Stochastic species distributions are driven by organism size.

The strengths of environmental drivers and biotic interactions are expected to show large variability across organism groups. We tested two ideas related to the degree of ecological determinism vs. stochasticity using a large data set comprising bacterio-, phyto-, and zooplankton. We expected that (1) there are predictable, size-driven differences in the degree to which planktonic taxa respond to different drivers such as water chemistry, biotic interactions, and climatic variables; and (2) species distribution models show lowest predictive performance for the smallest taxa due to the stochastic distributions of microbes. Generalized linear models (GLMs), generalized additive models (GAMs), and generalized boosted methods (GBMs) were constructed for 84 species to model their occurrence as a function of eight predictors. Predictive performance was measured as the area under the curve (AUC) of the receiver-operating characteristic plot and true skill statistic (TSS) using independent model evaluation data. We found that the model performances were typically remarkably low for all planktonic groups. The proportion of satisfactory models (AUC > 0.7) was lowest for bacteria (11.1% of the models), followed by phyto- (24.2%) and zooplankton (38.1%). The occurrences of taxa within all planktonic groups were related to climatic variables to a certain degree, but bacteria showed the strongest associations with the climatic variables. Moreover, zooplankton occurrences were more related to biotic variables than the occurrences of smaller taxa, while phytoplankton occurrences were more related to water chemistry. We conclude that the occurrences of planktonic taxa are highly unpredictable and that stochasticity in occurrences is negatively related to the organism size perhaps due to efficient dispersal and fast population dynamics among the smallest taxa.