Discussion
Here we utilize marine imagery collected from an AUV to demonstrate that an NTR has a substantial localised effect on the formation of barrens by a range extending urchin species in a region that is experiencing rapid climate change. We find a doubling in the odds of the presence of barrens in reference sites outside the NTR; however, although the NTR had consistently lower presence of barrens through time, we also find that the data collected to date does not indicate that the rate of increase in barrens inside the NTR is different to that outside. Therefore, our results imply that the NTR may have conferred resilience in terms of the resistence to the initial establishment of barrens, but that there is currently no evidence of recovery inside the NTR once barrens are established. Preventing the phase-shift from healthy rocky reef ecosystems to large areas of urchin barrens that have occurred elsewhere on the east coast of Tasmania is likely to require the ongoing maintenance of low densities of urchins (see Ling, 2014). Therefore, continued monitoring of this study region is necessary to establish whether the NTR maintains a lower prevalence of barrens or a differing rate of barrens expansion. Monitoring data could also be used to set trigger points for management responses such as the spatial targeting of an emerging urchin fishery (see Cresswell et al., 2019).
Our results regarding the rate of increase in barrens through time are in concurrence with other recent findings that demonstrate an alarming expansion of C. rodgersii barrens in this section of coastline (Ling and Keane, 2018). Ling and Keane (2018) found that barrens were not detected in SCUBA and towed video surveys in this region in 2001/2, but were approximately 2% of surveyed habitat in 2016/17. We found that over our survey period the median probability of any given image being classified as an urchin barren was increasing by 31.9% per year. While the proportion of barrens habitat was still lower than 10% for any of our sites in 2016, the rapid increase in barrens over the 5 year period provides a concerning trajectory. Robust monitoring programs such as the current AUV program are necessary to inform the effective adaptive management of nearshore habitats and economically important fisheries that depend upon them.
We demonstrate that a rugosity measure derived from multibeam data and a quadratic depth term are both strongly correlated with urchin barren presence and therefore may aid in the fine-scale prediction of areas likely to see ongoing increases in the presence of barrens habitat. This information could be used in the spatial planning of target areas for conservation efforts. Our research demonstrates that imagery collected by AUVs provides a benthic monitoring platform capable of tracking important ecological changes thereby providing important and timely information for the management of the marine environment.