Temperate functional niche availability not resident-invader competition shapes tropicalisation in reef fishes

This study examines how the tropicalisation of shallow reefs changes functional niches for fishes in Japan and Australia. They discover that functional niches in tropical-temperate transitional communities are asynchronously invaded by tropical species, mediated more by habitat availability than competition with resident temperate species. Temperate reefs are at the forefront of warming-induced community alterations resulting from poleward range shifts. This tropicalisation is exemplified and amplified by tropical species' invasions of temperate herbivory functions. However, whether other temperate ecosystem functions are similarly invaded by tropical species, and by what drivers, remains unclear. We examine tropicalisation footprints in nine reef fish functional groups using trait-based analyses and biomass of 550 fish species across tropical to temperate gradients in Japan and Australia. We discover that functional niches in transitional communities are asynchronously invaded by tropical species, but with congruent invasion schedules for functional groups across the two hemispheres. These differences in functional group tropicalisation point to habitat availability as a key determinant of multi-species range shifts, as in the majority of functional groups tropical and temperate species share functional niche space in suitable habitat. Competition among species from different thermal guilds played little part in limiting tropicalisation, rather available functional space occupied by temperate species indicates that tropical species can invade. Characterising these drivers of reef tropicalisation is pivotal to understanding, predicting, and managing marine community transformation.

Automated summary

This study investigates the impact of tropicalisation on functional niches for fishes in shallow reefs in Japan and Australia. The researchers find that functional niches in transitional communities are invaded by tropical species at different times, with habitat availability playing a more important role than competition with resident temperate species. The invasion of tropical species into temperate ecosystems indicates the need to understand and manage marine community transformation.