Nutrient conditions determine the strength of herbivore-mediated stabilizing feedbacks in barrens

13 pages, 5 figures, 5 tables, supporting information https://doi.org/10.1002/ece3.9929.-- Data Availability Statement: The data used in this study will be made publicly available at the Dryad open repository (https://datadryad.org/stash)

Abiotic environmental conditions can significantly influence the way species interact. In particular, plant–herbivore interactions can be substantially dependent on temperature and nutrients. The overall product of these relationships is critical for the fate and stability of vegetated ecosystems like marine forests. The last few decades have seen a rapid spread of barrens on temperate rocky reefs mainly as a result of overgrazing. The ecological feedbacks that characterize the barren state involve a different set of interactions than those occurring in vegetated habitats. Reversing these trends requires a proper understanding of the novel feedbacks and the conditions under which they operate. Here, we explored the role of a secondary herbivore in reinforcing the stability of barrens formed by sea urchin overgrazing under different nutrient conditions. Combining comparative and experimental studies in two Mediterranean regions characterized by contrasting nutrient conditions, we assessed: (i) if the creation of barren areas enhances limpet abundance, (ii) the size-specific grazing impact by limpets, and (iii) the ability of limpets alone to maintain barrens. Our results show that urchin overgrazing enhanced limpet abundance. The effects of limpet grazing varied with nutrient conditions, being up to five times more intense under oligotrophic conditions. Limpets were able to maintain barrens in the absence of sea urchins only under low-nutrient conditions, enhancing the stability of the depauperate state. Overall, our study suggests a greater vulnerability of subtidal forests in oligotrophic regions of the Mediterranean and demonstrates the importance of environment conditions in regulating feedbacks mediated by plant–herbivore interactions

J Boada acknowledges funding from the Spanish Ministry of Science and Innovation (FJC2018-035566-I) and the European Commission – European Union's Horizon 2020 MSCA – SHIFT2SOLVE-1030591. T Alcoverro acknowledges funding from the Spanish Ministry of Science and Innovation – Project UMBRAL (CTM2017-86695-C3-1R). F. Bulleri acknowledges support from the FutureMARES project, funded from the European Union's Horizon 2020 research and innovation program under grant agreement No. 869300

With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

Peer reviewed