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Dispersal and body size in aquatic invertebrates

Active dispersers in freshwaters

Freshwater habitats are typically isolated, and relatively short-lived on geolog-

ical timescales (Moss,1998; Ribera, Foster & Vogler,2003), and so freshwater

organisms need to retain the ability to disperse, at least occasionally, if they are

to persist. As noted by Darwin (1859), many freshwater taxa have relatively wide

ranges, suggesting that they are capable of moving long distances. Movement

between sites is achieved by active or passive means (Fig.10.1; Bilton, Freeland &

Okamura, 2001 ), although the dichotomy between these is not clear cut (see

below).

Active dispersal in freshwaters is achieved by self-generated movement of

individuals, which may use sensory cues to seek out new areas of habitat or

patches occupied by conspecifics. Insects constitute the overwhelming majority

of actively dispersing freshwater organisms, and here movement between sites

occurs during the adult stage. One obvious factor that could influence the

dispersal ability of such flying taxa is their ability to travel large distances.

Rotifera

Copepoda

Hydrachnidia

Cladocera

Ostracoda

Branchipoda

Diptera

Plecoptera

Ephemeroptera

Trichoptera

Hemiptera

Coleoptera

Odonata

0.1 1 10 102 103

Approx. body length (mm)

Taxon

Figure 10.1Approximate size ranges of adults in selected groups of freshwater

invertebrates, divided into active and passive dispersers. Passive dispersers (open bars)

generally have smaller modal body sizes than active dispersers (shaded bars). The shaded

area indicates the size range in which the transition to cosmopolitanism has been

predicted to occur (Finlay, 2002 ), whilst the dotted line indicates the size below which

flying insects are frequent members of the aerial plankton (see text).

188 S. D. RUNDLEET AL.