Sample size estimation through power analysis is a fundamental tool in planning an ecological study, yet there are currently no well-established procedures for when multivariate abundances are to be collected. A power analysis procedure would need to address three challenges: designing a parsimonious simulation model that captures key community data properties; measuring effect size in a realistic yet interpretable fashion; and ensuring computational feasibility when simulation is used both for power estimation and significance testing. Here we propose a power analysis procedure that meets these challenges with accompanying R software (ecopower). Our simulation model uses a Gaussian copula model, and expert opinion is leveraged to simplify effect size specification into “increasers”, “decreasers” or “no effect” taxa. Computational issues are addressed by using a critical value approach, reducing computation time from days to minutes. The procedure is demonstrated by estimating the sample size required to detect changes for fish abundances.

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. Despite the loss of kelp, we found that species richness and overall abundance of fishes (measured as probability of occurrence) 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 during this period, while only four 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 suggest that changes in pathways of energy flow are a major impact of climate change in tropicalised systems that can lead to increased fish biomass, as planktonic inputs become less important and a higher proportion of algal productivity gets consumed locally by increasingly abundant herbivores.