Estimating ecosystem function: contrasting roles of closely related herbivorous rabbitfishes (Siganidae) on coral reefs

Abstract

We explored the role of behaviour and trophic ecology in driving differences in ecosystem function between 2 closely related species. We examined the relationships between diet, feeding rate, alimentary tract structure and patterns of digestion for 2 reef herbivores commonly found on the Great Barrier Reef, Siganus doliatus and S. lineatus. Despite their similar morphology, the 2 species exhibited distinct feeding behaviours and significantly different feeding rates, diets and movements of digesta through the alimentary tract. S. doliatus displayed a typical herbivore diurnal feeding pattern, taking an average 9.7 bites min–1 over the main part of the feeding day and with a diet dominated by red thallate algae (primarily Laurencia spp., Eucheuma sp., Halymenia sp. and Gracilaria sp.) and red and green filamentous algae. S. lineatus was not observed taking a single bite from the reef substratum in >100 h of underwater observations. The stomach contents of S. lineatus were dominated by amorphous organic matter (detritus). Gut passage rates suggest that S. lineatus is feeding nocturnally or during crepuscular periods. We suggest that these 2 species have distinct functional roles, with S. doliatus being a grazer of reef turf algae and S. lineatus primarily a grazer of off-reef detrital aggregates. This versatility of ecosystem function in closely related species provides further evidence that functional roles do not necessarily divide along taxonomic lines. The results highlight the importance of validating ecosystem function on a species-by-species basis.