Partitioning patterns of fish and sea urchin herbivory from tropical to temperate reefs

Herbivory can structure vegetation and affect the states of sunlit ecosystems. On shallow tropical and temperate reefs, herbivory can inhibit macroalgal dominance and promote turf-dominated benthic states. However, standardised comparisons of tropical and temperate herbivory are primarily focussed on fish, leaving the relative contributions of sea urchins poorly understood. Here, we examined thermal realm (tropical vs temperate) and depth (2, 5, 8 and 12-m) effects on relative herbivory patterns and herbivore distributions, using standardised macroalgal assays (N = 1,340) and underwater visual surveys (N = 67 surveys) at 49 sites across six locations spanning approximately 29° of latitude in eastern Australia. Assays were deployed to partition the relative intensity of herbivory using two partitioning treatments: 1) assays were suspended above the benthos allowing only fish consumption (water-column herbivory), and 2) assays were laid flat on the benthos enabling fish and urchin consumption (benthic herbivory). Assay results showed higher fish herbivory on tropical reefs than on temperate reefs, whereas relative benthic herbivory did not differ between thermal realms. Variation in assay consumption across depth was limited. Results from assays and surveys aligned, with higher herbivorous fish abundance on tropical reefs and higher urchin abundance on temperate reefs. Relative patterns in fish and urchin herbivory between thermal realms underscores differences in how herbivory occurs and where in the water-column it predominates, highlighting a key axis of variation for tropicalizing reefs.