Non-trophic Interactions Along a Latitudinal Gradient

Rising global sea surface temperatures are shifting marine species distributions, driving the tropicalisation of temperate regions. This process introduces tropical species into new environments, potentially altering existing community structures and fostering interactions among species with different thermal affinities. This study examines whether, how, and how much coral and temperate reef fishes living among coral communities interact along a tropicalisation gradient in the Ryukyu Islands and Japan’s east coast. We filmed species interactions at 30 locations and performed a series of statistical tests to assess whether species with different thermal affinities interacted and how environmental factors, such as habitat type, shaped these interactions. Additionally, we investigated changes in the likelihood and frequency of cooperative and competitive interactions for tropical species in response to thermal stress using Generalized Linear Mixed-Effects Models. Our findings indicate that species with similar thermal affinities were more likely to interact in mixed communities than species with different thermal affinities, and that tropical and temperate species coexisted without significant competition. This result suggests that mixed communities were structured by species’ functional niches rather than competitive exclusion. We also found that increased thermal stress was associated with a higher frequency of cooperative interactions, supporting the Stress Gradient Hypothesis where species with differing thermal affinities more commonly engaged in cooperative interactions than competitive ones. Finally, our results showed that coral habitats facilitated more diverse and frequent interactions, while turf habitats exhibited fewer less diverse interactions. These findings suggest that tropical and temperate species largely remain separate when cohabitating and that collaborative interactions are helping to support the expansion of tropical species ranges. Future research should explore longer-term trends and broader geographic areas to fully understand the effects of tropicalisation on marine community dynamics.