Latitudinal gradients in herbivorous and detritivorous reef fish productivity

Sterling Tebbett, Helen Yan, Scott Bennett, Graham Edgar, Scott Ling, Rick Stuart-Smith & David Bellwood

Abstract:

The abundance of herbivorous fishes is known to vary strongly with latitude. However, our understanding of this pattern is largely based on the examination of nominally herbivorous fishes (i.e. both herbivores and detritivores) as a single group. Therefore, we do not know how this collective classification may have confounded our understanding of distribution patterns, nor how different trophic pathways function across latitudes and associated temperature gradients. This constrains our ability to predict how tropicalising reefs may function as oceans warm, especially following range extensions of tropical taxa. Here, we explored the productivity of seven groups of roving nominally herbivorous fishes across eastern (34° of latitude; ~3800 km) and western (23°; ~2600 km) Australia, with specific consideration of the herbivore versus detritivore dichotomy across key spatial and environmental gradients. In terms of the total nominally herbivorous fish community, we found near-continuous declines in species richness with increasing distance from the equator, while total biomass and productivity were maintained across nearly 30° of latitude in eastern Australia. However, when we separated herbivores and detritivores, we found detritivore productivity dominated the tropics but declined abruptly in temperate regions, with this decline closely correlated with decreasing temperature in a synchronous manner along both coastlines. No such synchronous relationship was observed between herbivore productivity and temperature. These results highlight the importance of the herbivore versus detritivore division in understanding reef trophodynamics across latitudes, and the importance of understanding fish-based detritivory when predicting how the trophodynamic functioning of reefs may change along warming coastlines.