, Volume 162, Issue 3, pp 733–745 | Cite as

Interaction intensity and importance along two stress gradients: adding shape to the stress-gradient hypothesis

  • Peter Christiaan le Roux
  • Melodie A. McGeoch
Community ecology - Original Paper


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 selagoAgrostis 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.


Biotic interactions Environmental gradient Facilitation Nurse plant Severity-interaction relationship 



Marion Burger, Natalie Haussmann, Mawethu Nyakatya and Tess Rautenbach are thanked for field assistance and two anonymous reviewers for their thoughtful comments. Financial support was provided by the South African National Research Foundation, the South African National Antarctic Program (grant number 2069543 and SNA 2004070900002) and Stellenbosch University. This research complied with the current laws of South Africa.

Supplementary material

442_2009_1484_MOESM1_ESM.doc (572 kb)
Supplementary material 1 (DOC 572 kb)


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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Peter Christiaan le Roux
    • 1
    • 2
  • Melodie A. McGeoch
    • 1
    • 3
  1. 1.Centre for Invasion Biology, Department of Conservation Ecology and EntomologyStellenbosch UniversityMatielandSouth Africa
  2. 2.Centre for Invasion Biology, Department of Botany and ZoologyStellenbosch UniversityMatielandSouth Africa
  3. 3.Cape Research CentreSouth African National Parks SteenbergSouth Africa

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