Organismal diversity encompasses the full taxonomic hierarchy and its components, from individuals upwards to populations, subspecies and species, genera, families, phyla, and beyond to kingdoms and domains. Measures of organismal diversity thus include some of the most familiar expressions of biodiversity, such as the numbers of species (i.e. species richness). Others should be better studied and more routinely employed than they have been thus far. Starting at the lowest level of organismal diversity, little is known about how many individual organisms there are at any one time, although this is arguably an important measure of the quantity and variety of life (given that, even if sometimes only in small ways, most individuals differ from one another). Nonetheless, the numbers must be extraordinary. The global number of prokaryotes has been estimated to be 4–6 x 1030 cells—manymillion times more than there are stars in the visible universe (Copley 2002)—with a production rate of 1.7 x 1030 cells per annum (Whitman et al. 1998). The numbers of protists is estimated at 104 107 individuals per m2 (Finlay 2004).Impoverished habitats have been estimated to have 105 individual nematodes per m2, and more productive habitats 106 107 per m2, possibly with an upper limit of 108 per m2; 1019 has been suggested as a conservative estimate of the global number of individuals of free-living nematodes (Lambshead 2004). By contrast, it has been estimated that globally there may be less than 1011 breeding birds at any one time, fewer than 17 for every person on the planet (Gaston et al. 2003). Individual organisms can be grouped into relatively independent populations of a species on the basis of limited gene flow and some level of genetic differentiation (as well as on ecological criteria).
The population is a particularly important element of biodiversity. First, it provides an important link between the different groups of elements of biodiversity. Second, it is the scale at which it is perhaps most sensible to consider linkages between biodiversity and the provision of ecosystem services (supporting services—e.g. nutrient cycling, soil formation, primary production; provisioning services—e.g. food, fresh water, timber and fiber, fuel; regulating services—e.g. climate regulation, flood regulation, disease regulation, water purification; cultural services—e.g. aesthetic, spiritual, educational, recreational; MEA 2005). Estimates of the density of such populations and the average geographic range sizes of species suggest a total of about 220 distinct populations per eukaryote species (Hughes et al. 1997). Multiplying this by a range of estimates of the extant numbers of species, gives a global total of 1.1 to 6.6 x 109 populations (Hughes et al. 1997), one or fewer for every person on the planet. The accuracy of this figure is essentially unknown, with major uncertainties at each step of the calculation, but the ease with which populations can be eradicated (e.g. through habitat destruction) suggests that the total is being eroded at a rapid rate.
People have long pondered one of the important contributors to the calculation of the total number of populations, namely how many different species of organisms there might be. Greatest uncertainty continues to surround the richness of prokaryotes, and in consequence they are often ignored in global totals of species numbers. This is in part variously because of difficulties in applying standard species concepts, in culturing the vast majority of these organisms and thereby applying classical identification techniques, and by the vast numbers of individuals. Indeed, depending on the approach taken, the numbers of prokaryotic species estimated to occur even in very small areas can vary by a few orders of magnitude (Curtis et al. 2002; Ward 2002). The rate of reassociation of denatured (i.e. single stranded) DNA has revealed that in pristine soils and sediments with high organic content samples of 30 to 100 cm^3 correspond to c. 3000 to 11 000 different genomes, and may contain 104 different prokaryotic species of equivalent abundances (Torsvik et al. 2002). Samples from the intestinal microbial flora of just three adult humans contained representatives of 395 bacterial operational taxonomic units (groups without formal designation of taxonomic rank, but thought here to be roughly equivalent to species), of which 244 were previously unknown, and 80% were from species that have not been cultured (Eckburg et al. 2005).
