Trophic interactions across 61 degrees of latitude in the Western Atlantic

Aim The aim was to evaluate the hypothesis that biotic interactions are more intense in the tropics using reef fishes (from both functional and taxonomic perspectives), the crucial consumers on most reefs. Location Fifteen reef locations between 34 degrees N and 27 degrees S in the Western Atlantic. Time period 2011-2014. Major taxa studied Reef fishes. Methods We quantified fish feeding pressure on the benthos across 61 degrees of latitude in the Western Atlantic via 1,038 10 min videos of 2 m(2) reef areas, where every fish feeding on the benthos was identified, had its total length estimated, and the number of bites on the reef substratum was counted. Fish were assigned to functional groups based on diet and feeding modes. Benthic cover estimates were also obtained through visual assessments from the videos. Results We documented feeding rates that were 2- to 22-fold higher in tropical versus extratropical locations. This pattern was driven mainly by an interaction between fish functional group and temperature, with herbivory dominating in tropical regions (c. 20-fold higher), shifting to omnivory in temperate regions of both Hemispheres. Feeding by invertivores was common across all latitudes. Consumer species composition differed between Northern and Southern Hemispheres, but functional groups were similar, and their feeding changed in a similar manner with temperature regardless of Hemisphere. Main conclusions Our results support the hypothesis that biotic interactions, especially plant-herbivore interactions, are more intense in the tropics. These findings help to explain the lower palatability of tropical versus extratropical seaweeds and suggest that herbivory is favoured in warm conditions but omnivory in colder waters. The functional approach indicated that factors associated with latitude or temperature have selected for similar feeding functions among the different species occupying these geographical regions. Understanding the shifting trophic interactions across latitudes might help to predict the impacts of global changes on ecosystem function as tropical species move polewards and contact temperate systems.