of microbial species are correlated (Fig.9.2 ). Unlike most macroscopic organisms,
whose dispersal and spatial distribution are determined by historical factors such
as continental drift, or physical barriers such as mountain ranges, these probably
have little influence on the geographical distribution of microbes. The microbial
species that thrive in a particular habitat are probably the result of habitat
properties alone and, with their capacity for ubiquitous dispersal, we can assume
that they thrive wherever their preferred habitat type is realized on Earth.Figure 9.1Random distribution of
soil protozoa – the spatial
distributions of rare and abundant
species of testate amoebae in a
grassland soil. Each line is the
regression through six data points
for each species. For clarity only
the six most abundant species and
the six rarest species are included.
The bold line represents the
equality of variance and mean. The
rare species may simply have been
transported randomly to the site
but unable to produce a population
in this particular soil habitat type.
Further details are in Finlayet al.
(2001).Figure 9.2The local and global
abundances of individuals in soil
protozoan species are significantly
correlated (p<0.001). This analysis is
based on frequency of detection
(a surrogate measure of abundance per
species) of 95 ciliated protozoan
species in 150 soil samples from a one-
hectare upland grassland in Scotland,
and of the same species in 606 soil
samples collected worldwide. Adapted
from Finlayet al.(2001).168 B. J. FINLAY AND G. F. ESTEBAN