Resilience and protection effects
Resilience can be framed in terms of a systems ability to both resist and recover from disturbance (see Hodgson et al., 2015), with disturbances pushing systems towards alternative stable states. Urchin barrens represent an alternative stable state in our study system, with high densities of urchins capable of overgrazing kelp, eventually pushing the system to an alternate state of widespread barrens. C. rodgersii urchin barrens are known to demonstrate a hysteresis, whereby the level of urchin density required to return reefs to productive kelp beds is much lower than that required to result in the phase shift to barrens (Ling et al., 2009). In areas of the east coast of Tasmania where urchins settled earlier, widespread barrens are now common (Ling and Keane, 2018). Therefore, understanding whether conservation measures such as NTRs can affect the ability of communities to both resist and recover from the establishment of C. rodgersii urchins is important.
Our results clearly demonstrate the resistance to the establishment ofC. rodgersii urchin barrens of a Tasmanian NTR when compared to nearby reference areas where fishing is permitted. Our finding is supported by a study examining a time series of SCUBA monitoring of NTR and reference sites across Tasmania that found that areas protected from fishing activities resisted the colonisation by C. rodgersii and facilitated greater cover of macroalgae (Bates et al., 2017). This suggests that NTRs offer a spatial management tool to aid in the expansion of barrens habitat on the east coast of Tasmania. While our study does not test the causal relationship for the lower presence, the higher density of the rock lobster J. edwardsii (and especially large lobsters) inside Tasmanian NTRs (Buxton et al., 2006, Barrett et al., 2009b, Edgar and Barrett, 1999) and the higher predation rates of urchins in those reserves by lobsters (Ling and Johnson, 2012) suggests that urchin densities are likely to be kept lower in the NTR through higher predation rates. The Governor Island NTR, established in 1991, was likely to have had a much higher density and larger size structure of lobsters than adjacent coastal waters when C. rodgersiiurchins began arriving in this area in significant numbers during the 1990’s (Johnson et al., 2005, Buxton et al., 2006). As J. edwardsii also shows high site fidelity (Barrett et al., 2009a), higher localised predation rates within the reserve are likely to have maintained lower levels of barrens.
Although our analysis demonstrates a strong protection effect in terms of resistance to the establishment of barrens, the data collected to date does not demonstrate that recovery is occurring inside the NTR. In fact, current rates of increase in barrens were not distinguishable between the NTR and reference sites, suggesting both are being pushed towards alternative stable states. The high temporal correlation in the presence of barrens is in agreeance with other studies that show that, once C. rodgersii urchins are established, recovery of kelp habitat is problematic (Ling, 2014, Ling et al., 2009). Whether the proportion of barrens habitat can be maintained at a level which prevents catastrophic phase shifts to an alternative stable state (see Ling, 2014), or perhaps even a recovery, requires ongoing monitoring and adaptive management. Monitoring approaches such as the AUV-based program reported here provide a useful platform for ongoing work and to test ideas around the resilience of NTRs to the effects of climate change.