Aim: Niche and dispersal processes influence biodiversity, but their relative importance along latitude is unclear. We predicted that: i) niche processes would dominate at high latitudes due to increased climatic stress, consistent with the physiological tolerance hypothesis and the Dobzhansky-MacArthur hypothesis and ii) dispersal limitation would prevail at low latitudes due to narrower niches and smaller range sizes, as postulated by the dispersal-ecological specialization tradeoff hypothesis, the latitude-niche breadth hypothesis, and Rapoport’s rule. 

Location: Central United States

Time Period: 1993-2019

Major taxa studied: Stream algae, insects, and fish

Methods: We examined the relative effects of environment (climate and physicochemistry) vs. space on stream biodiversity in seven latitudinal zones, spanning 19 latitudinal degrees. In each zone, species richness (α-diversity) was analyzed with multiple regression and variance partitioning. Compositional dissimilarity (β-diversity) within zones was assessed with distance-based RDA and variance partitioning.

Results: For α-diversity, latitudinal variability of niche and dispersal processes conformed to our predictions in all three groups, except for dispersal processes in insects. However, the drivers of β-diversity did not follow our predictions. The latitude-niche breadth hypothesis and Rapoport’s rule were weakly supported only in fish.

Main Conclusions: The importance of niche and dispersal processes varied predictably along the latitudinal gradient only for α-diversity. However, the niche effects were driven mostly by physicochemistry, and the dispersal effects were not always linked with ecological specialization and range size. This suggests that climate-based biodiversity theories do not have particular relevance for the streams in our study. Niche processes had a greater impact than dispersal processes across species groups and diversity metrics, emphasizing the primary role of the environment.

Datasets containing 2687 algal sites, 3809 insect sites, and 2753 fish sites from streams in the conterminous United States were used to measure species range size and niche breadth. Smaller subsets from the middle United States (349 algal, and 350 insect and fish sites) were used to determine the relative importance of niche and dispersal processes on α- and β-diversity along a latitudinal gradient, comprising seven latitudinal zones. 

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