species-level OTUs in human ~1,000: genus-level OTUs in worm 832: phylotypes in termite hindgut 216 OTUs/phylotypes per individual
||Shapira M. Gut Microbiotas and Host Evolution: Scaling Up Symbiosis. Trends Ecol Evol. 2016 Jul31(7):539-549. doi: 10.1016/j.tree.2016.03.006 p.539 3rd paragraphPubMed ID27039196
|| T.D. Luckey Introduction to intestinal microecology Am. J. Clin. Nutr., 25 (1972), pp. 1292-1294  C.A. Lozupone, et al. Diversity, stability and resilience of the human gut microbiota Nature, 489 (2012), pp. 220-230 doi: 10.1038/nature11550  M. Berg, et al. Assembly of the Caenorhabditis elegans gut microbiota from diverse soil microbial environments ISME J. (2016), doi: 10.1038/ismej.2015.253  F. Warnecke, et al. Metagenomic and functional analysis of hindgut microbiota of a wood-feeding higher termite Nature, 450 (2007), pp. 560-565 DOI: 10.1038/nature06269PubMed ID4639749, 22972295, 26800234, 18033299
||P.539 3rd paragraph: "While examples of species evolution driven by specific pathogens or mutualists are numerous, the full picture is likely more complex, as animals (and plants) harbor complex communities comprising diverse microbes, some more adapted to their host, others generalists, or transient, representing a broad spectrum of potential contributions. In animals, the most extensive microbiota is that of the gut. The human gut microbiota is estimated to include 10^14 bacteria, with approximately 1000 species-level operational taxonomic units (OTUs) per individual [primary sources 6, 7]. Smaller organisms also harbor diverse communities: the Caenorhabditis elegans gut microbiota was found to include 832 genus-level OTUs, with 32 of these present in all worm populations, and the termite hindgut was reported to harbor 216 different phylotypes [primary sources 8, 9]. Thus, the potential diversity of host–symbiont interactions in the gut alone is immense."