experiments such as these are not carried out more frequently, especially when
the rewards (for example, discovering the true dimensions of microbial eukary-
ote diversity in a very small sample) are so easily gained.Dispersal of free-living protists is essentially random
Any contender for a randomly distributed free-living species will have small body
size, high absolute abundance, be easily dispersed and have minimal or non-
existent social or parasitic behaviour. Probably all soil protozoa fit the bill. They
are adapted for life in soil, they form protective cysts (Finlay & Fenchel,2001 ),
there is evidence they spend much of their time as cysts, and the cysts remain
viable for long periods of time (e.g. 49 years, Goodey,1915 ). Thus, encysted soil
protozoa could fit the criteria for random distribution. To test this hypothesis, we
examined the possibility that the rarer testate amoebae species in a one-hectare
grassland site in the southern uplands of Scotland could be randomly distributed
(for a review see Estebanet al., 2006 ). We compared the spatial distributions of six
rare species and six abundant species, obtaining a mean and variance for each
species across the 25 plots in which the grassland site was divided, and repeating
this on six sampling occasions (Finlayet al., 2001 ). We found that the rare and
abundant species did appear to have different spatial distributions. Like most
animal species the abundant testate species had aggregated distributions
(Fig.9.1 ), whereas the distributions of rare species were close to random (log-
variance was closely proportional to the log-mean, the intercept was small, and
we concluded that the distributions of these species were effectively random).
Unlike the abundant species, with sustained population growth at this site, the
rare species were probably transported randomly to the site but unable to initiate
population growth in this particular habitat type.Cosmopolitan distribution implies the existence of similar species
inventories in similar habitats irrespective of geographical distance
This could be tested by investigating the ciliated protozoa living in a habitat that
is separated from Northern Europe by geographical barriers and great distance –
the sediment of a Holocene volcanic crater-lake in Australia (Finlayet al., 1999;
Estebanet al., 2000). Of the 85 ciliate species recorded, all were already known
from northern Europe by the year 1935 except one that was described in 1985.
As the crater is only 2 km from the Southern Ocean, seaspray renders the water
brackish (2–5%), and eight of the species recorded were typical of brackish
waters. One of them (Tracheloraphis caudata) was a typical marine interstitial
ciliate. This species is fragile and it does not form protective cysts – so, it was
unclear how it may have reached the crater-lake sediment. The same can also be
said for the anaerobic ciliatePlagiopyla frontata, a marine ciliate that contains
unmistakable endosymbiotic methanogenic bacteria in both Northern Europe
and Australia.174 B. J. FINLAY AND G. F. ESTEBAN