Aim To understand the ecological and historical/evolutionary processes underlying an inverse latitudinal gradient of richness (LGR) using crustacean peracarid species as a model group.
Location The Pacific coast of South America, along the Chilean coast between 18° S and 56° S.
Methods The LGR was evaluated using a dataset including 320 marine peracarid species reported for the coasts of Chile. Five ecological hypotheses invoking a relationship between species richness and present-day conditions were tested: species–energy, species–area, Rapoport rescue effect, mid-domain geometric constraint and niche breadth. Historical/evolutionary hypotheses (i.e. biogeographic conservatism, and diversification rates) were indirectly tested by analysing the latitudinal variation in the taxonomic distinctness, the taxonomic conservatism of the midpoint of the latitudinal range and the degree of nestedness at different taxonomic levels.
Results Richness increased poleward, varying approximately eightfold, following an inverse LGR coupled with an increase in bathymetric distribution. Overall this inverse LGR seems robust to uncertainties in the completeness of the species inventory. We found support for only two of the five ecological hypotheses tested: species–area and Rapoport rescue effect. Historical/evolutionary hypotheses seemed important in structuring the richness pattern, as indicated by the higher taxonomic distinctness in the southern region, the strong taxonomic inertia in the mean range size and the high degree of nestedness of assemblages at different taxonomic levels.
Conclusions When combined, these results underscore the importance of long-term processes and historical/evolutionary explanations for the inverse LGR, conceptualized in what we term the ‘out of the deep south’ hypothesis that involves the effects of both biogeographic niche conservatism and evolutionary rates. We propose that the southern region may be a source of evolutionary novelties and/or exhibit higher diversification rates (i.e. higher speciation/lower extinction rates). Furthermore, phylogenetic conservatism of latitudinal range may limit the geographic expansion of these new taxa towards the depauperated northern region.