Passive dispersers in freshwaters
In terms of overland, inter-site dispersal, both wind and animal vectors have
been implicated in the passive transport of freshwater invertebrates (Dumont,
1994 ; Dahms,1995; Korovchinsky & Boikova, 1996 ; Maguire,1963). Anostracan
eggs may be dispersed by the wind (Riddoch, Mpoloka & Cantrell,1994;
Brendonck & Riddoch,1999), and such movement is likely to occur in other
taxa with small, desiccation-resistant stages. Transport by mobile animal vectors
has also been noted in a number of freshwater invertebrates (Bohonak,
1999 ; Bohonak, Smith & Thornton,2004 ) including the movement of resting
stages or individuals on and in waterfowl and other aquatic vertebrates
(Proctor & Malone,1965 ;Biltonet al., 2001 ; Green & Figuerola,2005 ). Whilst
being classed as passive, from the point of view of the dispersed organism, such
movement differs fundamentally from wind-mediated transport in that the
vector itself is dispersing actively, which may have important consequences
for population structure and dynamics (Bohonaket al., 2004 ; Figuerola, Green &
Michot,2005 ).
The majority of such passive dispersal in freshwater metazoans occurs during
early life-history stages, and often involves resting eggs or cysts, which are
resistant to desiccation and other adverse environmental conditions (Bilton
et al., 2001). In many respects dispersal in such animals resembles dispersal in
higher plants via seeds (De Stasio,1989; Levinet al., 2003). Given the constraints
of dispersal of propagules via wind, rain or animal vectors, these taxa would be
expected to be relatively small. On the basis of the apparent ubiquity of many
microbial eukaryotes Finlay (2002) andFinlay and Esteban(this volume) propose
a transition between species with a biogeography and those which are ubiqui-
tous at between 1 and 10 mm in length (see Fig.10.1). In the case of testate
amoebae, Wilkinson ( 2001 ) shows that this transition apparently occurs at
around 100mm, and indeed most of the organisms within the shaded area of
Fig. 10.1 do have biogeographies, i.e. they arenotcosmopolitan. As in the case of
marine larval dispersers (see below), however, it is difficult here to identify the
life-history stage where body size may be most relevant in shaping the evolution
of dispersal. As with many wind-dispersed plant taxa (Fenner & Thompson,
2005 ), passive dispersers may be under selection for small propagule size,
since these may be better dispersed. Unfortunately, data on propagule size in
passively-dispersing freshwater metazoans are lacking or disparate (but see
Poulin,1995), making explicit tests of the relationship between propagule size
and dispersal ability impossible at present. However, passive dispersal may also
be related to the number of propagules produced, with fecundity related pos-
itively to dispersal potential. At the same time, if fecundity scales positively
with body size in these passively dispersing freshwater taxa, we would predict
a positive relationship between adult body size and dispersal ability, with
larger-bodied taxa producing more propagules and being more widespread190 S. D. RUNDLEET AL.