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).