Endemic species
Endemic protist species – if they exist – would seriously challenge the theory of
ubiquitous dispersal. Until recently, the large and distinctive ciliateLoxodes rex
had been found only in freshwater in tropical Africa (Dragesco,1970), where it
was considered to be an example of an endemic ciliate. However, 30 years later,
it was discovered thriving in a pond in Thailand (Estebanet al., 2001), where it
joined an expanding collection of large ciliate species previously believed to be
endemics – a status they all subsequently lost with additional sampling effort.
Neobursaridium gigaswas originally recorded from Argentina (Balech, 1941 ), and
subsequently in Uganda (Nilsson,1962). The very large (2–3mm)Avelia martini-
censisfrom the West Indies (Nouzare`de, 1975) was found in Brazil and, later, in
Saudi Arabia.Condylostoma reichifrom Israel (Wilbert & Kahan,1981), was found
in tropical Africa (Dragesco & Dragesco-Kerne ́is, 1986).
These few examples show that it is difficult to find protist species whose
status as ‘endemics’ stands the test of time. Disproof of ubiquitous dispersal
might have been provided by the protist community described by Gajewskaja
(1933) from Lake Baikal – the world’s oldest and deepest lake. But the dozen or so
new ‘forms’ and new species have all been found in other parts of the world.
Loxodes rexwould, because of its very large size (2 mm long), fragility, and lack of
cyst formation, have been a prime candidate for an endemic ciliate, but it is not
and it seems to thrive in tropical freshwater habitats on different continents
(Estebanet al., 2001). In contrast, the unusual desmids of Tasmania (Tyler,1996)
indeed appear to be endemic. A possible explanation, however, is that unusual
habitats (supporting unusual, even endemic, semi-aquatic plants) provide local
microhabitats for particular, globally rare, micro-organisms. Thus, the unusual
desmids may still disperse over a large area, but remain rare or undiscovered
outside their place of discovery.
It is generally agreed that the composition of diatom communities is domi-
nated by cosmopolitan species with high dispersal ability (e.g. Soinen, Paavola &
Muotka, 2004 ), but the relative importance of (a) physical barriers that may limit
species dispersal, and (b) the extent to which diatoms have limited access to
suitable habitat, was unknown. Recent analyses (Potapova & Charles,2002)
suggest that the availability of suitable habitat is the principal determinant of
diatom community structure in rivers. Further, if the presence of a diatom in a
habitat is limited primarily by the quality of the local environment rather than
by physical barriers to dispersal, the diatom can potentially be found wherever
the same habitat type occurs. In a quantitative approach using canonical corres-
pondence analysis, Potapova and Charles ( 2002 ) showed that habitat factors
consistently explained more variation than physical barriers. At the national
(USA) scale, more than two-thirds of explicable variation in diatom species
composition could be attributed to habitat factors, with less than one-third
attributable to the role of physical barriers and other spatial factors.
BODY SIZE AND BIOGEOGRAPHY 175