Plants and animals possess biogeographies, e.g. monkey puzzle (Araucaria araucana) trees are only found in South America and kangaroos only live in Australia; in contrast microbes do not possess biogeographies ... or at least that’s the common assumption. Microbes are so small and their populations are so huge that no mountain range, desert or ocean is a big enough barrier to prevent their dispersal to every corner of the planet. Every microbe will therefore be found wherever there is suitable habitat, e.g. if there are similar Scottish and Australian lakes, the same diatoms will be present in each. That’s the theory (e.g. Finlay 2002), often paraphrased as the 'everything is everywhere' paradigm. But - no surprises here - the real picture appears to be much more complex! Some of the main weaknesses in the theory of ubiquitous microbial dispersal are:
- a coarse species concept is often used, e.g. S. pupula in its traditional sense is indeed found all over the world (i.e. it is described as a cosmopolitan species akin to sparrows and pigeons). However, we know from our research (e.g. Evans et al. 2008) that “S. pupula” is composed of at least 40 different species and the key question is whether each of these is cosmopolitan or more restricted in its distribution.
- researchers who push the “everything is everywhere” theory have, in the past at least, relied on the literature to tell them whether species are widely distributed. This is an unsafe approach because of the historical and current inadequacy of species concepts.
- whenever a diatom or other microbe is suggested to be restricted or endemic to a particular area, the 'everything is everywhere' proponents say that the researchers just haven’t looked hard enough elsewhere, i.e. they have undersampled. We believe that this counter argument is over-used; for example, many diatoms need to find a partner of the opposite sex in order to complete the life cycle, which puts some constraints on the degree of rarity they can tolerate. If potential mates are too far apart, they will never find each other and sexual reproduction will be impossible (even if diatoms secrete compounds that they can 'sniff' to detect each other). It looks like polar bears are soon going to experience the same difficulties.
- no-one knows how microbes disperse, so how are they getting to every corner of the planet? It’s generally assumed that they are blown around, or that birds or bugs carry them, but for many diatoms this does not seem feasible, because they seem to be unable to tolerate drying for any significant period of time. Empirical research in this field has seriously lagged behind theory.
- most of the theory is extrapolations
of work conducted on ciliates and microscopic animals, the majority of
which do possess stages in their life cycles that are tolerant of
drying. The biology of particular groups of organisms may be as
important as organism size in controlling dispersal.