Functional redundancy increases towards the tropics in lake phytoplankton

It has been suggested that the overwhelming number of species in tropical ecosystems consist largely of "neutral" and functionally equivalent species. In phytoplankton, differences in functionality have been shown to be clearly distinguishable from morphological traits. Here we examine whether the increase in species towards the tropics goes together with an increase in functional richness or not. We analyse the latitudinal distribution of phytoplankton morphology-based functional groups (MBFG), the within-group richness and community morphological traits in 83 shallow lakes across South America (5-55°S). We further looked into explaining environmental variables. Despite the increment in species richness towards the (sub)tropics, the average number of MBFG remained constant. Furthermore, size average and variance decreased towards warmer regions. In warm lakes, phytoplankton communities were species rich but redundant in terms of belonging to a MBFG. Increasing species richness only translated into increasing number of rare species in some of the MBFG. In contrast, cold lakes were species poor but less redundant (i.e. essential to maintain the number of MBFG) and had a higher morphological variability. Our results support the hypothesis of higher functional redundancy in warmer areas and the relevance of increasing herbivory in colder regions as a main driving process of latitudinal patterns.