The importance of rugosity and depth for the presence of barrens
Habitat complexity is an important factor in the distribution of many species and if quantified adequately over large areas may provide a means to better predict species distributions. For C. rodgersiiurchins, the importance of habitat complexity that provides refuge for urchins has been demonstrated in diver-based studies (e.g., Ling and Keane, 2018, Johnson et al., 2005). Our results demonstrate that a measure of habitat complexity derived from bathymetric mapping provides an important covariate for barrens presence and thus could be a useful predictor over large areas. Further exploration of the use of multibeam mapping derived complexity measures and the optimal scale at which to quantify complexity for C. rodgersii would help identify areas that should receive targeted efforts to control the expansion of urchin barrens habitat. For example, predictive maps that incorporate habitat complexity could be used to better understand the likely future extent and spatial distribution of barren formation, and inform adaptive management responses, such as a subsidised urchin fishery (see Cresswell et al., 2019).
Our findings regarding the depth distribution of urchin barrens in Tasmania agrees with previous research that shows that while barrens do extend into deeper areas of reef (> 40 metres), the majority of barrens are expected to form in 10 – 30 metres (Johnson et al., 2005, Ling and Keane, 2018, Perkins et al., 2015). The strong negative coefficient for depth-squared indicates a negative quadratic relationship of barrens presence with depth. Therefore, our results are in agreeance with ecological expectations of the distribution of barrens across our study sites, with lower presence in both deep and shallow images.