9780521861724htl 1..2

produce long-lived diapausing egg banks (De Stasio,1989; Hairstonet al., 1995;

Ca ́ceres,1998 ; Brendonck & De Meester,2003 ; Vandekerkhoveet al., 2005 ), which

lower local extinction risk. Such dormancy can be seen to represent dispersal

in time, and theoretical and empirical data predict trade-offs between spatial

and temporal dispersal (Venable & Lawlor,1980 ; Levin, Cohen & Hastings,1984 ;

Hairston & Ca ́ceres,1996 ), suggesting that dispersal rate may actually be lower in

taxa whose eggs can remain dormant for long periods. This may impact on the

timing of local colonization events (Ca ́ceres & Soluk,2002 ), but over larger spatial

and temporal scales the production of resting propagules is likely to lead to larger

ranges due to increased propensity for long-distance dispersal.

Adult body size is well documented for the canthocamptid species considered

by Rundleet al.(2000; 2002b), and reanalysis of these data reveal a consistent

tendency for widespread taxa to be larger than their restricted relatives, at both

a global and European scale (Fig.10.4). Mechanisms that may underlie this

apparent trend are unclear, but may relate to relative reproductive output: if

larger-bodied taxa have higher fecundity this may increase the likelihood of

long-range dispersal. Studies of other groups such as Cladocera would be useful

here, as would comparative data on egg size, survivorship and estimates of

effective dispersal distances.

As discussed above, the dispersal biology of many passively-dispersing fresh-

water taxa has similarities with that in seed plants. Levinet al.(2003) note

that modelling studies of seed dispersal demonstrate that both mean dispersal

distance and the overall distribution of dispersal distance are critical in

0.4

0.5

0.6

0.7

0.8

European Global

Adult body length (mm)

Figure 10.4Mean body size (SE) of freshwater canthocamptid copepods, which can be

considered as geographically restricted, i.e. found in one area of endemicity (white bars),

or widespread, i.e. found in>1 area of endemicity (grey bars), at global and Western

European scales. Widespread species were significantly larger for both global

(T 40 ¼2.02, p¼0.015) and European (T 81 ¼1.99, p<0.050) data.

198 S. D. RUNDLEET AL.