Taxonomic Diversity: The Shifting Baseline of Biogeography
21
because we cannot yet determine how far we are from a final listing of species, however
we decide to define that unit.
In fact, how many “species” may there be in the sea? It is certainly unhelpful to speak
in terms of billions—the most extreme case I have seen is a claim for 1 × 10
13 species. The
extraordinarily high numbers projected for some groups of organisms have been arrived
at by what has been called statistical legerdemain, also practiced in the enumeration of
terrestrial insects. Extrapolation from the fauna of 19 forest trees in Panama gave an
estimate of 30 million insects globally and, similarly, by extrapolating from a few box-core
samples yielding 800 species of small metazoa, Grassle and Maciolek (1992) derived an
estimate of about 10 million marine species worldwide. I prefer the “expert opinion”
reasoning of John Briggs (1994), who, by extrapolating from the taxonomic literature,
arrived at a total of fewer than 200,000 potential species of marine eukaryotes: Porifera
9000, Cnidaria 9000, Nematoda 35,000, Annelida 15,000, Arthropoda 37,000, Mollusca
29,000, Bryozoa 15,000, Chordata 15,000, remainder 14,000. I have suggested that there
are perhaps 3000 Linnaean species of metazooplankton (Longhurst, 2001), an estimate
that sits well with the ∼4000 species each of planktonic protists and of phytoplankton
proposed by others.
Unfortunately, the lack of closure in taxonomic procedures has important consequences for marine biogeographers. A typical example is what lies behind a statement of
John McGowan, who wrote in 1971 that “holoplanktonic zooplankton are taxonomically
well-known at the species level.” This was surely an overoptimistic statement that is
worth brief reexamination. The taxonomic status of copepods of the genus Calanus, as it
has developed since McGowan’s comment, is probably typical of copepod genera. Two
North Atlantic species are surely the best known of all zooplankters, but C. finmarchicus
(Gunnerus, 1770) and C. helgolandicus (Claus, 1863) were not clearly distinguished until
the work of Fleminger and Hulseman (1977). They were, for a long time, thought to be
geographical races (or subspecies) of a single cosmopolitan species. Bucklin et al. (1995)
suggested that we should recognize 14 species of Calanus, of which 3 (C. hyperboreus,
C. simillimus, and C. propinquus) are morphologically distinct from the remainder, which
are themselves distinguishable only by expert analysis of fine differences in secondary sexual characters of the exoskeleton. These comprise two species groups: a small arcto-boreal
group (C. finmarchicus, C. glacialis, and C. marshallae) and a larger group of the midlatitudes (C. helgolandicus, C. pacificus, C. australis, C. orientalis, C. euxinus, C. aguihensis,
C. chilensis, and C. sinicus).
But there are also significant differences between individuals of C. pacificus from the
California coast, from the North Pacific gyre, and from Puget Sound, so that subspecies
rank is accorded to the populations of these three regions. This arrangement recalls the
work of Fleminger and Hulseman (1977), who referred to 10 morphologically distinguishable forms of C. helgolandicus from eastern and western Atlantic regions. In neither
case do we know if the characters—morphological or genetic—lie along gradients or are
discontinuous, and we know very little of the extent to which species of Calanus may
have sympatric distributions. We should not be surprised that authors of papers on the
ecology of Calanus still feel constrained to explain in detail the taxonomic status of their
material. If this is the unhappy situation for what is perhaps the most extensively studied
genus of marine plankton, you may imagine the status of less-studied genera. And, don’t
forget, there are almost 200 other genera of calanoid copepods to be specified, not to
speak of all those cyclopoids and harpacticoids.
Of course, much can be done once a group has been critically revised in the classical
manner, as was done for chaetognaths and siphonophores by Alvarino (1965, 1971) or
for Clausocalanus by Frost and Fleminger (1968). Alvarino reduced the 106 species of
chaetognaths listed in 1935, and the 70–80 in a 1961 listing, to only 52 species. To follow
the trail of a chaetognath species quoted in the literature so as to integrate all information
21
because we cannot yet determine how far we are from a final listing of species, however
we decide to define that unit.
