Our understanding of the global magnitude of biodiversity, even considering the level for which we have the best information, species, is still very limited. Complete catalogs of the described, valid species exist for only a few groups of organisms, and the total can only be estimated. This figure is estimated to be around 1.5 million and includes exclusively eukaryotic organisms (basically plants, some groups of animals, and some fungi). The number and definition of prokaryotic species and viruses are still very limited. It is not surprising, therefore, that current estimates of the total number of species in the world span over 2 orders of magnitude (106-108). Methods employed to estimate global diversity can be classified in three broad categories: (1) ratios of known to unknown groups, (2) extrapolation from samples, and (3) expert opinions by scientists who understand the level of diversity in a particular group of organisms well, or combinations thereof. An example of the first approach is the estimation carried out by ecologists Nigel Stork and Kevin Gaston in 1990. They first noted that the ratio of the number of species of butterflies to all species of insects on the well-known British fauna is 67:22 000. Assuming that such a relationship was robust overall, and based on an estimate of 15 000-20 000 species of butterflies worldwide, they scaled up to a total figure between 4.9 and 6.6 million species of insects. The second approach is illustrated by the study carried out by entomologist Terry Erwin at a rainforest site in Panama. Erwin performed a systematic sampling using a fogging protocol targeted on the beetle fauna living in the canopy of the tree Luehea semannii. Based on the magnitude of the fauna collected and using a series of assumed relationships involving the ratio between beetles and other types of arthropod species, the degree of host specificity of arthropods, and the expected number of tree species to be found in the Tropics, Erwin came up with the astounding estimate of 30 million species of tropical arthropods. Robert May attempted an estimate based on the known relationship between body-size category and number of species of animals. Assuming that such relationship holds for individuals as small as 0.2 mm, May calculated that there might be about 10 million species of animals. All these estimations are based on a series of assumptions that await confirmation. However, there is some level of agreement that the plausible number of global species falls within a range of 5-15 million and a recent review suggests a best guess of around 7 million species.
Yet, new species are continuously described sometimes as a consequence of refinements in the taxonomic classifications but commonly as the result of real discoveries from surveys in nature. Current rates of publication of new species are significant, including some 13 000 animal species per year. This makes it evident that the task of describing the total number of species on Earth will not be completed for many decades, even assuming they can all be collected and analyzed by the appropriate experts before they go extinct due to anthropogenic impact. These discoveries are not restricted to small-sized organisms but also include species of large mammals such as monkeys and deer. Even more, exploration of remote areas sometimes results in the discovery of organisms that generate a biodiversity revolution because of their degree of biological novelty. Examples of this are the plant Lacandonia schismatica, found two decades ago in southeast Mexico. This plant was found to belong into an entirely new taxonomic family (Lacandonianceae), due to its spectacular characteristics, including that it presents its reproductive organs with a spatial disposition the other way around from any other known flowering plant: male structures toward the center of the flowers and female structures at the periphery! Equally spectacular as taxonomic novelty is the discovery, a few years ago, of an insect in Africa that combines characteristics of a cricket and a stick insect, so different to any other insect that it required the creation of a new taxonomic order (Mantophasmatodea). All these discoveries underscore the level of uncertainty we have about the magnitude of biodiversity. This uncertainty becomes more striking when we take into consideration that species richness is only one facet of biodiversity, as discussed earlier.
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