Stable isotopes reveal a consistent consumer-diet relationship across hundreds of kilometres

Characterising geographical variation in trophic interactions is an important step towards understanding the consequences of changes in food webs. We characterised geographical variation across similar to 1000 km in stable isotope values (delta N-15 and delta C-13) of the widely distributed sea urchin Heliocidaris erythrogramma and macroalgae that are its potential food. Variation in delta N-15 of H. erythrogramma and macroalgae was typically greatest among locations separated by hundreds of km, while variation in delta C-13 was typically greatest: among reefs separated by a few km. Geographical variation in delta N-15 and delta C-13 of H. erythrogramma was well-explained by variation in delta N-15 and delta C-13 of the kelp Ecklonia radiata. The slope and intercept of regressions of stable isotope ratios of H. erythrogramma on kelp were within the range predicted if the sea urchin's diet was exclusively kelp. These results are consistent with high reliance on a single diet across hundreds of km, revealing that H. erythrogramma likely relies predominantly on a single food source, viz. E. radiata, across southwestern Australia.