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The processes involved in shaping latitudinal‐diversity gradients (LDGs) have been a longstanding source of debate and research. Climatic, historical and evolutionary factors have all been shown to contribute to the formation of LDGs. However, meta‐analyses have shown that different clades have LDG slopes that may vary in more than one order of magnitude. Such large variation cannot be explained solely by climatic or historical factors (e.g. difference in surface area between temperate and tropical zones) given that all clades within a geographic region are subject to the same conditions. Therefore, biotic processes intrinsic to each taxonomic group could be relevant in explaining rate differences in diversity decline across latitudinal gradients among groups. In this study, we developed a model simulating multiple competing species subjected (or not) to a demographic Allee effect. We simulated the range expansion of these species across an environmental gradient to show how these two overlooked factors (competition and Allee effects) are capable of modulating LDGs. Allee effects resulted in a steeper LDG given a higher probability of local extinction and lower colonization capacity compared to species without Allee effects. Likewise, stronger competition also led to a steeper decline in species diversity compared to scenarios with weaker species antagonistic interactions. This pattern occurred mostly due to the strength of priority effects, wherein scenarios with strong competition, species that dispersed earlier in the landscape were able to secure many patches whereas late‐arriving species were progressively precluded from expanding their ranges. Overall, our results suggest that the effect of biotic processes in shaping macroecological patterns could be more important than it is currently appreciated. [ABSTRACT FROM AUTHOR]

The hierarchical branching nature of river networks can have a strong influence on the assembly of freshwater communities. This unique structure has spurred the development of the network position hypothesis (NPH), which states that the strength of different assembly processes depends on the community position in the river network. Specifically, it predicts that 1) headwater communities should be exclusively controlled by the local environment given that they are more isolated and environmentally heterogeneous relative to downstream reaches. In contrast, 2) downstream communities should be regulated by both environmental and dispersal processes due to increased connectivity given their central position in the riverscape. Although intuitive, the NPH has only been evaluated on a few catchments and it is not yet clear whether its predictions are generalizable. To fill this gap, we tested the NPH on river dwelling fishes using an extensive dataset from 28 French catchments. Stream and climatic variables were assembled to characterize environmental conditions and graph theory was applied on river networks to create spatial variables. We tested both predictions using variation partitioning analyses separately for headwater and downstream sites in each catchment. Only 10 catchments supported both predictions, 11 failed to support at least one of them, while in 7 the NPH was partially supported given that spatial variables were also significant for headwater communities. We then assembled a dataset at the catchment scale (e.g. topography, environmental heterogeneity, network connectivity) and applied a classification tree analysis (CTA) to determine which regional property could explain these results. The CTA showed that the NPH was not supported in catchments with high heterogeneity in connectivity among sites. In more homogeneously connected catchments, the NPH was only supported when headwaters were more environmentally heterogeneous than downstream sites. We conclude that the NPH is context dependent even for taxa dispersing exclusively within streams. [ABSTRACT FROM AUTHOR]