Abstract

Aim: We evaluated the bathymetric gradient of benthic polychaete species richness from the Chilean coast, as well as its possible underlying causes. We tested three possible hypotheses to explain the richness gradient: (1) Rapoport's effect; (2) the mid-domain effect (MDE); and (c) the source-sink hypothesis. Location: South-eastern Pacific coast of Chile. Methods: The bathymetric gradient in richness was evaluated using the reported ranges of bathymetric distribution of 498 polychaete species, from the intertidal to abyssal zone (c. 4700 m). Rapoport's effect was evaluated by examining the relationship between bathymetric mid-point and bathymetric range extent, and species richness and depth. The MDE was tested using the Monte Carlo simulation program. The source-sink hypothesis was tested through nestedness analysis. Results: Species richness shows significant exponential decay across the bathymetric gradient. The pattern is characterized by a high presence of short-ranged species on the continental shelf area; while only a few species reach abyssal depths, and they tend to show extremely wide bathymetric ranges. Our simulation analyses showed that, in general, the pattern is robust to sampling artefacts. This pattern cannot be reproduced by the MDE, which predicts a parabolic richness gradient. Rather, results agree with the predictions of Rapoport's effect. Additionally, the data set is significantly nested at species, genus and family levels, supporting the source-sink hypothesis. Main conclusions: The sharp exponential decay in benthic polychaete richness across the bathymetric gradient supports the general idea that abyssal environments should harbour fewer species than shallower zones. This pattern may be the result of colonization-extinction dynamics, characterized by abyssal assemblages acting as 'sinks' maintained mainly by shallower 'sources'. The source-sink hypothesis provides a conceptual and methodological framework that may shed light on the search for general patterns of diversity across large spatial scales. © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd.