Species Diversity Within and Among Ecosystems - Ecological Systems - page 256

Table 14.1 Estimates of number of eukaryote species globally, following [ 10 ]
Estimate References Method

30 million Erwin [ 17 ] Extrapolation from samples

3-5 million Raven [ 18 ] Ratios known: unknown species

10-80 million Stork [ 19 ] Extrapolation from samples

4.9-6.6 million Stork and Gaston [ 20 ] Ratios known: unknown species

1.84-2.57 million Hodkinson and Casson [ 21 ] Ratios known: unknown species

5 million Hodkinson [ 22 ] Ratios known: unknown species

4-6 million Novotny et al. [ 23 ] Extrapolation from samples

7.4-10 million Mora et al. [ 16 ] Extrapolation from samples

reproductive characteristics. In addition, most populations within a species share
common ancestors with other populations in the very recent past, which in turn
causes them to not be significantly differentiated [ 10 ].
Since the middle of the twentieth century, the most commonly accepted definition
has been the biological species concept [ 10 ],whichdefinesaspeciesasagroupof
interbreeding natural populations whose members are unable to successfully reproduce
with members of other such groups [ 11 ]. As species based on this definition have
natural and objective boundaries due to gene flow [ 12 ], they provide natural units for
biodiversity assessment [ 5 ]. While the approach of classifying species will influence
both conservation priorities and the scale of intervention [ 5 ], the important thing for
conservationists is that the commonly understood units can be defined and
interventions implemented and assessed. There will always be issues with using species
as a basis for biodiversity assessment, but many of these can be overcome [ 5 , 10 , 13 ].
The next question that often arises is how many species exist on Earth. While
there are no reliable estimates for prokaryotes [ 10 , 14 , 15 ], many estimates exist for
eukaryotic species. Recent studies [ 16 ] predict 8.7 million species globally, of

which 2.2 million are marine species. Earlier estimates have varied greatly, with

most falling between five and ten million (see Table 14.1 ). However, these
estimates could be much higher if poorly known groups such as deep-sea
organisms, algae, or fungi have more species than currently believed [ 10 ]. Incom-
plete sampling, lack of robust extrapolation approaches, controversy over the
underlying assumptions, and subjectivity are all cited reasons for the uncertainty
[ 16 ]. However, with only around 1.7 million species currently described [ 16 ] and
only 15,000 newly described species each year [ 25 ], the key point is that a major
knowledge gap still exists in this area; there is a likelihood of losing species to
extinction before ever describing them.

Patterns of Species Diversity

Despite the uncertainty around how to define a species and the total number of
species on the planet, the diversity of species is evident in the world around us.
Species are unevenly distributed across the globe [ 10 , 26 ] in terms of both