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.