A one-hectare freshwater pond will support something in the region of 10^18
bacteria, 10^16 protists and 10^11 small animals (Finlay & Maberly, 2000 ).
Most of the organisms that Leeuwenhoek observed were protozoa, micro-
algae, and a diversity of mixotrophic protists that mostly lie in the size range
2–200mm. Flagellates are very small (mostly2–5mm, and almost all< 20 mm).
Most amoebae are 5–50mm, and most ciliates are 15–200mm. Thus the protozoa,
together with meiofauna such as rotifers, tardigrades and gastrotrichs (almost
all<1 mm), have body sizes roughly 100 times bigger than the bacteria on which
many of them feed.
Compared with macroscopic animals, protozoa are extremely abundant: one
gram of soil typically contains around 15 000 naked amoebae (Finlayet al., 2000 ),
and every millilitre of fresh- or seawater on the planet supports 10^2 to 10^6
heterotrophic flagellates (Berninger, Finlay & Kuuppo-Leinikki,1991). The abso-
lute abundance of organisms in any aquatic habitat is typically astronomical
(Finlay, 2002 ) – indeed the areal abundance of individuals is more than, say, ten
orders of magnitude greater than that of average-sized mammal species
(Fig. 9.3). At the global scale, the number of individuals of a typical protozoan
species is virtually incalculable.
One consequence of great abundance is that the probability of dispersal is
significantly elevated. An individual protozoon may not survive transport
through groundwaters, or attached to a duck’s foot, or carried across the
Atlantic in a harmattan dust cloud. However, population sizes with astronomi-
cal dimensions must increase the probability that geographical barriers will be
crossed in the course of time, leaving few if any absolute barriers to (free-living)
microbial dispersal.Figure 9.3Body size and areal abundance of
free-living organisms, adapted from Finlay
(2002), where more detailed information is
available. In the microbial eukaryotes, each
data point represents a species. The ‘mammals’
data are included only to establish the existence
of a consistent pattern stretching over 12 orders
of magnitude in abundance.170 B. J. FINLAY AND G. F. ESTEBAN