The Rare Biosphere, 2011" and it discusses some of the issues associated with the long tail of rare organisms that might exist, especially microbes. It is worth a look.
From their page:
The microbial world represents the last truly unexplored frontier in the diversity of life on Earth. New environmental sampling technologies have revealed a wealth of rare microbial species in the soil, ocean, even our own bodies that were effectively cloaked from previous sampling methods by more abundant species. Dubbed the rare biosphere, these microbial species, while individually rare, collectively account for more than 75% of the biomass of some microbial communities, yet little is known about them. This rare biosphere represents a treasure trove of genetic novelty that may possess numerous unique bioprocesses and biomaterials. These rare species may play keystone roles in microbial communities and act as a reservoir of genetic diversity. But how can scientists effectively study the rare biosphere? In April 2009 the American Academy of Microbiology convened a colloquium to explore this question. Based on that colloquium, this report analyzes the current state of study of the rare biosphere and identifies where gaps in knowledge exist. The report concludes that the Herculean task of studying the rare biosphere requires an international collaborative effort and additional environmental sampling, coupled with a focus on advancing sequencing and data analysis technologies. With less than 1% of microbial species able to be grown in the laboratory, the prospects of new discoveries in the rare biosphere seem as vast as microbial diversity itself.