Crait Jamie R, Ben-David Merav
Department of Zoology and Physiology, University of Wyoming, Laramie 82071, USA.
Ecology. 2007 Apr;88(4):1040-52. doi: 10.1890/06-0078.
Animals that deposit aquatically derived nutrients on terrestrial landscapes link food webs and affect a variety of in situ processes. This phenomenon, however, is poorly documented in freshwater habitats, especially where species introductions have drastically changed an ecosystem's trophic structure. In this study, we used stable isotopes to document water-to-land nutrient transport by river otters (Lontra canadensis) around Yellowstone Lake, an ecosystem recently altered by nonnative species invasions. We then investigated the effects of otter fertilization on plant growth and prevalence at latrine (scent-marking) sites and evaluated how the recent changes to the lake's food web could influence these plant responses. Values of delta15N were higher on latrines compared to non-latrine sites in five of seven sample plant taxa. Additionally, latrine grasses had higher percentage N than those from non-latrines. Foliar delta15N positively related to fecal deposition rate for some plants, indicating that increased otter scent-marking led to a rise in these N values. Logistic regression models indicated that otters selected for well-shaded latrines with access to foraging. Atypical latrines, misclassified as non-latrines by the regression models, had values of delta15N similar to correctly classified latrines, suggesting that site effects alone cannot explain elevated N values at otter latrine sites. No difference in plant diversity or percent cover of N-fixing taxa occurred between latrine and nonlatrine sites, though specific genera did differ between site types. Measurements of shoot lengths indicated increased growth of some latrine currants (Ribes sp.). In Yellowstone Lake, a twofold reduction in otter numbers could result in an even greater decline in nutrient deposition at latrines, as otters may become less social in a system with decreased prey availability. Our results highlight the role of animals in linking aquatic and terrestrial habitats in inland freshwater systems and suggest that ongoing changes in the trophic structure of Yellowstone Lake could have unexpected ramifications well beyond the lake itself.
在陆地上沉积源自水生环境养分的动物，会连接食物网并影响各种原位过程。然而，这种现象在淡水生境中记录较少，尤其是在物种引入已大幅改变生态系统营养结构的地方。在本研究中，我们使用稳定同位素来记录黄石湖周边水獭（加拿大水獭）从水到陆地的养分传输情况，黄石湖是一个最近因外来物种入侵而改变的生态系统。然后，我们研究了水獭在排便（气味标记）地点施肥对植物生长和分布的影响，并评估了该湖食物网最近的变化如何影响这些植物反应。在七个样本植物分类群中的五个中，与非排便地点相比，排便地点的δ15N值更高。此外，排便地点的草类含氮百分比高于非排便地点的草类。对于某些植物，叶片δ15N与粪便沉积速率呈正相关，这表明水獭气味标记增加导致这些氮值升高。逻辑回归模型表明，水獭选择有觅食通道且遮荫良好的排便地点。被回归模型误分类为非排便地点的非典型排便地点，其δ15N值与正确分类的排便地点相似，这表明仅地点效应无法解释水獭排便地点氮值升高的现象。尽管特定属在不同地点类型之间存在差异，但排便地点和非排便地点在植物多样性或固氮类群的覆盖百分比方面没有差异。茎长度测量表明，一些排便地点的茶藨子（醋栗属）生长增加。在黄石湖，如果水獭数量减少一半，可能会导致排便地点的养分沉积下降得更多，因为在猎物可获得性降低的系统中，水獭可能会变得不那么群居。我们的研究结果突出了动物在连接内陆淡水系统中水生和陆地生境方面的作用，并表明黄石湖营养结构正在发生的变化可能会产生远远超出湖泊本身的意外影响。