In fact, how many “species” may there be in the sea? It is certainly unhelpful to speak
in terms of billions—the most extreme case I have seen is a claim for 1 × 10
13 species. The
extraordinarily high numbers projected for some groups of organisms have been arrived
at by what has been called statistical legerdemain, also practiced in the enumeration of
terrestrial insects. Extrapolation from the fauna of 19 forest trees in Panama gave an
estimate of 30 million insects globally and, similarly, by extrapolating from a few box-core
samples yielding 800 species of small metazoa, Grassle and Maciolek (1992) derived an
estimate of about 10 million marine species worldwide. I prefer the “expert opinion”
reasoning of John Briggs (1994), who, by extrapolating from the taxonomic literature,
arrived at a total of fewer than 200,000 potential species of marine eukaryotes: Porifera
9000, Cnidaria 9000, Nematoda 35,000, Annelida 15,000, Arthropoda 37,000, Mollusca
29,000, Bryozoa 15,000, Chordata 15,000, remainder 14,000. I have suggested that there
are perhaps 3000 Linnaean species of metazooplankton (Longhurst, 2001), an estimate
that sits well with the ∼4000 species each of planktonic protists and of phytoplankton
proposed by others.
Unfortunately, the lack of closure in taxonomic procedures has important consequences for marine biogeographers. A typical example is what lies behind a statement of
John McGowan, who wrote in 1971 that “holoplanktonic zooplankton are taxonomically
well-known at the species level.” This was surely an overoptimistic statement that is
worth brief reexamination. The taxonomic status of copepods of the genus Calanus, as it
has developed since McGowan’s comment, is probably typical of copepod genera. Two
North Atlantic species are surely the best known of all zooplankters, but C. finmarchicus
(Gunnerus, 1770) and C. helgolandicus (Claus, 1863) were not clearly distinguished until
the work of Fleminger and Hulseman (1977). They were, for a long time, thought to be
geographical races (or subspecies) of a single cosmopolitan species. Bucklin et al. (1995)
suggested that we should recognize 14 species of Calanus, of which 3 (C. hyperboreus,
C. simillimus, and C. propinquus) are morphologically distinct from the remainder, which
are themselves distinguishable only by expert analysis of fine differences in secondary sexual characters of the exoskeleton. These comprise two species groups: a small arcto-boreal
group (C. finmarchicus, C. glacialis, and C. marshallae) and a larger group of the midlatitudes (C. helgolandicus, C. pacificus, C. australis, C. orientalis, C. euxinus, C. aguihensis,
C. chilensis, and C. sinicus).
But there are also significant differences between individuals of C. pacificus from the
California coast, from the North Pacific gyre, and from Puget Sound, so that subspecies
rank is accorded to the populations of these three regions. This arrangement recalls the
work of Fleminger and Hulseman (1977), who referred to 10 morphologically distinguishable forms of C. helgolandicus from eastern and western Atlantic regions. In neither
case do we know if the characters—morphological or genetic—lie along gradients or are
discontinuous, and we know very little of the extent to which species of Calanus may
have sympatric distributions. We should not be surprised that authors of papers on the
ecology of Calanus still feel constrained to explain in detail the taxonomic status of their
material. If this is the unhappy situation for what is perhaps the most extensively studied
genus of marine plankton, you may imagine the status of less-studied genera. And, don’t
forget, there are almost 200 other genera of calanoid copepods to be specified, not to
speak of all those cyclopoids and harpacticoids.
Of course, much can be done once a group has been critically revised in the classical
manner, as was done for chaetognaths and siphonophores by Alvarino (1965, 1971) or
for Clausocalanus by Frost and Fleminger (1968). Alvarino reduced the 106 species of
chaetognaths listed in 1935, and the 70–80 in a 1961 listing, to only 52 species. To follow
the trail of a chaetognath species quoted in the literature so as to integrate all information