the key effect of body size in these passive dispersing species is upon fecundity.
In marine systems it is also possible that the main consequences of body size for
dispersal act through dictating reproductive-dispersive strategies.
Conclusions
In the case of freshwater environments the data we present suggest that there
may be some positive relationships between range size and body size or traits
that may be linked to dispersal. For passive dispersers, rather than metabolic
costs associated directly with dispersal there may be a link between propagule
output and body size – a possibility worth further investigation. For at least some
active dispersers, there is evidence that wing size rather than body size per se
is of particular importance. We would suggest that aquatic insects could provide
a valuable model system in which scaling laws for flight (e.g. Schilder & Marden,
2004 ) could be used alongside those for metabolism for making robust macro-
ecological predictions.
In the marine environment links between dispersal, range and body size are
most likely to be moderated by developmental mode and lifetime reproductive
output. Smaller organisms, tending towards viviparity or brooding, invest more
in each individual offspring and have more rapid generation times. Such a
strategy is likely to be less successful for dispersing over long distances than
the possession of a planktonic stage. Body size is likely to manifest itself more
through relationships with fecundity, as larger organisms will be able to
produce greater numbers of eggs. This is, however, less likely to result in strong
range size–body size correlations due to the kind of allometries in fecundity and
size of area for brooding described by Strathmann and Strathmann (1982). In
intermediate size organisms that produce lecithotrophic larvae the size and
number of dispersing particles will dictate range, with the trade-off between
maternal investment (size, and hence developmental time spent in the plank-
ton; Levitan, 2000 ) and fecundity being key. Larger organisms will be able to
generate larger numbers of eggs irrespective of egg size, and this effect of
fecundity is likely to result in any observed trends between body size and range.
In marine planktotrophs there is both theoretical (Levitan,2000) and empir-
ical (Eckert,1999) evidence that no general body size–egg/larval size relation-
ship exists, although some taxa do seem to indicate such a trend (see Figs.10.5 &
10.6). No inherent ‘dispersability–body size’ relationship can be predicted
for individual species groups, as is the case of freshwater dispersers. Body
size–range size relationships in these taxa may instead be determined by
fecundity, which in turn is likely to scale directly with body size, invoking the
same basis in metabolic rules as proposed for other macroecological patterns
(Gillooly,et al., 2001; Marquetet al., 2005).
In summary, we hope that we have demonstrated that the exploration of
macroecological patterns in aquatic invertebrates can be profitable and that it
BODY SIZE, DISPERSAL AND RANGE SIZE IN AQUATIC INVERTEBRATES 203