Community assembly seeks to understand patterns of diversity across multiple scales and ecological processes, integrating ideas from biogeography and community ecology. At the regional scale, differential dispersal abilities of species are hypothesized to influence patterns of community diversity. At the local scale, species interactions are thought to shape the composition and diversity of organisms in communities. Previous studies have demonstrated the importance of both processes in shaping patterns of diversity, and that the relative importance of these processes are dependent on the spatial scale, environment, and organisms under investigation. However, most research has focused on a singular trophic level, largely ignoring the role of trophic interactions in assembly. Thus, our study seeks to better understand the relative importance of dispersal and species (trophic) interactions in a food web context. We measured dispersal rates in a trophically diverse group of protist species, using a two-patch system which allowed active dispersal between connected bottles. We estimated species interaction strengths for all pairwise species combinations across a range of densities. Finally, we parametrized spatially explicit models to predict how species interactions and dispersal jointly influence the assembly of food webs.
Results/Conclusions:
Protist dispersal rates scale with functional (trophic) groups, where trophic position, body size, and growth rate show unique patterns. Smaller bacterivores (~40-80 μm) with high growth rates (>1.5 1/d) displayed the highest dispersal rates. This was followed by larger bacterivores (>100 μm), mixtrophs, and lastly omnivores which displayed the lowest dispersal rates. Species within the same trophic group tend to display negative interactions strengths. Across trophic groups, mixotrophs display positive interactions with both large and small bacterivores, and omnivores (Blepharisma sp.) display negative interactions strengths with the bacterivores they consume. In particular, omnivory in this system shows differential functional responses (Type II vs. Type III) based on the size of prey (larger vs. smaller bacterivores), suggesting a lag in switching food sources between bacteria and bacterivores. Modeling assembly, we show how functional diversity in the regional pool and the underlying spatial connectivity of the system interact to produce differential patterns of synchronous vs. asynchronous local food web dynamics. Future experiments with protists will test these predictions from modeling simulations to further disentangle the relative roles of species interactions and dispersal in structuring patterns of diversity.