Ocean acidification represents a pervasive environmental change that is predicted to affect a wide range of species1,2, yet our understanding of the emergent ecosystem impacts is very limited. Many studies report detrimental effects of acidification on single species in lab studies, especially those with calcareous shells or skeletons3,4,5. Observational studies using naturally acidified ecosystems have shown profound shifts away from such calcareous species6,7,8, and there has been an assumption that direct impacts of acidification on sensitive species drive most ecosystem responses. We tested an alternative hypothesis that species interactions attenuate or amplify the direct effects of acidification on individual species9,10,11,12. Here, we show that altered competitive dynamics between calcareous species and fleshy seaweeds drive significant ecosystem shifts in acidified conditions. Although calcareous species recruited and grew at similar rates in ambient and low pH conditions during early successional stages, they were rapidly overgrown by fleshy seaweeds later in succession in low pH conditions. The altered competitive dynamics between calcareous species and fleshy seaweeds is probably the combined result of decreased growth rates of calcareous species, increased growth rates of fleshy seaweeds, and/or altered grazing rates13. Phase shifts towards ecosystems dominated by fleshy seaweed are common in many marine ecosystems14,15,16, and our results suggest that changes in the competitive balance between these groups represent a key leverage point through which the physiological responses of individual species to acidification could indirectly lead to profound ecosystem changes in an acidified ocean.
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We thank E. Sanford and S. Palumbi for comments that improved earlier versions of this manuscript. We are grateful to the staff from the Ischia benthic ecology group of the Stazione Zoologica Anton Dohrn, and especially M. C. Buia and L. Porzio for field/lab assistance and advice regarding algae, and to M. Lorenti, B. Iacono and Capt. V. Rando for their field assistance. A. Haupt was instrumental in the field. A. Callea (molluscs), G. Innocenti (crustaceans/echinoderms) and J. F. Sartone (algae) helped in classifications. This research was supported by a National Science Foundation GRF (K.J.K.), Stanford University Chambers Fellowship (F.M.), and the Stazione Zoologica Anton Dohrn.
The authors declare no competing financial interests.
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Kroeker, K., Micheli, F. & Gambi, M. Ocean acidification causes ecosystem shifts via altered competitive interactions. Nature Clim Change 3, 156–159 (2013). https://doi.org/10.1038/nclimate1680
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