Harrison Susan, Grace James B, Davies Kendi E, Safford Hugh D, Viers Joshua H
Department of Environmental Science and Policy, University of California, Davis, California 95616, USA.
Ecology. 2006 Mar;87(3):695-703. doi: 10.1890/05-0778.
Exotic species have been observed to be more prevalent in sites where the richness of native species is highest, possibly reflecting variation among sites in resources, propagule supply, heterogeneity, or disturbance. However, such a pattern leaves unclear whether natives at species-rich sites are subject to especially severe impacts from exotics as a result. We considered this question using path models in which relationships between exotic cover and native richness were evaluated in the presence of correlated environmental factors. At 109 sites on serpentine soils across California, USA, exotic cover was positively correlated with total native herbaceous richness and was negatively correlated with the richness of both serpentine-endemic and rare native herbs. However, in path models that accounted for the influences of soil chemistry, disturbance, overstory cover, and regional rainfall and elevation, we found no indication that exotic cover reduced any component of native herb richness. Rather, our results indicated similarities and differences in the conditions favoring exotic, native, endemic, and rare species. Our results suggest that, in spite of some localized impacts, exotic species are not exerting a detectable overall effect on the community richness of the unique native flora of Californian serpentine.
据观察，外来物种在本地物种丰富度最高的地点更为普遍，这可能反映了不同地点在资源、繁殖体供应、异质性或干扰方面的差异。然而，这样一种模式尚不清楚物种丰富地点的本地物种是否因此受到外来物种特别严重的影响。我们使用路径模型来考虑这个问题，在该模型中，外来物种覆盖度与本地物种丰富度之间的关系在相关环境因素存在的情况下进行评估。在美国加利福尼亚州各地的109个蛇纹石土壤地点，外来物种覆盖度与本地草本植物总丰富度呈正相关，与蛇纹石特有和稀有的本地草本植物丰富度呈负相关。然而，在考虑了土壤化学、干扰、上层林冠覆盖以及区域降雨和海拔影响的路径模型中，我们没有发现外来物种覆盖度降低本地草本植物丰富度任何组成部分的迹象。相反，我们的结果表明了有利于外来物种、本地物种、特有物种和稀有物种的条件的异同。我们的结果表明，尽管存在一些局部影响，但外来物种并未对加利福尼亚蛇纹石独特本地植物群的群落丰富度产生可检测到的总体影响。
