Introduction
Distributional changes of marine species associated with climate change
are being documented worldwide (e.g. Poloczanska et al., 2013, Perry et
al., 2005). In cases where newly arriving species play a dominant
ecological role, dramatic ecosystem changes or “phase shifts” may
occur with major impacts on endemic communities. Understanding how
conservation measures may confer resilience, that is, the ability to
resist and recover from disturbances (Holling, 1973, Hodgson et al.,
2015), is therefore a key conservation priority in the face of climate
change (Bates et al., 2019). Conservation measures that may confer
resilience, such as the protection of habitats or species from
additional human pressures (Carpenter et al., 2001), are typically
legislated and managed on smaller scales than those over which climate
change impacts occur. Therefore, studies that examine the efficacy of
localised conservation measures in protecting habitats and biodiversity
are required.
No-take reserves (NTRs) are areas where fishing pressure is
reduced/removed. NTRs provide researchers with a means of examining the
resilience of ecosystems that may have more “intact” communities
compared to fished areas. The greater abundance and size of predators in
some NTRs has been linked to increased community stability when compared
to adjacent fished areas (e.g. Bates et al., 2017, Bates et al., 2013,
Mellin et al., 2016, Strain et al., 2019) as well as possible dampening
of marine pest outbreaks (Vanhatalo et al., 2017). Ecological theory
developed from invasion ecology suggests that that the top-down control
of higher trophic level predators can aid in reducing the establishment
and persistence of invasive or range extending species (e.g., Worm et
al., 2006, Byrnes et al., 2007, Bulleri et al., 2009, Vanhatalo et al.,
2017). However, the number of empirical studies examining the effect of
NTRs on local ecosystem resilience to climate-driven invasive species
impacts is limited by the technical difficulties associated with
acquiring high-quality time-series monitoring data in the marine
environment.
Here, autonomously acquired marine imagery is coupled with
spatio-temporal statistical analyses to assess how an NTR mediates local
ecosystem resilience in a regional hotspot of global climate warming.
The marine region of south east of Australia, and in particular the east
coast of Tasmania, has been identified as having warming above the
global average (Ridgeway, 2007, Dunstan et al., 2018). The strengthening
of the East Australian Current (EAC) has resulted in increased ocean
temperatures and an extended poleward penetration of the current over
the last 60 years. This EAC extension has coincided with the arrival of
many new species from sub-tropical latitudes into this temperate
mid-latitude region (Robinson et al., 2015). Perhaps the most
ecologically important new arrival is the long-spined sea urchinCentrostephanus rodgersii (Agassiz). The first records of this
species in northern Tasmania were recorded in the late 1970s, and
sightings have been increasingly moving poleward through time (Ling and
Keane, 2018). This urchin species is an ecosystem engineer, capable of
transforming productive kelp beds and invertebrate covered reefs into
bare rock barrens with major impacts on biodiversity and flow-on effects
for economically important rock lobster and abalone fisheries (Ling,
2008).
Dive-based and towed video surveys of urchin densities and barrens
habitat on the east coast of Tasmania have highlighted concerning trends
over the last 15 years, with an estimated increase in C.
rodgersii numbers of approximately 50% and an overall four-fold
increase in barrens habitat between 2001 and 2016 (Ling and Keane, 2018,
Johnson et al., 2005). However, Tasmanian NTRs may offer resilience to
barrens formation and expansion due to the higher density of predators
able to control urchin densities (Bates et al., 2017, Ling and Johnson,
2012, Ling et al., 2009). The main potential predator of C.
rodgersii is the rock lobster Jasus edwardsii , a species that is
the target of commercial and recreational fisheries. Higher densities of
lobsters inside Tasmanian NTRs have been shown to result in three to
seven times higher predation rates, depending on habitat and potential
refuge for urchins (Ling and Johnson, 2012). Monitoring the distribution
of C. rodgersii across its extended range and the extent and rate
of expansion of barrens habitat as driven by climate change, and whether
the spread of barrens may be mitigated by NTR establishment, is
therefore a high priority management issue from both a conservation and
fisheries perspective.
Long term monitoring of urchin benthic habitat over large spatial
extents is a challenging problem. The use of imagery as a monitoring
tool for the marine benthos has been increasing over recent decades.
Sampling platforms such as Autonomous Underwater Vehicles (AUVs), towed
and drop camera systems and Remotely Operated Vehicles (ROVs) have
allowed collection of large amounts of data over greater areas and
deeper depths than have been traditionally surveyed using diver-based
approaches (Pizarro et al., 2013, Durden et al., 2016). Imagery or video
footage from these platforms can typically be geolocated, allowing for
spatial components to be incorporated into subsequent analyses;
including the co-location of observations with mapping products such as
multibeam sonar (e.g., Hill et al., 2014) and the analysis of spatial
patterns in distributions of target species (e.g., Perkins et al.,
2018). NTR monitoring programs utilising these platforms such as
Australia’s Integrated Marine Observing System (IMOS) AUV program and
California’s Marine Protected Area ROV monitoring program now have data
spanning over a decade in some reserves. As these programs are located
across regions experiencing climate change impacts, an opportunity
exists to utilise the available monitoring data to compare NTRs and
adjacent fished areas through time.
Here we use a Bayesian hierarchical modelling approach to analyse the
presence of C. rodgersii barrens using a time series of marine
imagery collected by an AUV from an east coast Tasmanian NTR and nearby
control sites. A survey of barrens inside and outside of the
long-established (> 25 year) Governor Island NTR using
benthic imagery from an AUV revealed that the NTR has a reduced presence
of barrens compared to nearby fished areas (Perkins et al., 2015);
however, analysis was based on a single set of survey data undertaken in
2010-11. Subsequent AUV surveys have been conducted across the same
sites, providing data to examine changes through time. The analysis of
this novel dataset of marine imagery examines: (i) the difference in
presence of urchin barrens inside the NTR compared to control sites
(i.e., the “protection effect”); (ii) whether the NTR mitigates the
rate of barrens expansion through time; (iii) whether depth and rugosity
are important ecological predictors of the presence of barrens; and (iv)
whether including spatial and temporal correlation structures improves
predictive capability.