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

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.