The ‘problem’ is readily illustrated with the following simple example (Finlay &
Fenchel,2004 ). Consider the aforementioned proposition ‘if it did exist else-
where, it would have been found already’. On the basis of results from previous
sampling campaigns, we might predict that a certain ‘rare’ protist species lives in
the oxygen-depleted hypolimnion of a productive freshwater pond. Priest Pot, in
the UK, is such a pond. It has an area of one hectare, and a 1.75m-deep hypo-
limnion holding roughly 6300tonnes of water. Assume that the protist species we
are looking for exists in the pond as a population of one million individuals. This
figure may appear to have little to do with the idea of rarity, but each individual
would, on average, be surrounded by 6.3 litres of water, giving it a volume
Figure 9.6Local species richness calculated as a ratio of the known or ‘best estimate’
global species richness for the same taxonomic group (adapted from Finlay & Fenchel,
2004 ). Moving from taxa with the largest to the smallest mean size, the symbols
represent, (a)Priest Pot(filled symbols) – name of taxon followed in parenthesis by
organism size (mm), and number of species in taxonomic group: fish (220, 5); Odonata
(43, 15); leeches (20, 10); Chironomidae (14, 12); Mollusca (9.8, 71); Tricladida (6, 3);
Ostracoda (0.804, 9); Nematoda (0.8, 71); Cladocera (0.73, 31); Rotifera (0.236, 22);
gastrotrichs (0.164, 6); testate amoebae (0.097, 39); ciliates (0.076, 274); euglenids (0.048, 21);
Synura(0.047, 6);Korotnevella(0.045, 6); naked amoebae (0.037, 64),Cochliopodium(0.03, 13);
heliozoans (scale-bearing; 0.0271, 57); diatoms (0.0234, 93);Mallomonas(0.016, 24);
heterotrophic flagellates (0.0158, 86),Spiniferomonas(0.006, 8);Paraphysomonas(0.005, 40);
Cyathobodo(0.005, 7);Luffisphaera(0.004, 15). (b)Niva^88 Bay(open symbols) – fish (165, 9);
polychaetes (45, 9); Mollusca (30, 11); Crustacea (13, 44); Nematoda (1.3, 55); Turbellaria
(1.2, 34); Ostracoda (0.557, 15); gastrotrichs (0.29, 4); ciliates (0.11, 201); dinoflagellates
(0.0446, 103); Prasinophyta (0.0337, 25); naked amoebae (0.0242, 42); flagellates (0.0077, 75).
The least squares regression through the combined data set is highly significant (p<0.001).
BODY SIZE AND BIOGEOGRAPHY 177