Do Iberian and European fish faunas exhibit convergent functional structure along environmental gradients?

We tested whether the functional structures of Mediterranean and temperate western European fish communities responded similarly along environmental gradients. The species pools of the 2 regions were quite different, with few species common to both regions. Each species was assigned to 1 trait for each of 6 guilds considered. We aggregated occurrences or densities of the species sharing the same traits and then computed 26 metrics describing the functional structure of fish assemblages. For each metric, we fitted and then compared 3 nested models. The 1(st) model related the metric to environmental variables without taking into account the region. Therefore, the response was assumed to be the same between regions. The 2(nd) model related the metric to the environmental variables with the region as an additive parameter. Therefore, the response was assumed to be similar between regions but with a constant deviation between them. The 3(rd) model took into account all interactions between the environmental variables and region. Therefore, the response to the environmental gradient was assumed to be different in the 2 regions. For the 17 metrics finally tested, 11 metrics responded similarly to environmental gradients but generally showed a constant deviation between the 2 regions, and responses of 6 metrics differed between the regions. Our results highlight the roles played by biogeographical factors and the environment on current community structure in 2 neighboring but ecologically distinct regions.