Harding Elaine K, Stevens Emiko
Department of Environmental Studies, University of California, Santa Cruz, Santa Cruz, CA, 95064, USA.
Oecologia. 2001 Nov;129(3):436-444. doi: 10.1007/s004420100729. Epub 2001 Nov 1.
In this study, we used the stable isotope ratios N/N and C/C to clarify the spatial and temporal patterns of small mammal derpredation by wide-ranging raptors across a salt marsh and grassland landscape. To determine whether clear isotope signals existed for the two adjoining habitats, and if these differences could allow for an assessment of the seasonal habitat origins of depredated rodents, primarily the vole, Microtus californicus, we investigated the nitrogen and carbon isotope ratios for three sets of data. First, to establish potential differences in isotope signatures between the two habitats, we collected plant species consumed by the vole. Second, we analyzed bone collagen from voles originating from known locations along a gradient from marsh to grassland. Finally, the signatures of these unpredated voles were then used to categorize isotope values of vole remains found in the pellets of their raptor predators. Results indicated that mean δN and δC were both higher in marsh plants than in grassland species. Although both isotopes showed a trophic level increase from plant to bone collagen, nitrogen provided the more informative signature when attempting to distinguish the habitat origins of voles. We found that the δN values of unpredated voles were significantly related to the distance of the specimen along the marsh-grassland interface, with higher values in the marsh habitat. Analysis of the depredated bones found in raptor pellets indicated that a greater percentage of voles originated in grassland during the spring/summer season (88%), as compared with the fall/winter period (60%). Finally, we compared this pattern of habitat and season-specific predation to another measure of predation risk, the relative encounter rates of raptors foraging for voles, and found that the two estimates were similar except during the winter season. This study demonstrates the usefulness of stable isotope analysis for elucidating seasonal patterns of predation across a complex landscape.
在本研究中，我们使用稳定同位素比率N/N和C/C来阐明大范围活动的猛禽在盐沼和草原景观中对小型哺乳动物捕食的时空模式。为了确定这两个相邻栖息地是否存在清晰的同位素信号，以及这些差异是否能够用于评估被捕食啮齿动物（主要是加州田鼠，Microtus californicus）的季节性栖息地来源，我们调查了三组数据的氮和碳同位素比率。首先，为了确定两个栖息地之间同位素特征的潜在差异，我们收集了田鼠食用的植物物种。其次，我们分析了来自沿着从沼泽到草原梯度的已知位置的田鼠的骨胶原。最后，这些未被捕食田鼠的特征随后被用于对在其猛禽捕食者粪便中发现的田鼠遗骸的同位素值进行分类。结果表明，沼泽植物中的平均δN和δC均高于草原物种。尽管两种同位素从植物到骨胶原都显示出营养级的增加，但在试图区分田鼠的栖息地来源时，氮提供了更具信息性的特征。我们发现，未被捕食田鼠的δN值与标本沿着沼泽 - 草原界面的距离显著相关，在沼泽栖息地的值更高。对猛禽粪便中发现的被捕食骨骼的分析表明，与秋冬季节（60%）相比，春夏季节有更大比例的田鼠起源于草原（88%）。最后，我们将这种栖息地和季节特异性捕食模式与另一种捕食风险衡量指标——猛禽捕食田鼠的相对遭遇率进行了比较，发现除了冬季外，这两种估计相似。这项研究证明了稳定同位素分析在阐明复杂景观中捕食季节性模式方面的有用性。
