le Roux Peter C, McGeoch Melodie A
Department of Conservation Ecology and Entomology, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa.
Oecologia. 2008 Apr;155(4):831-44. doi: 10.1007/s00442-007-0954-1. Epub 2008 Feb 6.
The stress-gradient hypothesis predicts that the intensity of interspecific positive interactions increases along environmental severity (i.e. stress and disturbance) gradients faster than the intensity of negative interactions. This study is the first to test if the stress-gradient hypothesis is supported for a location in the climatically extreme and species-poor sub-Antarctic. To do so, we investigate the fine-scale spatial distribution of plant species across altitude- and aspect-related abiotic severity gradients on a scoria cone on Marion Island. A clear altitudinal severity gradient was observed across the scoria cone, with lower temperatures, stronger winds and greater soil movement at higher altitudes. The altitudinal severity gradient was matched by stronger interspecific spatial association between the four dominant species at higher altitudes and in areas of lower vegetation cover. This suggests that, relative to the intensity of competition, the intensity of facilitation is greater under more severe conditions, supporting the stress-gradient hypothesis at the community level (i.e. for multiple pairs of species) and corroborating its usefulness for predicting variation in plant interactions at high latitudes and altitudes. Furthermore, the directional intraspecific aggregation and interspecific association plant cover patterns found within the gradient suggest that protection from the prevailing wind and from burial by loose substrate are the dominant facilitative mechanisms. Thus, plants benefit from the presence of neighbours when they provide shelter and substrate stability, and the relative intensity of this positive interaction is greatest at higher altitudes, and varies between species pairs. This study, therefore, not only provides support for the stress-gradient hypothesis in the sub-Antarctic, but also demonstrates fine-scale directional spatial patterns between multiple species nested within the severity gradient.
胁迫梯度假说预测，种间正相互作用的强度沿环境严峻程度（即胁迫和干扰）梯度增加的速度，要快于负相互作用的强度。本研究首次检验了在气候极端且物种匮乏的亚南极地区的一个地点，胁迫梯度假说是否成立。为此，我们调查了马里恩岛上一个火山渣锥上，植物物种在与海拔和坡向相关的非生物严峻程度梯度上的精细尺度空间分布。在整个火山渣锥上观察到了明显的海拔严峻程度梯度，海拔越高温度越低、风力越强且土壤移动越大。海拔严峻程度梯度与四种优势物种在较高海拔和植被覆盖较低区域之间更强的种间空间关联相匹配。这表明，相对于竞争强度，在更严峻的条件下促进作用的强度更大，在群落水平上（即对于多对物种）支持了胁迫梯度假说，并证实了其在预测高纬度和高海拔地区植物相互作用变化方面的有用性。此外，在该梯度内发现的定向种内聚集和种间关联植物覆盖模式表明，免受盛行风影响和防止被松散基质掩埋是主要的促进机制。因此，当邻居提供庇护和基质稳定性时，植物会从中受益，这种正相互作用的相对强度在较高海拔处最大，并且在不同物种对之间有所不同。因此，本研究不仅为亚南极地区的胁迫梯度假说提供了支持，还展示了嵌套在严峻程度梯度内的多个物种之间的精细尺度定向空间模式。
