Global biodiversity patterns are often driven by diff erent environmental variables at diff erent scales. However, it is still controversial
whether there are general trends, whether similar processes are responsible for similar patterns, and/or whether
confounding eff ects such as sampling bias can produce misleading results. Our aim is twofold: 1) assessing the global
correlates of diversity in a group of microscopic animals little analysed so far, and 2) inferring the infl uence of sampling
intensity on biodiversity analyses. As a case study, we choose rotifers, because of their high potential for dispersal across
the globe. We assembled and analysed a new worldwide dataset of records of monogonont rotifers, a group of microscopic
aquatic animals, from 1960 to 1992. Using spatially explicit models, we assessed whether the diversity patterns conformed
to those commonly obtained for larger organisms, and whether they still held true after controlling for sampling intensity,
variations in area, and spatial structure in the data. Our results are in part analogous to those commonly obtained for
macroorganisms (habitat heterogeneity and precipitation emerge as the main global correlates), but show some divergence
(potential absence of a latitudinal gradient and of a large-scale correlation with human population). Moreover, the eff ect
of sampling eff ort is remarkable, accounting for 50% of the variability; this strong eff ect may mask other patterns such
as latitudinal gradients. Our study points out that sampling bias should be carefully considered when drawing conclusions
from large-scale analyses, and calls for further faunistic work on microorganisms in all regions of the world to better
understand the generality of the processes driving global patterns in biodiversity