Environmental Engineering Reference
In-Depth Information
habitat that are short lived and unpredictable. Take, for example, the southern dam-
selfl y, Coenagrion mercuriale , which has fl ying adults but aquatic eggs and larvae.
The species is restricted in Britain to the highly fragmented habitat of small streams
and ditches associated with ancient water meadows that have long been managed
by periodic grazing or cutting (Purse et al., 2003). Within these habitats, the aquatic
larvae are further restricted to unshaded fl owing water with perennial herbaceous
vegetation. Such sites have recently been grazed or cut but will alter with time
(t h rough a proce s s c a l le d ecological succession - Chapter 8) to a shady state unfa-
vorable to the damselfl ies. Persistence of the species within the shifting mosaic of
favorable habitat patches depends on adult colonization of newly created patches,
counterbalancing the loss of subpopulations from patches as they become unsuita-
ble. But to what extent do the poor powers of fl ight of adults, which only live for 7
or 8 days, limit dispersal to new suitable patches?
Purse's team captured adults in some water meadows in Wales and released them
again after marking on the forewing and thorax so they could be individually identi-
fi ed when subsequently recaptured. As in our bellbird example, this allowed the net
lifetime movements of individuals to be determined (Figure 4.3) - you can see that
the damselfl ies operate over a markedly smaller range than do the birds. Purse's
team concluded that management effort should be focused on locations where a
cluster of habitable sites occur within 1-3 km of each other. In other locations, where
there are only a few, relatively isolated sites, suitable habitat is so fragmented that
reintroduction will not lead to the establishment of stable populations. The authors
also suggest that hedges should be removed between any existing population and
empty, but suitable sites, to facilitate stepping-stone dispersal movements. This is
analogous, but again on a much smaller scale, to the corridors linking core panda
habitats.
45
40
35
30
25
20
15
10
5
0
0
75
150
225
300
375
450
525
600
675
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825
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Net lifetime movement (m)
Fig. 4.3 Percentage distribution of net lifetime movements for male (open bars) and female damselfl ies (closed bars) at a site
in Preseli, Wales. Most recaptured individuals dispersed over short distances but a few moved up to 1 km. (After Purse
et al., 2003.)
 
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