Facilitation of Australia’s southernmost reef-building coral by sea urchin herbivory
Competition for space between corals and macroalgae represents a key threatening process for coral reefs, yet the influence of climate change on this competitive interaction is poorly understood, particularly at the poleward margins of coral distribution. Here we describe the discovery of Australia’s southernmost hermatypic corals and explore novel dynamics facilitating the presence and extent of high-latitude coral communities. Examination of 607 shallow reef sites across temperate Australia revealed hard corals to be negatively associated with increasing kelp bed cover, but positively associated with increasing sea surface temperature, herbivorous fishes, grazing sea urchins, and increasing cover of turf algae, which proliferates in the absence of kelp. However, the nature of these effects varied across different regions of temperate Australia consistent with regional variability in the presence/absence of key functional groups for temperate reefs, such as guilds of subtropical herbivorous fishes and/or prevalence of overgrazing sea urchins. For the southernmost coral communities, in eastern Bass Strait Tasmania, the dominant reef-building coral Plesiastrea versipora was negatively associated with kelp and positively associated with the southward range-extending diadematid sea urchin Centrostephanus rodgersii, which has caused extensive kelp bed overgrazing since first locally reported in 1974. Facilitation of coral establishment was strongest on overgrazed barrens where urchin density was relatively low, but sufficient to maintain the reef kelp-free, while corals were less frequent at high urchin densities and completely absent from barrens colonised by intensively grazing limpets. In contrast to tropical Australian coral reefs and other temperate regions (e.g. Western Australia), assays of herbivory confirmed sea urchin grazing, not herbivorous fishes, as chiefly responsible for kelp consumption within this high-latitude system. Size structure of P. versipora in eastern Bass Strait was dominated by small colonies (~ 20 cm2), suggesting an expanding population at the poleward edge of the species’ range. Nevertheless, colonies up to a maximum area of 500 cm2 were observed, which are likely > 40 yrs old based on growth rates established in warmer waters. This research highlights novel patterns and processes structuring the interface between subtropical and temperate reef communities under climate change and specifically highlights the role of herbivores in releasing corals from competition with kelp under warming ocean regimes.
KeywordsNovel ecosystem Tropicalization Regime-shift Temperate reefs Climate change Kelp beds
Support was provided by ARC Linkage Project LP15010076 to GJE and NSB, and ARC Discovery Project DP170104668 to GJE and SDL. German Soler and Kate Fraser assisted in the field and Antonia Cooper assisted with management of the long-term data set. Australian Temperate Reef Collaboration field surveys were supported by Tasmanian Parks and Wildlife, Parks Victoria, New South Wales Department of Primary Industries, South Australian Department of Environment, Water and Natural Resources, and Western Australian Department of Biodiversity, Conservation and Attractions.
All authors designed and performed field sampling; SDL wrote the draft manuscript; all authors edited the manuscript and acquired funding for the research.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Johnson CR, Ling S, Ross D, Shepherd S, Miller K (2005) Establishment of the long-spined sea urchin (Centrostephanus rodgersii) in Tasmania: first assessment of potential threats to fisheries. Fisheries Research and Development Corporation Final Report, Project No. 2001/044, University of Tasmania, Hobart, Tasmania, Australia Google Scholar
- Johnson CR, Banks SC, Barrett NS, Cazassus F, Dunstan PK, Edgar GJ, Frusher SD, Gardner C, Haddon M, Helidoniotis F (2011) Climate change cascades: shifts in oceanography, species’ ranges and subtidal marine community dynamics in eastern Tasmania. Journal of Experimental Marine Biology and Ecology 400:17–32CrossRefGoogle Scholar
- Ling S, Scheibling R, Rassweiler A, Johnson C, Shears N, Connell S, Salomon A, Norderhaug K, Pérez-Matus A, Hernández J (2015) Global regime shift dynamics of catastrophic sea urchin overgrazing. Philosophical Transactions of the Royal Society B: Biological Sciences 370:20130269CrossRefGoogle Scholar
- Ling SD, Mahon I, Marzloff M, Pizarro O, Johnson C, Williams S (2016) Stereo-imaging AUV detects trends in sea urchin abundance on deep overgrazed reefs. Limnology and Oceanography: Methods 14:293–304Google Scholar
- Ridgway K (2007) Long‐term trend and decadal variability of the southward penetration of the East Australian Current. Geophys Res Lett 34:L13613. https://doi.org/10.1029/2007GL030393
- Vergés A, Doropoulos C, Malcolm HA, Skye M, Garcia-Pizá M, Marzinelli EM, Campbell AH, Ballesteros E, Hoey AS, Vila-Concejo A (2016) Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp. Proc Nat Acad Sci 113:13791–13796CrossRefGoogle Scholar