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.

Species redistributions are causing novel interactions and leading to profound regime shifts globally. In temperate reefs, the range expansion of tropical herbivorous fish has been linked to the disappearance of temperate kelps, but consequent effects on resident fish communities are not clear. Here, we show overall increases in species richness and abundance (measured as probability of occurrence) of both tropical and temperate fishes identified on video surveys over a 17-year period of kelp loss. Tropical herbivores increased most markedly as kelp declined, while temperate planktivores declined, a potential consequence of tropicalisation not previously identified that suggests important changes to energy pathways. We identified 22 tropical and temperate species from four trophic guilds that significantly increased in occurrence, and only four temperate species that 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.