Natural and anthropogenic climate variability shape assemblages of range-extending coral-reef fishes

Aim: Climate change is redistributing species globally, resulting in altered community structures and ecosystem functioning. The current paradigm is that species should track temperature isoclines along latitudinal and depth gradients to remain within their thermal niches. However, the many exceptions to this rule point to complex ecological and environmental processes often overlooked in statistical models predicting species redistributions. We tested the contributions of natural versus anthropogenic climate change to the long-term spatio-temporal dynamics of assemblages of range-shifting tropical fishes at the leading edge of redistribution fronts. Location: East coast of Australia. Taxon: Tropical coral-reef fishes. Methods: We analysed 16 years (2003–2018) of tropical species occurrences at two temperate locations using traditional diversity metrics (richness, accumulation curves and β-diversity). We also quantified the role of primary environmental covariates and estimated species associations using joint species distribution models. Results: We reveal that tropical species richness has increased in this temperate ecosystem over time. Furthermore, we show that the richness of tropical vagrant species increased with the sea-surface temperature experienced by both local vagrants and their putative source populations at the southern Great Barrier Reef, which accounted for 23.1% and 22.1% of the explained variance, respectively. We also detected a signal from El Niño-Southern Oscillation, as species turnover and richness peaked during the strong La Niña event of 2010–2011. Main conclusions: While the increases in ocean temperature and strength of the surface ocean current due to anthropogenic climate change are gradually favouring the poleward redistribution of tropical species, natural climatic oscillations can have a strong additive effect by rapidly modifying the pool of incoming species and potentially disrupting local communities.