大鵟的食性改变:来自稳定性碳同位素的证据

通过对大规模灭鼠后的海北高寒草甸生态系统大鵟、小型哺乳类以及雀形目鸟类肌肉稳定性碳同位素比值的测定发现，大篱的肌肉稳定性碳同位素比值介于-22．60‰和-23．10‰之间；小型哺乳类和主要雀形目鸟类肌肉稳定性碳同位素比值分别介于-25．57‰和-25．78‰以及-24．81‰和-22．51‰之间，且它们之间差异性显著。基于碳同位素的分馏模式(即动物和它食物之间稳定性碳同位素分馏在1‰～2‰之间)，我们推断经大规模灭鼠后，大篱的食性发生了较大变化，其食物主要来源于高寒草甸的雀形目鸟类，而非原来的小型哺乳类。通过稳定性同位素营养级模型的运算发现，大蔫处在4．23左右的营养级；雀形目鸟类处在2．4到3．39左右的营养级，而小型哺乳类则处在1．45到1．82左右的营养级。进而采用稳定性同位素质量平衡模型计算得出，大篱的食物由35．04％的小型哺乳类和64．96％的雀形目鸟类所组成，进一步说明小型哺乳类在大量灭鼠后仅占大篱食物的很小一部分。由此可见，采取大规模的化学灭鼠，不仅降低了小型啮齿类天敌——大鵟的数量，而日使得其食谱发毕了巨大改变而转向草甸主要雀形目鸟类[动物学报49(6)：764～768，2003]。

Diet shift of upland buzzards (Buteo hemilasius ):evidence from stable carbon isotope ratios

We measured the stable carbon isotope ratios for muscle of the upland buzzards (Buteo hem ilasius), plateau pika (Ochotona curzoniae), root vole (Microtus oeconomus ), plateau zokor (Myospalax fontanierii ) and passerine bird species at the Haibei Alpine Meadow Ecosystem Research Station (HAMERS), and provided diet information of upland buzzards with the measurement of stable carbon isotopes in tissues of these consumers. The results showed that δ13C values of small mammals and passerine bird species ranged from - 25.57‰ to - 25.78‰ ( n = 12), and from - 24.81‰ to -22.51‰ (n = 43), respectively. δ13C values of the upland buzzards ranged from - 22.60‰ to - 23.10‰ when food was not available. The difference in δ13C values (2.88‰ ± 0.31‰) between upland buzzards and small mammals was much larger than the differences reported previously, 1‰ -2‰, and showed significant difference, while 1.31‰ ± 0.34‰ between upland buzzard and passerine bird species did not differ from the previously reported trophic fractionation difference of 1‰ - 2‰. Estimation of trophic position indicated that upland buzzards stand at trophic position 4.23, far from that of small mammals, I. E. , upland buzzards scarcely captured small mammals as food at the duration of food shortage. According to isotope mass balance model, small mammals contributed 7.89 ％ to 35.04 ％ of carbon to the food source of the upland buzzards, while passerine bird species contributed 64.96 ％ to 92.11％. Upland buzzards turned to passerine bird species as food during times of shortage of small mammals. δ13C value, a useful indicator of diet, indicates that the upland buzzards feed mainly on passerine bird species rather than small mammals due to "you are what you eat" when small mammal preys are becoming scarce [Acta Zoologica Sinica 49 (6): 764 - 768, 2003].