9780521861724htl 1..2

with body length for either species comparisons or using contrast scores gen-

erated by independent contrasts analysis, which suggests that it is an increase in

wing size rather than size per se that is correlated with increased range size in

these freshwater invertebrates.

Passive dispersers in freshwaters

Empirical estimates of dispersal are eitherdirect, involving mark-release-recapture,

originally developed for population size estimation (Angelibert & Giani,2003 ;

Southwood & Henderson,2000 ), or indirect, typically employing genetic analyses

(see Biltonet al., 2001 ; Bullock, Kenward & Hails,2002 ;Nathanet al., 2003for

reviews). Given the difficulty of obtaining direct estimates, many of the data on

dispersal of aquatic invertebrates relate to the feasibility of dispersal rather than

the frequency of successful dispersal events (Biltonet al.,2001 ).Thisisparticularly

true of passive freshwater dispersers, whose small, cryptic propagules do not lend

themselves to direct study. In recent years, a combination of observational data

and population genetic analysis have greatly expanded our understanding of

dispersal in organisms such as bryozoans and zooplankton (see De Meester

et al., 2002 ;Okamura&Freeland,2002 ; Green & Figuerola,2005 ; Figuerolaet al.,

2005 ), and several experimental studies have examined the relative ability of

b)

0

0.4

0.8

1.2

1.6

0 0.01 0.02 0.03 0.04 0.05

log wing length contrasts

log range contrasts

a)

4.5

5.0

5.5

6.0

6.5

7.0

7.5

1.2 1.25 1.3 1.35 1.4

log wing length

log range

Figure 10.3The relationship between: (a) wing length (mm) and range size (km^2 ); and (b) wing

length contrasts and range size contrasts from comparative analyses forEnallagma

damselflies in North America. Analysis includes only species in the USA and Canada and

range size data were collected as for Fig.10.2. Contrast scores, representing independent,

ancestral trait values were derived using Comparative Analysis by Independent Contrasts.

Stepwise regressions (using body length and wing length and contrast scores for both

as predictors) demonstrated significant relationships in both instances. The best model

for the species comparison included both wing length and body length: R^2 ¼0.57;

y¼16.1x10.2xþ0.71; F2,24¼16.9; p<0.001, but the plot shown is for wing length

only (a) y¼7.84x4.13, F1,24¼17.7, p<0.001. For contrast scores only wing length was

included in the model: (b) y¼24.15x, F1,20¼60.1, p<0.0001; here regression for contrast

scores was forced through the origin (hence, there is no constant in the equation) and

the ordinate axis was scaled to be positive.

196 S. D. RUNDLEET AL.