Centre for Invasion Biology, Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland, South Africa.
Oecologia. 2010 Mar;162(3):733-45. doi: 10.1007/s00442-009-1484-9. Epub 2009 Nov 10.
The stress-gradient hypothesis (SGH) predicts that the community-wide prevalence of positive interactions, relative to negative interactions, is greater under more severe environmental conditions. Because the frequency of positive and negative interactions within a community is the aggregate of multiple pair-wise interactions, one approach to testing the SGH is to examine how pair-wise interactions vary along severity gradients. While the SGH suggests that the net outcome of an interaction should monotonically become more positive with increasing environmental severity, recent studies have suggested that the severity-interaction relationship (SIR) may rather be unimodal. We tested which of the proposed shapes of the SIR best fits the variation in the interaction between two species along two types of severity gradients on sub-Antarctic Marion Island. This was done by comparing the performance of the grass Agrostis magellanica in the presence and absence of the cushion plant Azorella selago, along both species' entire altitudinal ranges (transects spanning 4-8 km), and along a shorter (transect = 0.4 km) wind exposure gradient. Along the altitudinal transects the relative intensity, but not the absolute intensity or the importance, of the Azorella selago-Agrostis magellanica interaction increased with altitude, consistently forming a plateau-shaped SIR with a positive asymptote. Thus, while the performance of Agrostis magellanica was negatively affected by Azorella selago at low altitudes, the grass benefited from growing on the cushion plant under greater environmental severity. Along the wind exposure gradient the intensity of the interaction also became more positive with increasing environmental severity for most performance measures. This suggests that the switch from a net negative to a net positive interaction can occur across both short and long distances. Therefore, this study provides strong evidence for a plateau-shaped SIR, and confirms that the SIR is unimodal along the particular non-resource severity gradients of this study.
应激梯度假说（SGH）预测，在更恶劣的环境条件下，相对于负面相互作用，社区内积极相互作用的总体流行率更高。由于社区内的积极和消极相互作用的频率是多个两两相互作用的总和，因此检验 SGH 的一种方法是检查两两相互作用如何沿着严重程度梯度变化。虽然 SGH 表明，随着环境严重程度的增加，相互作用的净结果应该单调地变得更加积极，但最近的研究表明，严重程度-相互作用关系（SIR）可能是单峰的。我们检验了在所研究的亚南极马里恩岛的两种严重程度梯度上，哪种 SIR 形状最适合两种物种之间相互作用的变化。这是通过比较垫状植物 Azorella selago 存在和不存在的情况下，两种物种的整个海拔范围内（跨越 4-8 公里的横断）以及较短的（横断=0.4 公里）风暴露梯度上的 Agrostis magellanica 草本植物的性能来实现的。沿着海拔横断，Azorella selago-Agrostis magellanica 相互作用的相对强度（而不是绝对强度或重要性）随着海拔的升高而增加，始终形成具有正渐近线的高原形 SIR。因此，虽然 Agrostis magellanica 的性能在低海拔地区受到 Azorella selago 的负面影响，但在更大的环境严重程度下，草本植物从生长在垫状植物上受益。沿着风暴露梯度，对于大多数性能测量，相互作用的强度随着环境严重程度的增加而变得更加积极。这表明，从净负相互作用到净正相互作用的转变可以在短距离和长距离内发生。因此，这项研究为高原形 SIR 提供了强有力的证据，并证实了 SIR 沿着这项研究的特定非资源严重程度梯度是单峰的。
