Résumé: Trail-based ecology has been developed for decades lo infer ecosystem responses to stressors based on the functional structure of communities, yet its value in species-poor systems is largely unknown. Here, we used an extensive clataset in a Spanish region highly prone to non-native fish invasions (15 catchments, N 389 sites) to assess for the first time how species-poor communities respond to large-scale environmental gradients using a taxonomic and functional trait-based approach in riverine fish. We examined total species richness and three functional trait-based indices available when many sites have <= 3 species (specialization, FSpe; onginaliy, FOri and entropy, FEnt). We assessed the responses of these taxonomic and functional indices along gradients of altitude, water pollution, physical habitat degradation and non-native fish biomass. Whilst species richness was relatively sensitive to spatial effects, functional diversity indices were responsive across natural and anthropogenic gradients. All four diversity measures declined with altitude but this decline was modulated by physical habitat degradation (richness, FSpe and FEnt) and the non-native total fish biomass ratio (FSpe and FOri) in ways that varied between indices. Furthermore, FSpe and FOri were significantly correlated with Total Nitrogen. Non-native fish were a major component of the taxonomic and functional structure of fish communities, raising concerns about potential misdiagnosis between invaded and environmentally-degraded river reaches. Such misdiagnosis was evident in a regional fish index widely used in official monitoring programs. We recommend the application of FSpe and FOri to extensive clatasets from monitoring programs in order to generate valuable cross-system information about the impacts of non-native species and habitat degradation, even in species-poor systems. Scoring non-native species apart from habitat degradation in the indices used to determine ecosystem health is essential to develop better management strategies. (C) 2013 Elsevier B.V. All rights reserved.

Résumé: The composition and spatial variability of ichthyoplankton assemblages were investigated at three shallow seamounts between latitudes 19 degrees S and 33 degrees S in the South West Indian Ocean (SWIO) – La Perouse (60 m), an unnamed pinnacle south of Madagascar, referred to hereafter as MAD-Ridge (240 m), and the Walters Shoal seamount (18 m). In all, 299 larvae (23 families, 54 species) were present at La Perouse, 964 larvae (58 families and 127 species) at MAD-Ridge, and 129 larvae (9 families, 24 species) at the Walters Shoal. Larvae of mesopelagic fish in the families Myctophidae and Gonostomatidae were the most dominant at all three seamounts. All developmental stages were present at each seamount, suggesting the larval pelagic phase of certain species occurs at the seamounts. A 'seamount effect' was detected only at MAD-Ridge where larval fish densities were significantly higher at summit stations. Overall, MAD-Ridge had much higher densities of fish larvae (157.0 larvae 100 m(-3)) than La Perouse (31.1 larvae 100 m(-3)) and the Walters Shoal (9.6 larvae 100 m(-3)). Our study demonstrates that ichthyoplankton communities at shallow seamounts in the SWIO are more influenced by their location relative to a landmass, and to oceanographic features such as currents, mesoscale eddies and water masses than the seamount latitude and topography itself.