to organisms with microbial dimensions will probably not break down at the
global scale (Finlayet al., 2001).Concluding remarks
It may be easier to detect the footprints of cosmopolitan distribution (e.g.
identical genotypes across the global scale) than to establish the cause of an
apparent biogeographic pattern. Take for example the relationship between
species richness and latitude. Most latitudinal gradients have been described
for macrofauna and plants, and species richness tends to be significantly higher
at low latitudes. In the case of (cosmopolitan) microbial eukaryotes, however, we
do not expect any discernible latitudinal gradient in species richness. Hillebrand
and Azovsky (2001 ) pursued an approach that nearly succeeded. Following
expectations, no correlation was found between latitude and species richness;
but in the southern hemisphere, there was a significant negative correlation –
probably an artefact based on some very low richness values from the Antarctic.
This whole area of science seems to have acquired the status of a ‘hot topic’, and
we can expect to see previously unforeseen developments in the near future.
We close here with three points:(1) Protists thrive wherever their preferred habitat requirements are met and there are no
barriers to dispersal. This is a key point. It is usually relatively straightforward
to observe microfaunal species thriving in their preferred habitat type,
but with few exceptions, this is not so straightforward with bacteria.
Nevertheless, Horner-Devineet al.(2004) discovered that sites sharing sim-
ilar environmental characteristics were also similar in bacterial taxonomic
composition – i.e. bacteria, like protists, thrive wherever their preferred
habitat is realized.
(2) With decreasing mean body size, local species richness becomes an increasing
fraction of global species richness. This helps to explain the relatively flat
taxon richness-area relationships in microbial eukaryotes (e.g. Green
et al., 2004), and the observation that local species richness (body sizes
0.004 mm) can be almost as great as global species richness.
(3) The undersampling problem. Historically, undersampling has been a major
problem, and in the ‘molecular age’ the almost infinitely variable riboso-
mal DNA sequences contribute little if anything to ease the problem. Now
that species-specific primers can be developed routinely, our experience is
that the scale of sampling protists and microfauna could potentially
increase dramatically.Acknowledgements
This work was carried out with financial assistance from the Natural
Environment Research Council (Marine and Freshwater Microbial Biodiversity182 B. J. FINLAY AND G. F. ESTEBAN