Résumé
The link between biodiversity and ecosystem functioning has been the topic of considerable research, but it remains unclear how biodiversity decline is compromising ecosystem functionality, particularly in the pelagic realm. Here, we explore how pelagic fish species diversity relates to functional diversity by sampling two locations, which, on the basis of biogeography, environmental conditions and human pressures, were expected to host pronounced differences in species composition and abundances and therefore functionality. Strings of five drifting mid-water Baited Remote Underwater Video Systems were used to survey pelagic vertebrate diversity and abundance in two isolated oceanic island systems, the Malpelo Fauna and Flora Sanctuary-a large, 25-year-old marine protected area-and an unprotected area in Cape Verde. Functional diversity, which offers insight into a community's resilience against disturbance, was analysed using six key functional traits of marine fishes. Abundance was recorded as MaxN, the maximum number of individuals of a given species in a single frame during the 2-h deployment time. Cape Verde showed high overall abundance (Total MaxN 873) and low biomass (3559 kg), with a predominance of smaller fishes. Malpelo showed high biomass (7839 kg) but lower abundance (Total MaxN 465), with a predominance of large species. Species and functional diversity were marginally different between locations. Multivariate analysis of species relative abundances showed significant divergence between locations, although community functional traits overlapped strongly, suggesting that both communities share a similar structure and vulnerability. The existence of a common functional 'backbone' in diverging species communities across the oceans, under different productivity regimes, and under different protection levels, suggests that although pelagic communities may differ considerably in terms of species composition, this does not translate into a differing functional structure and resilience potential. Whether this vulnerability is a common feature of pelagic communities and how this contrasts with benthic systems warrants further research.