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

Abstract

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.

Refuge effects of Juncus effusus in grazed, subtropical wetland plant communities

“…This index is centered on zero, with positive values indicating facilitation and negative values indicating competition. We analyzed the relationship between RII and grazing intensity (cows/ha and vegetation height) with linear regressions assuming the beta distribution following le Roux and McGeoch (2010) and Smithson and Verkuilen (2006), including pasture-type as a fixed factor.…”

“…Of the few studies that have assessed plant interactions along stress gradients, most have only been able to compare two (high versus low) points along the gradient (Brooker et al , 2008. More studies are needed that sample entire stress gradients Michalet et al 2006;le Roux and McGeoch 2010).…”

Positive plant interactions in the Iberian Southeast: Mechanisms, environmental gradients, and ecosystem function

“…This severity-interaction relationship pattern has been widely supported by empirical evidence (Callaway, 2007). However, and based on recent empirical data, other authors proposed that positive interactions may increase with environmental severity until they reach a plateau, which is also a monotonic pattern (see reviews in Kawai and Tokeshi, 2007;le Roux and McGeoch, 2010), take a hump-shaped pattern e i.e., negative interactions at both low and high levels of environmental severity and positive interactions at intermediate levels (Kawai and Tokeshi, 2007;Maestre and Cortina, 2004a,b;Michalet et al, 2006), or follow a combination of the above patterns.…”

Does extreme environmental severity promote plant facilitation? An experimental field test in a subtropical coastal dune

“…Other studies have found that, at the stressful end of the gradient, the intensity of the positive net interaction reaches an asymptote (Kawai and Tokeshi 2007;le Roux and McGeoch 2010) or may wane and become neutral (Forey et al 2010;Maalouf et al 2012) or even negative (de Bello et al 2011;Koyama and Tsuyuzaki 2013). Some authors have proposed that the decrease in the net interaction most likely occurs because facilitators are less successful in ameliorating stress levels under very severe conditions, while competition for resources may decrease, remain constant or even increase with stress (Michalet et al 2006;Maestre et al 2009).…”