The ubiquitous dispersal of microbes initiates a train of patterns and pheno-
mena, such as low rates of allopatric speciation, species extinction and endem-
ism, as described below.
The cosmopolitan–biogeography transition
Over the past 200 years or so, luminaries such as Darwin, Ehrenberg and
Beijerinck formed an opinion based largely on anecdotal evidence that small
species had cosmopolitan distribution and large species did not. Until quite
recently, the body-size range of the transition zone remained unknown,
although Lawton (1998) surmised that it might be in the region of 1 mm.
The largest (and least abundant) macroscopic organisms may not be cosmo-
politan, but the smallest (and most abundant), ranging from bacteria (Glo ̈ckner
et al., 2000; Hagstro ̈m, Pinhassi & Zweifel, 2000 ; Massana, DeLong & Pedro ́s-Alio ́,
2000 ) to protists (Fenchel, Esteban & Finlay, 1997 ; Finlay & Clarke,1999a, b;
Darlinget al., 2000; Lee & Patterson,2000; Finlayet al., 2001; Fenchel & Finlay,
2004 ), tardigrades (Morgan & King,1976), nematodes (Abebe & Coomans, 1995 )
and rotifers (Shiel & Green,1996) do in many cases appear to be cosmopolitan.
This suggests that the global biota can be divided into two broad groups – small
species each consisting of a single, cosmopolitan metapopulation, and larger
species composed of one or more regionally restricted metapopulations. There
must be a crossover body-size range – a cosmopolitan-biogeography transition –
but neither the steepness of this transition zone, nor the organism size range in
which it occurs, was known.
Finlay and Fenchel (2004) obtained extensive inventories of free-living eukar-
yote species from two contrasting aquatic habitats – a brackish, two-hectare,
estuary-like habitat (Niva^88 Bay, north of Copenhagen) that is permanently cov-
ered by the waters of the Øresund, and a eutrophic freshwater pond in the
English Lake District, Priest Pot (Finlay & Maberly, 2000 ). The pond consists of
one hectare of open water, surrounded by roughly one hectare of floating bog.
By restricting taxonomic coverage to aquatic eukaryotes, it was possible to apply
a single species concept (the morphospecies) to all biota. Finlay, Fenchel and
their collaborators searched all accessible resources for geographic records
worldwide of the species recorded in Priest Pot and Niva^88 Bay, but the level of
undersampling of meiofauna and protists created difficulties. Much published
information is available for the northern hemisphere, but some taxonomic
groups (e.g. marine ciliates, rotifers and gastrotrichs) have hardly been studied
in the southern hemisphere. Almost all geographical records for southern hemi-
sphere chrysomonads, heliozoans and heterotrophic flagellate, for example,
can be attributed to the efforts of four or five workers in the last three decades.
The authors obtained extensive inventories of naked amoebae in both Priest Pot
and Niva^88 Bay, but there has been only very limited sampling and species-level
identification of naked amoebae outside of the Holarctic.
BODY SIZE AND BIOGEOGRAPHY 171