Acknowledgements
This research was supported by Lenfest Ocean Program Grant to SJG and LBC and a Sloan Research Fellowship to SJG. We are grateful to participants in the 2019 Traitspace workshop at Chicheley Hall in Buckinghamshire, UK for feedback and discussion that has greatly enriched and improved this study.
Text Box 1: Re-examining the past to advance trait-based predictions for the future: Trait-based drivers of ecosystem re-assembly following over-exploitation and climate change
The tropicalisation of temperate reefs in coastal southeastern Australia as a result of ongoing global change represents a prominent example of abiotic and biotic traits facilitating species’ establishment in novel habitats, resulting in local community phase-shifts (Figure 7). Since 1910, average ocean warming of >1˚C and a strengthening boundary current in the region have enabled poleward colonization of numerous tropical and sub-tropical ‘vagrants’, as thermal barriers to their dispersal have been eroded (Poloczanska et al. 2007, Figueira & Booth 2010; Figueira et al. 2009, Verges et al. 2014). While many species have successfully dispersed to cooler latitudes, not all become established and fewer still go on to affect the form and function of novel ecosystems. However, the range expansion of an ecosystem engineer - the diadematid sea urchin Centrostephanus rodgersii - from tropical reefs off New South Wales (NSW) to temperate reefs off Tasmania has had major ecological repercussions, threatening kelp forest ecosystems in this region already affected by ocean warming (Johnson et al. 2005, Ling et al. 2009a).
Several abiotic, dispersal, and trophic traits likely simultaneously contributed to the successful range and abundance expansion of C. rodgersii under changing environmental conditions. Namely, the species possesses high dispersal potential in the larval phase (Andrew & Byrne 2001; Johnson et al. 2005), being morphologically adapted for long distance travel in the plankton, resilient to low food quality and quantity, and with long pelagic larval duration (> 100 days; Emlet et al. 2002, Soars et al. 2009). In addition, C. rodgersii is a moderately fast growing sea urchin, likely reaching a critical size for predator avoidance within 4–5 years even in novel higher-latitude ecosystems, they are nocturnally active and hide in complex reef habitat by day (Ling et al. 2009b, Andrew & Byrne 2001).
While the removal of thermal barriers combined with dispersal-related life history traits facilitated initial range expansion by C. rodgersii , these changes also coincided with the prior growth overfishing of a key bottom-dwelling predator capable of foraging on temperate reef invertebrates including urchins - the spiny lobsterJasus edwarsii . Both a reduction in abundances and body sizes (which influences their amount of crushing force on prey) of these predators may have effectively reduced predation pressure on colonizing sea urchins on Tasmanian reefs to levels that facilitated urchin establishment and population growth. Combined with changing abiotic conditions, predator loss from the system likely resulted in over-grazing of kelp beds, and loss of kelp-dominated ecosystems in this region (Barrett et al. 2009, Ling et al. 2009b).