Effects Research Laboratory, 200 SW 35th St., Corvallis, OR, 97333, USA.
Oecologia. 2005 Aug;144(4):520-7. doi: 10.1007/s00442-004-1816-8. Epub 2005 Feb 16.
Stable isotope mixing models are often used to quantify source contributions to a mixture. Examples include pollution source identification; trophic web studies; analysis of water sources for soils, plants; or water bodies, and many others. A common problem is having too many sources to allow a unique solution. We discuss two alternative procedures for addressing this problem. One option is a priori to combine sources with similar signatures so the number of sources is small enough to provide a unique solution. Aggregation should be considered only when isotopic signatures of clustered sources are not significantly different, and sources are related so the combined source group has some functional significance. For example, in a food web analysis, lumping several species within a trophic guild allows more interpretable results than lumping disparate food sources, even if they have similar isotopic signatures. One result of combining mixing model sources is increased uncertainty of the combined end-member isotopic signatures and consequently the source contribution estimates; this effect can be quantified using the IsoError model (http://www.epa.gov/wed/pages/models/isotopes/isoerror1_04.htm). As an alternative to lumping sources before a mixing analysis, the IsoSource mixing model (http://www.epa.gov/wed/pages/models/isosource/isosource.htm) can be used to find all feasible solutions of source contributions consistent with isotopic mass balance. While ranges of feasible contributions for each individual source can often be quite broad, contributions from functionally related groups of sources can be summed a posteriori, producing a range of solutions for the aggregate source that may be considerably narrower. A paleo-human dietary analysis example illustrates this method, which involves a terrestrial meat food source, a combination of three terrestrial plant foods, and a combination of three marine foods. In this case, a posteriori aggregation of sources allowed strong conclusions about temporal shifts in marine versus terrestrial diets that would not have otherwise been discerned.
稳定同位素混合模型常用于量化混合物的来源贡献。例子包括污染源识别；营养网研究；土壤、植物或水体水源分析等等。一个常见问题是来源过多，无法得到唯一解。我们讨论两种解决此问题的替代方法。一种选择是先验地将具有相似特征的来源合并，使来源数量足够少以提供唯一解。仅当聚类来源的同位素特征无显著差异且来源相关，从而合并后的来源组具有某种功能意义时，才应考虑合并。例如，在食物网分析中，将营养级内的几个物种归为一组比将不同的食物来源归为一组能得到更具可解释性的结果，即使它们具有相似的同位素特征。合并混合模型来源的一个结果是合并后的端元同位素特征以及由此产生的来源贡献估计的不确定性增加；这种效应可以使用IsoError模型（http://www.epa.gov/wed/pages/models/isotopes/isoerror1_04.htm）进行量化。作为混合分析前合并来源的替代方法，IsoSource混合模型（http://www.epa.gov/wed/pages/models/isosource/isosource.htm）可用于找到与同位素质量平衡一致的所有可行的来源贡献解。虽然每个单独来源的可行贡献范围通常可能相当宽泛，但功能相关的来源组的贡献可以事后求和，得出汇总来源的一系列解，这些解可能会窄得多。一个古人类饮食分析的例子说明了这种方法，其中涉及一种陆地肉类食物来源、三种陆地植物食物的组合以及三种海洋食物的组合。在这种情况下，来源的事后汇总使得能够就海洋饮食与陆地饮食的时间变化得出强有力的结论，否则这些结论是无法辨别的。
