Understanding the causes of spatial variation in species richness is a major research focus of biogeography, macroecology and conservation. However, analyses have been limited to taxa with well-known distributions - mainly vertebrates, plants and some insects. The distribution patterns of most other organisms are still undocumented, thus limiting the generality of the conclusions of such studies.
Microscopic organisms can provide further, independent evidence for or against the global correlates of species richness of better-studied groups. Biodiversity patterns of microorganisms may differ from those of macroscopic organisms because of their high abundances, the existence of resting stages, and their capacity for frequent and long-distance dispersal.
One problem with the analysis of geographical patterns in microorganisms is that their diversity can be too high to assess, especially over wide scales, so biogeographical hypotheses cannot be tested globally. Indeed, macroecological analyses of protists or prokaryotes have so far been difficult or inconsistent due to lack of reliable data. Our approach was to use a different model taxon, which (i) has the microbial features of small size, high abundance, presence of resting stages, frequent and long-distance dispersal, but also (ii) shows a much lower overall diversity than protists and prokaryotes, and for which (iii) faunistic data are already available. Monogonont rotifers fulfill all these requirements as a model organism for ecogeographical studies of microscopic diversity.
We tested the consistency of the correlates of global species richness described so far for other organisms by assembling and analysing a global dataset of monogonont rotifer distribution records. We tested the generality of the previously known patterns and correlates of species richness at different spatial resolutions. We also assessed the influence of sampling intensity, which is often not considered in diversity studies and may lead to invalid results.
Sampling intensity was indeed the most important variable influencing species richness, in what we could call the ‘rotiferologist’ effect. The geographical units with highest recorded diversity were mostly in Europe, but even here, countries such as Bulgaria, Denmark, Greece and Portugal had less than 60 recorded species. For the richest geographical units, cumulative species curves did not reach an asymptote, suggesting that new species can still be found even in these areas. Thus, even for a group of organisms not as diverse as prokaryotes or protists and for which many (nearly 47,000) records are now available, results from global analyses can be misleading if sampling bias is not considered.
After controlling for sampling intensity, habitat diversity and rainfall were the main environmental correlates of rotifer diversity. Both are in line with species richness drivers suggested for other taxa, namely habitat heterogeneity and environmental productivity. Interestingly, these variables have different roles in explaining rotifer species richness at different resolution scales, confirming the scale-dependence in the patterns and determinants of diversity. The latitudinal gradient in species richness commonly documented for other organisms was not apparent in our dataset, although this might be an artefact of sampling intensity. Other common correlates of species diversity in other groups, such as altitude, area and human population, were also not significant in our models.
We stress the need for macroecological analyses to explicitly consider sampling intensity as a source of error. We also remark that faunistic data for rotifers and other inconspicuous organisms are still far from being able to document their diversity patterns in a reliable way. We thus call for more long-term studies on rotifers, especially in understudied areas such as northern Asia, southern Africa and the central part of North America. Such studies, on rotifers and on other microscopic organisms worldwide, would be pivotal to assess the generality of the processes driving global patterns in biodiversity.
The data we gathered, including both the global rotifer distribution records and the eco-geographical variables at different resolution levels, are provided with the article and can also be accessed from our Resources section. This is the first time that a database of worldwide distribution records is made available for any group of microorganisms, and we hope it will be useful for further research.
Participating institutions: Imperial College London, ‘Rui Nabeiro’ Biodiversity Chair (CIBIO, University of Évora), Swedish Museum of Natural History, Belgian Biodiversity Platform
Article reference: Fontaneto D., Barbosa A.M., Segers H. & Pautasso M. (2012) The ‘rotiferologist’ effect and other global correlates of species richness in monogonont rotifers. Ecography 35: 174-182