Likewise, samples from leaves were estimated to harbor at least 95 to 671 bacterial species from each of nine tropical tree species, with only 0.5% common to all the tree species, and almost all of the bacterial species being undescribed (Lambais et al. 2006). On the basis of such findings, global prokaryote diversity has been argued to comprise possibly millions of species, and some have suggested it may be many orders of magnitude more than that (Fuhrman and Campbell 1998; Dykhuizen 1998; Torsvik et al. 2002; Venter et al. 2004). Although much more certainty surrounds estimates of the numbers of eukaryotic than prokaryotic species, this is true only in a relative and not an absolute sense. Numbers of eukaryotic species are still poorly understood. A wide variety of approaches have been employed to estimate the global numbers in large taxonomic groups and, by summation of these estimates, how many extant species there are overall. These approaches include extrapolations based on counting species, canvassing taxonomic experts, temporal patterns of species description, proportions of undescribed species in samples, well-studied areas, well-studied groups, speciesabundance distributions, species-body size distributions, and trophic relations (Gaston 2008). One recent summary for eukaryotes gives lower and upper estimates of 3.5 and 108 million species, respectively, and a working figure of around 8 million species. Based on current information the two extremes seem rather unlikely, but the working figure at least seems tenable. However, major uncertainties surround global numbers of eukaryotic species in particular environments which have been poorly sampled (e.g. deep sea, soils, tropical forest canopies), in higher taxa which are extremely species rich or with species which are very difficult to discriminate (e.g. nematodes, arthropods), and in particular functional groups which are less readily studied (e.g. parasites). A wide array of techniques is nowbeing employed to gain access to some of the environments that have been less well explored, including rope climbing techniques, aerial walkways, cranes and balloons for tropical forest canopies, and remotely operated vehicles, bottom landers, submarines, sonar, and video for the deep ocean.
Molecular and better imaging techniques are also improving species discrimination. Perhaps most significantly, however, it seems highly probable that the majority of species are parasites, and yet few people tend to think about biodiversity from this viewpoint. How many of the total numbers of species have been taxonomically described remains surprisingly uncertain, in the continued absence of a single unified, complete and maintained database of valid formal names. However, probably about 2 million extant species are regarded as being known to science (MEA 2005). Importantly, this total hides two kinds of error. First, there are instances in which the same species is known under more than one name (synonymy). This is more frequent amongst widespread species, which may show marked geographic variation in morphology, and may be described anew repeatedly in different regions. Second, one name may actually encompass multiple species (homonymy). This typically occurs because these species are very closely related, and look very similar (cryptic species), and molecular analyses may be required to recognize or confirm their differences. Levels of as yet unresolved synonymy are undoubtedly high in many taxonomic groups. Indeed, the actual levels have proven to be a key issue in, for example, attempts to estimate the global species richness of plants, with the highly variable synonymy rate amongst the few groups that have been well studied in this regard making difficult the assessment of the overall level of synonymy across all the known species.
Equally, however, it is apparent that cryptic species abound, with, for example, one species of neotropical skipper butterfly recently having been shown actually to be a complex of ten species (Hebert et al. 2004). New species are being described at a rate of about 13 000 per annum (Hawksworth andKalin-Arroyo 1995), or about 36 species on the average day. Given even the lower estimates of overall species numbers this means that there is little immediate prospect of greatly reducing the numbers that remain unknown to science. This is particularly problematic because the described species are a highly biased sample of the extant biota rather than the random one that might enable more ready extrapolation of its properties to all extant species. On average, described species tend to be larger bodied, more abundant and more widespread, and disproportionately from temperate regions. Nonetheless, new species continue to be discovered in even otherwise relatively well-known taxonomic groups. New extant fish species are described at the rate of about 130–160 each year (Berra 1997), amphibian species at about 95 each year (from data in Frost 2004), bird species at about 6–7 each year (Van Rootselaar 1999, 2002), and terrestrial mammals at 25–30 each year (Ceballos and Ehrlich 2009).
Recently discovered mammals include marsupials, whales and dolphins, a sloth, an elephant, primates, rodents, bats and ungulates. Given the high proportion of species that have yet to be discovered, it seems highly likely that there are entire major taxonomic groups of organisms still to be found. That is, new examples of higher level elements of organismal diversity. This is supported by recent discoveries of possible new phyla (e.g. Nanoarchaeota), new orders (e.g. Mantophasmatodea), new families (e.g. Aspidytidae) and new subfamilies (e.g. Martialinae). Discoveries at the highest taxonomic levels have particularly served to highlight the much greater phyletic diversity of microorganisms compared with macroorganisms. Under one classification 60% of living phyla consist entirely or largely of unicellular species (Cavalier-Smith 2004). Again, this perspective on the variety of
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