Effects of habitat on predation of ecologically important sea urchin species on east coast Australian temperate reefs in tethering experiments

The expansion and persistence of urchin-dominated habitats at the expense of macroalgal cover has long been of interest in marine ecology. Macroalgal habitats are considered to harbor a high abundance of urchin predators that protect these habitats from grazing. We tested this concept using tethering experiments in south-eastern Australia to investigate whether predation on urchins is higher and faster within macroalgal habitats (dense seaweed) than in barrens-mosaic habitats (primarily bare rock with the presence of turfing algae, ascidians and macroalgae). We also assessed whether the sympatric species, the diadematoid Centrostephanus rodgersii and the echinometrid Heliocidaris erythrogramma differ in the rates at which they are preyed upon. As smaller urchins are generally considered to be easier prey, we also investigated the influence of urchin size on predation risk. We tethered a total of 96 urchins (48 C. rodgersii and 48 H. erythrogramma) in macroalgal and barrens-mosaic habitats at 4 locations (n = 24 of each species per habitat) and revisited the tethers to record mortality. A damage index assigned to urchin remains was used to infer likely sources of predation (lobster or fish), and we recorded predators in the area. We found no difference in predation between habitat types. Urchins persisted significantly longer at one location where we recorded a less diverse predator guild. Overall, we found no evidence that habitat, urchin species or size play a role in predicting rates of predation on urchins. Our observations suggest that the local predator guild plays an important role in determining predation rates on urchins.