Swiss Federal Institute of Technology (ETH Zurich), CH-8092 Zürich, Switzerland.
Proc Natl Acad Sci U S A. 2011 Jan 11;108(2):656-61. doi: 10.1073/pnas.1013136108. Epub 2010 Dec 27.
Nonnative species richness typically declines along environmental gradients such as elevation. It is usually assumed that this is because few invaders possess the necessary adaptations to succeed under extreme environmental conditions. Here, we show that nonnative plants reaching high elevations around the world are not highly specialized stress tolerators but species with broad climatic tolerances capable of growing across a wide elevational range. These results contrast with patterns for native species, and they can be explained by the unidirectional expansion of nonnative species from anthropogenic sources at low elevations and the progressive dropping out of species with narrow elevational amplitudes--a process that we call directional ecological filtering. Independent data confirm that climatic generalists have succeeded in colonizing the more extreme environments at higher elevations. These results suggest that invasion resistance is not conferred by extreme conditions at a particular site but determined by pathways of introduction of nonnative species. In the future, increased direct introduction of nonnative species with specialized ecophysiological adaptations to mountain environments could increase the risk of invasion. As well as providing a general explanation for gradients of nonnative species richness and the importance of traits such as phenotypic plasticity for many invasive species, the concept of directional ecological filtering is useful for understanding the initial assembly of some native floras at high elevations and latitudes.
非本地物种丰富度通常会随着环境梯度（如海拔）而下降。通常认为，这是因为很少有入侵者具有在极端环境条件下成功生存所需的适应能力。在这里，我们表明，在世界各地到达高海拔地区的非本地植物并不是高度特化的耐胁迫物种，而是具有广泛气候耐受性的物种，能够在广泛的海拔范围内生长。这些结果与本地物种的模式形成鲜明对比，可以通过非本地物种从低海拔的人为来源单向扩张和具有较窄海拔幅度的物种逐渐消失来解释，我们称之为定向生态过滤。独立的数据证实，气候广适性物种已成功地在更高海拔的更极端环境中殖民化。这些结果表明，入侵抗性不是由特定地点的极端条件赋予的，而是由非本地物种的引入途径决定的。在未来，更多具有专门的生理生态适应能力的非本地物种的直接引入可能会增加入侵的风险。除了为非本地物种丰富度梯度以及表型可塑性等特征对许多入侵物种的重要性提供一般解释外，定向生态过滤的概念还可用于理解某些高山和高纬度地区本地植物群的初始组合。
