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 percieved 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 promnent temperate marine habitats 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 (Wernberg et al 2011b). 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.

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 project will quantify recent changes in the distribution and abundance of temperate and tropical organisms in southwestern Australia. It will also identify subsequent changes in ecological processes (recruitment, herbivory) with the potential to promote or retard ecosystem recovery.

 

 
Figure 1. Three parrotfishes roaming a turf-covered reef landscape where there were once kelps and temperate seaweeds.