Range contraction of kelp forests and tropicalisation of Australia's temperate marine environments

A project undertaken by the UWA Oceans Institute & School of Plant Biology, Univresity of Western Australia, and supervised by Thomas Wernberg

Co-investigator: Scott Bennett, University of Western Australia

Global warming is causing temperate and tropical species to retract and expand their ranges, respectively. Range-changes are usually perceived as a continuous process but in reality discrete events often drive successive local extirpations or colonizations. The ecological effects from these events are disproportionately large when habitat-forming species that support a myriad of other species, are lost.

Kelp forests (Ecklonia radiata) are prominent temperate marine habitats in Australia and New Zealand. Like forests on land they play a pivotal role as food and shelter for other organisms, including unique fish and invertebrate communities. The critical ecological importance of kelp forests implies that their local extirpation has the potential to cause wholesale shifts in the community structure and ecosystems services provided by local reefs, including catastrophic impacts on valuable temperate reef fisheries (click here to watch a short video about Australia’s Great Southern Reef).

In 2011, southwestern Australia experienced an unprecedented ‘marine heat wave’ where sea temperatures soared to 3-5 °C above normal, and remained elevated for several weeks. Prior to the heatwave, kelp forests covered over 70% of reef surfaces at typical coastal locations in the region.  Observations to date suggest the heatwave and subsequent warm summers resulted in widespread loss of kelps and an increase in tropical seaweeds, fish and invertebrates. This process is known as ‘tropicalisation’ and it is an increasingly common phenomenon. This project aimed to quantify recent changes in the distribution and abundance of temperate and tropical organisms in southwestern Australia. It also identifed subsequent changes in ecological processes (recruitment, herbivory) with the potential to promote or retard ecosystem recovery.

Our research established that kelp forests had contracted ~100 km south during the heatwave (Wernberg et al. 2016). At the same, turf algae proliferated and warm water seaweeds, fishes, invertebrates and corals increased while cool water species decreased (Bennett et al. 2015, Wernberg et al. 2016, Zarco-Perello et al. 2107, Tuckett et al. 2017) (click here to watch a short video about the devastation from the 2011 marine heatwave).

On some of the worst affected reefs, herbivory from warm-water fishes increased by ~400%, rivalling grazing rates on healthy coral reefs and making it highly unlikely kelp forests could recover (Bennett et al. 2015). However, tropical rabbitfish also recruited where the kelp forests remained reasonably intact and here they have decimated kelp cover y as much as ~60% on some reefs (Zarco-Perello et al. 2107).

Translocation experiments and physiological experiments revealed a link between temperature anomalies, thermal safety margins and the physiological performance of seaweeds in warming water (Bennett et al. 2015, Xiao et al. 2015, Wernberg et al. 2016).

Peer reviewed publications

Please click the links below to read the abstracts or visit https://wernberglab.org/publications/ to request pdf’s of these papers.
Bennett S, Wernberg T, Bijo AJ, de Bettignies T, Campbell AH (2015) Central and rear edge populations can be equally vulnerable to warming. Nature Communications, 6:10280
Xiao X, de Bettignies T, Olsen Y, Agusti S, Duarte C, Wernberg, T (2015) Sensitivity and acclimation of three canopy-forming seaweeds to UVB radiation and warming. PLoS One, 10(12): e0143031.
Bennett S, Wernberg T, Harvey ES, Santana-Garcon J, Saunders B (2015) Tropical herbivores provide resilience to a climate mediated phase-shift on temperate reefs. Ecology Letters, 18: 714-723.
Wernberg T, Bennett S, Babcock RC, de Bettignies T, Cure K, Depczynski M, Dufois F, Fromont J, Fulton CJ, Hovey RK, Harvey ES, Holmes, Kendrick GA, Radford B, Santana-Garcon J, Saunders BJ, Smale DA, Thomsen MS, Tuckett CA, Tuya F, Vanderklift MA, Wilson SK (2016) Climate driven regime shift of a temperate marine ecosystem. Science, 353: 169-172.
Wernberg T, de Bettignies T, Bijo AJ, Finnegan PM (2016) Physiological responses of habitat-forming seaweeds to increasing temperatures. Limnology & Oceanography, 61(6): 2180-2190.
Zarco-Perello S, Wernberg T, Langlois T, Vanderklift MA (2017) Tropicalization strengthens consumer pressure on habitat-forming seaweeds. Scientific Reports, 7: 820.
Tuckett CA, de Bettignies T, Fromont J, Wernberg T (2017) Expansion of corals on temperate reefs: direct and indirect effects of marine heatwaves. Coral Reefs, in press, doi: 10.1007/s00338-017-1586-5.

Other publications and media

Wernberg T, Smale DA (2015) Marine heatwaves threaten underwater forests. The Conversation 6/2/15. Web: https://theconversation.com/marine-heatwaves-threaten-the-future-of-underwater-forests-37154
Wernberg T, Hobday A, Johnson C, Poloczanska E, Bennett S, Connell S (2015) Australia’s ‘other reef’ is worth more than $10 billion per year – have you heard of it? The Conversation 17/8/15. Web: https://theconversation.com/australias-other-reef-is-worth-more-than-10-billion-a-year-but-have-you-heard-of-it-45600
Vergés A, Wernberg T (2015) Climate change: underwater forest decline – loss of kelp forest places commercial fisheries at risk. HOT TOPICS in Ecology, Ecological Society of Australia, 8/10/15. Web: http://www.ecolsoc.org.au/hot-topics/climate-change-underwater-forest-decline
Bennett S, Wernberg T, Andrews S (2016). Introducing Australia’s Great Southern Reef. YouTube video 16/1/16. Web: https://www.youtube.com/watch?v=clWCpejKrEc
Wernberg T, Bennett S, Andrews S (2016) The demise of kelp forests in Western Australia. YouTube video 7/7/16. Web: https://www.youtube.com/watch?v=wEfpDLAUpBs
Bennett S, Santana-Garcon J, Wernberg T (2016) A marine heatwave has wiped out a swathe of WA’s undersea kelp forest. The Conversation 8/7/16. Web: https://theconversation.com/a-marine-heatwave-has-wiped-out-a-swathe-of-was-undersea-kelp-forest-62042

 

Figure 1. Unique animals in Australian kelp forests. Slate pencil urchins, Harlequin fish and a kelp research diver (photos T. Wernberg, T. Wernberg, J. Costa).

 

Figure 2. Extensive underwater forests of Australian kelp (top, photo (c) J. Costa) were wiped out in 2011 under a marine heatwave. The kelps died from extreme temperatures and, at the same time small turf-forming seaweed proliferated and tropical fish herbivores (bottom, photo (c) S. Bennett) increased in abundance. The turf seaweeds and grazing fishes now prevent the return of kelp forests.

 

Figure 3. Infographic illustrating the impacts of the heatwave, kelp loss and tropicalisation of temperate reefs