Tropicalization of fish assemblages in temperate biogeographic transition zones

Biogeographic transition zones in marine temperate systems are often hotspots of biodiversity, with high levels of resilience to short-term climate shifts due to naturally occurring cyclic oscillations of oceanographic conditions. However, these environments are likely vulnerable to a steady global warming scenario in which these cyclical conditions could be disrupted. Here, we evaluate how changes in local oceanography affect the structure of rocky reef fish assemblages over a period of 50 yr in a biogeographic transition zone. Using a 12 yr time series of rocky reef fish assemblage structure, we identified the set of oceanographic variables that most influenced assemblage dynamics. Descriptive and predictive models (multivariate regression trees) were compared to observed data using the area under the curve. Winter northward wind stress and sea surface temperature (SST) were the most important drivers of change in assemblage structure. Only warmer years had indicator species with warm-temperate or tropical affinities. A fish assemblage 'tropicalization' index was developed in response to both local-spatial resolution and short-term environmental variation (1993-2011), and to regional spatial resolution and long-term SST (1960-2012). Predictive modelling for the last 50 yr revealed that species with tropical affinities have increased in frequency compared to cold-temperate species, coinciding with the trend of increasing mean winter SST. Since the mid-1980s, warm-temperate and tropical species have responded rapidly to more frequent warm winters, suggesting that species distributions are shifting polewards. Our results support a hypothesis that cold species retreat more slowly than the advance of warm species. We discuss the importance of transition zones as ` barometers' of climate change.