CHAPTER NINE
Body size and biogeography
B.J.FINLAY
Natural Environment Research Council, UKG.F.ESTEBAN
Natural Environment Research Council, UKIntroduction
June 9th, having received early in the morning some rain-water in a dish...and exposed it to the air
about the third story of my house...I did not think I should then perceive any living creatures therein;
yet viewing it attentively, I did, with admiration, observe a thousand of them in one drop of water,
which were the smallest sort that I had seen hitherto.
(From a letter written in 1676 by Antonie van Leeuwenhoek, who had a passion for
designing and building ‘magnifying glasses’.)
Leeuwenhoek was almost certainly the first person to see protozoa and other
microfauna, and the first to record their huge population sizes. He could not
explain how the microbes got into the ‘dish of rainwater’, and this rather
disappointing level of understanding has not changed much in 300 years.
The debate is rather polarized. On one hand are those who draw attention to
the possibility that a significant proportion of free-living microbial species may
be geographically restricted (examples include Mann & Droop,1996; Foissner,
1999 ). On the other hand are those who recognize that this is at odds with the
alternative hypothesis that neutral dispersal of small organisms is driven by
extraordinarily large numbers of the individuals themselves. Although the
probability of an individual microbe being transported over great distance is
vanishingly small, a multitude of interacting processes and events, both com-
mon and rare (hurricanes, transport in wet fur and feathers, etc.), in combina-
tion with huge population sizes, are expected to drive large-scale dispersal
across all spatial scales. A fitting analogy is the purchase of lottery tickets –
buying many tickets increases the probability of winning. Similarly, if a species-
population is big enough, some individuals will, for purely statistical reasons, be
transported over great distances.
Over time, therefore, the dispersal of microbes may be essentially random,
with the rate and scale of dispersal determined mainly by global population size
(Finlay, Monaghan & Maberly,2002 ). This view is supported by the discovery of
‘signatures’ of randomness (Fig.9.1 ) in the spatial distribution of soil protozoan
species, and by the observation that local and global abundances of a wide range
Body Size: The Structure and Function of Aquatic Ecosystems, eds. Alan G. Hildrew, David G. Raffaelli and Ronni
Edmonds-Brown. Published by Cambridge University Press.#British Ecological Society 2007.