The list of the most frequently occurring diatom species consists solely of
taxa with cosmopolitan distribution. This is presumably because common
species are adapted to the most common habitat, which in the case of the United
States and Europe is a nutrient-enriched, moderately alkaline river of the tem-
perate climatic zone. Abundance of this habitat type in one continent increases
the probability of dispersal to other parts of the world – with (a) the consequence
that locally abundant diatom species tend to be globally abundant (Finlayet al.,
2002 ), and (b), the corollary, that rare habitats such as oligotrophic waters,
support much lower dispersal. The thesis that locally abundant or rare species
tend to be globally abundant or rare is a recurring theme within this chapter and
in macroecology more generally (Bell, 2001 ). See, for example, exactly the same
phenomenon revealed within the soil protozoa (Fig.9.2).Local:global species ratios
Within any taxonomic group of small organisms, a large proportion of the
global species richness is found in a local area, and the local:global species
ratio for that group tends to be very high (Finlay & Clarke, 1999a, b;
Hillebrand & Azovsky,2001). For example, the percentage of the global species
pool recorded from Priest Pot is>15% for ciliates,>50% for heliozoa and>80%
for chrysomonads (all recorded species were actually recorded from roughly one
tenth of a cubic centimetre of sediment). It follows that, if cosmopolitan distri-
bution is the product of high absolute abundance, populations of the larger,
non-cosmopolitan organisms will have low local:global ratios. This appears to be
the case – the percentage of the global species pool of aquatic insects recorded in
Priest Pot is<0.001, but with decreasing mean body size, local species richness
represents an increasing fraction of global species richness, and it approaches
100% in the case of protozoan species with body sizes0.004mm. Finlay and
Fenchel (2004) found that a systematic relationship does exist. Even in the
case of two small aquatic ecosystems – one freshwater, the other marine – the
local:global species ratio increases with decreasing organism size, incorporating
the combined freshwater and marine species in a single relationship (Fig.9.6).Undersampling
The problem of undersampling persists because sampling regimes are often
inadequate for detecting protists and other microfauna. This is especially so for
the rarer species. The frequently cited argument of undersampling – ‘this
species must be an endemic because it is morphologically so distinctive that if
it existed elsewhere, it would have been recorded by now’ – might be convincing
for large mammal species but, with respect to the protists, it evades the crucial
question. It can confidently be demonstrated that a large mammal species does
not exist outside of its endemic range, but how are we to demonstrate that a
particular protist species does not exist elsewhere on the planet?176 B. J. FINLAY AND G. F. ESTEBAN