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.