Crossing gradients of consumer pressure and physical stress on shallow rocky reefs: a test of the stress‐gradient hypothesis

Abstract

Summary1. The stress-gradient hypothesis predicts an increase in the importance and intensity of positive species interactions towards extreme ends of gradients generated by either physical stress or consumer pressure. However, little attention has been devoted to assessing how the co-occurrence of different gradients of stress and variations in the abundance of the benefactor can influence switches in species interactions. 2. On shallow rocky reefs, we assessed shifts in the effects of different covers of Vermetid tube-building gastropods (benefactor) on macroalgae (beneficiary), under experimental conditions generated by crossing a gradient of consumer pressure (sea urchin density) and a gradient of physical stress (sediment deposition). 3. Negative effects of Vermetids on macroalgae in the absence of herbivores switched to positive at intermediate grazing pressure, but sedimentation and benefactor cover determined their intensity. Thus, association with Vermetids provides macroalgae with a refuge from herbivores. When consumer pressure was the greatest, facilitation persisted both at natural and moderately enhanced sedimentation if the benefactor cover was reduced. When the benefactor monopolized space, facilitation was only observed at natural levels of sedimentation. Thus, the relationship between the outcome of the benefactor-beneficiary interaction (expressed as the Relative Interaction Index) and consumer pressure varied from linear to asymptotic or quadratic, according to sedimentation levels and benefactor abundance. 4. Synthesis. These results show that shifts in the direction and intensity of species interactions are regulated by the interplay of biological and physical factors. In addition, they suggest that densitydependent processes are more likely to shape species interactions at extreme ends of gradients of stress.

Drought and grazing combined: Contrasting shifts in plant interactions at species pair and community level

“…Also contradicting the predictions from the original SGH, studies in grazed ecosystems have reported an increase in facilitation intensity from low to high grazing pressure but a decrease again with further increases in grazing pressure (Smit et al, 2009). Such collapses in facilitation intensity at high grazing pressure have been observed in both terrestrial (Brooker et al, 2006;Graff et al, 2007;Saiz and Alados, 2012;Smit et al, 2007) and aquatic ecosystems (Bulleri et al, 2011;Le BagoussePinguet et al, 2012;Levenbach, 2009). Plants can protect neighbours by physically sheltering them from herbivore damage, but when grazing pressure becomes very high, facilitative interactions might wane, because the nurse itself gets damaged by grazing or trampling (Michalet et al, 2014), or because consumers might start searching more intensively for resources (Soliveres et al, 2011a).…”

Direct plant–plant facilitation in coastal wetlands: A review

“…Facilitating tolerance of other biotic and abiotic stresses such as sea waves, pollination or dispersal stress has also been proposed as mechanisms of species interaction (Table 1). However, the cumulative effects of cooccurring environmental stresses, particularly abiotic and biotic stresses on species interactions, which may play an important role in community assembly are still poorly understood (Bulleri et al, 2011).…”

Mechanisms of plant–plant interactions: concealment from herbivores is more important than abiotic-stress mediation in an African savannah

“…concealment), in which neighbours reduce herbivory on focal individuals by making them less conspicuous or accessible ( [6], see [7] for a recent review). We know little about the relative strength of these two facilitative mechanisms, or how they interact, since few experimental studies incorporate both variable stress levels and herbivory ( [1], but see [4,8,9]). …”