One of the more awkward and contentious problems for supporters of
the ‘restricted distribution’ of microbial species has been the difficulty in
establishing the existence of microbial ‘endemics’. This problem may be insur-
mountable. We can be confident in the knowledge that a particular large
mammal species does not exist outside of its ‘endemic range’, but it is probably
impossible to demonstrate that a microbial species does not exist elsewhere in
the biosphere. Moreover, if the geographical ranges of microbial ‘endemics’
cannot be established, the term ‘endemic microbial species’ is probably
meaningless.
The fundamental characteristics of biodiversity at the microbial and macro-
scopic levels (Table9.1) are markedly different – indeed they appear to represent
two distinctly different ‘provinces’ of biodiversity. That most macroscopic
organisms have biogeographies is well established, but the case for ubiqui-
tous neutral dispersal and cosmopolitan distribution driven by population
abundance in free-living organisms<1 mm has not yet received unanimous
support.
The absolute abundance of microbial species-populations
Various independent studies confirm that the abundance of individuals within
morphospecies is inversely related to body size, and that microbial species
therefore contain extremely large numbers of individuals (see for example
Damuth1981; Schmid, Tokeshi & Schmid-Araya,2000; Finlay,2002). This
is readily confirmed with empirical evidence from any small ecosystem.
Table 9.1The two provinces of biodiversity – adapted from Finlay, Esteban and
Fenchel ( 2004 ).
Macro-organisms Micro-organismsAbsolute abundance Low High
Rates of dispersal Low High
Rates of allopatric speciation High Low
Rates of species extinction High Low
Relative number of endemics High Low/None
Global number of morphospecies High Low
Proportion of global species pool found locally Low High
Cryptic persistence of species Variable High
Persistence of specific morphotypes over
geological time
Low HighLarge-scale distribution determined by historical
factors e.g. continental drift
High LowBODY SIZE AND BIOGEOGRAPHY 169