planktotrophic propagules would be low given the attrition rates normally
encountered in the plankton. Lecithotrophs and brooders might also be
expected to face a higher extinction risk depending upon the availability of
their habitats, and might be less likely to form widespread ‘open’ populations
(Berryman,2002; Camus & Lima, 2002 ; Grimm, Reise & Strasser, 2003 ). In
general then, the trend is for increasing degrees of lecithotrophy, brooding
and viviparity as organisms get smaller, with larger organisms in general tend-
ing towards planktotrophic development and broadcast spawning.
We, therefore, would expect planktotrophic organisms to be the most widely
dispersed marine forms with the greatest range sizes; this is supported for
several published studies of both living (Scheltema, 1989 ; Kohn & Perron,
1994 ; Emlet, 1995 ) and fossil (Hansen, 1978 ; Jablonski, 1986 ) marine inverte-
brates. Lecithotrophic and brooding species should be less widely distributed,
which is supported by the analyses presented by Emlet ( 1995 ). The picture is
confused somewhat by the increased probability of passive transport in smaller
lecithotrophic propagules and smaller brooding or direct-developing indivi-
duals, such that the overall particle size-range size distribution might be expected
to be ‘U’- or ‘J’-shaped. However, if selection for intermediate egg size in plank-
totrophic species is indeed strong (sensuLevitan,2000 ), then it is likely that the
number, and planktonic duration, of dispersing propagules is the primary
constraint upon dispersal in planktotrophic forms. Many taxa demonstrate
trade-offs between egg number and size (see Llodra, 2002 for review), and
significant relationships between adult female size and fecundity. Thus, larger
individuals will tend to produce more eggs, and ultimately become more suc-
cessful at dispersal through weight of numbers of dispersing propagules in a
fashion similar to that proposed for passively-dispersing freshwater taxa.
Dispersal, macroecology and body size: case studies
Active dispersers in freshwaters
...several species of Sympetrum and Aeshna have most often been seen engaging in mass flights,
sometimes settling in large numbers on ships at sea or flying in swarms, together with other migrating
insects, through high mountain passes. (Askew, 1988 .)
As the above quote suggests, the exemplary active dispersers in freshwater
insects are the dragonflies and damselflies (Fig.10.1), the adults of many species
being known to disperse long distances. This ability to disperse is due to their
highly aerodynamic wings, efficient flight muscles and very high flight muscle
to body mass ratios (up toc. 60%; Marden,1987). Despite the general efficiency of
odonate flight, however, there are substantial differences between species in
traits related to power production and efficiency that may have implications in
terms of dispersal potential and could relate to body size or other morphological
characteristics related to size (see above).
BODY SIZE, DISPERSAL AND RANGE SIZE IN AQUATIC INVERTEBRATES 193