The search for unknown biodiversity.

I n a world being rapidly transformed by human activities, an alarming possibility is that many species might disappear before we have a chance to study or even scientifically describe them. This possibility goes beyond a simple desire to document biodiversity, because unknown species could have important benefits for humanity. For instance, who might have imagined that an obscure herb endemic to Madagascar, the rosy periwinkle (Catharanthus roseus), would yield the only known treatment for childhood leukemia (1)? How prevalent are undescribed species, and where do they occur? Are they largely concentrated in areas that we already consider conservation priorities, or are they often found elsewhere—meaning that we may need to identify additional priority areas? These questions motivated the study by Joppa et al. (2) in PNAS. With considerable pluck in the face of data limitations and important uncertainties, they attempt to estimate the number of undescribed flowering plant species across much of the terrestrial world (2). Joppa et al. (2) conclude that the bulk of undescribed species are found in socalled biodiversity hotspots. Twenty-five hotspots were initially identified by Myers et al. (3) in a seminal paper that has had a major impact on global conservation strategies (4) and is the most highly cited article in the fields of ecology and environmental science (5). Myers et al. (3) defined hotspots by intersecting areas with large numbers of endemic vascular plant and terrestrial vertebrate species with those areas that had suffered severe (>70%) habitat loss. Remaining native habitats in the hotspots that they identified span just 1.4% of the planet’s land surface but sustain nearly one-half of all known plants and over one-third of all vertebrates (3). Hectare for hectare, this finding makes these hotspots the most biologically important real estate on Earth—assuming that large numbers of unknown species do not occur in other locales. In an effort to discern where unknown plant species occur, Joppa et al. (2) devise a statistical model that estimates, for a total of 50 broad geographic regions worldwide, the rate at which new species are being described scientifically. Based on their earlier work (6, 7) and expert opinion, they estimate that around 15% of all flowering plant species remain undescribed globally (2). The present study suggests that different geographic regions vary widely in the rate of new species discovery (2), and Joppa et al. (2) assume in their analysis that those areas with higher recent rates have a higher proportion of undiscovered species. It is apparent that the study by Joppa et al. (2) is a prime example of best available data inference. Among the taller hurdles that they face are the following challenges (2).