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.
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
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).