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Tropicalisation and kelp loss shift trophic composition and lead to more winners than losers in fish communities


Smith, Shannen (2022), Tropicalisation and kelp loss shift trophic composition and lead to more winners than losers in fish communities, Dryad, Dataset,


Climate-mediated species redistributions are causing novel interactions and leading to profound regime shifts globally.  For species that expand their distribution in response to warming, survival depends not only on their physiological capacity, but also on the ability to coexist or be competitive within the established community. In temperate marine reefs from around the world, the range expansion of tropical species, known as ‘tropicalisation’, has been linked to the disappearance of temperate habitat-forming kelps and shifts to dominance by low-biomass turfing algae. The consequences of these range expansions and habitat changes on resident fish communities are, however, unclear. Here, we use data derived from baited remote underwater video (BRUV) surveys to analyse changes in diversity and abundance of marine fishes over a 17-year period in warming reefs that have experienced kelp loss (occurring c. 2009). Despite the loss of kelp, we found that species richness and overall abundance of fishes (measured as probability of occurrence and relative abundance), including both tropical and temperate species, increased through time. We also found dramatic shifts in the trophic composition of fish assemblages. Tropical herbivorous fish increased most markedly through time and temperate-associated planktivores were the only group that declined, a potential consequence of tropicalisation not previously identified. At the species level we, identified 22 tropical and temperate species from four trophic guilds that significantly increased in occurrence, while only three species (all temperate associated) declined. Morphological trait space models suggest increases in fish diversity and overall occurrence are unlikely to be driven by uniqueness of traits amongst tropical range expanders. Our results show more winners than losers and suggest that pathways of energy flow will change in tropicalised systems, as planktonic inputs become less important and a higher proportion of algal productivity gets consumed locally by increasingly abundant herbivores.


Data collected as part of a lomg term BRUV survey effort in the Solitary Islands Marine Park. We specifically selected for reefs where kelp loss is recorded.

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