Crossing gradients of consumer pressure and physical stress on shallow rocky reefs: a test of the stress-gradient hypothesis
Article first published online: 4 NOV 2010
© 2010 The Authors. Journal of Ecology © 2010 British Ecological Society
Journal of Ecology
Volume 99, Issue 1, pages 335–344, January 2011
How to Cite
Bulleri, F., Cristaudo, C., Alestra, T. and Benedetti-Cecchi, L. (2011), Crossing gradients of consumer pressure and physical stress on shallow rocky reefs: a test of the stress-gradient hypothesis. Journal of Ecology, 99: 335–344. doi: 10.1111/j.1365-2745.2010.01750.x
- Issue published online: 22 DEC 2010
- Article first published online: 4 NOV 2010
- Received 14 April 2010; accepted 4 October 2010Handling Editor: Ray Callaway
- consumer pressure;
- density-dependent processes;
- environmental stress;
- plant–herbivore interactions;
- rocky reefs;
- stress-gradient hypothesis
1. 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 density-dependent processes are more likely to shape species interactions at extreme ends of gradients of stress.