The ecology and conservation of threatened damselflies

in which the species can exist (see Chapter 3). Although natural habitat generally
supports the more enduring populations, transient populations often occur in areas
such as active quarry sites, bomb craters (Fox and Cham 1994), motorway storm water
retention pools (Scher and Thièry 2005) and even wheel ruts (Smallshire and Swash
2004). As conditions are highly unstable in these types of habitat there will be
constantly changing opportunities for colonisation, enabling progressive movements
across the landscape.
In addition to the lack of evidence for long range dispersal from inter-site movements,
there is also no evidence for the proposed long range dispersal mechanism of upward
flight. Despite over 3000 man-hours spent in the field, over two flight seasons and in
two geographical regions, not a single instance of upward flight, as described
previously (Fox 1989; Cham 1993), was witnessed. During the study periods, weather
conditions varied, but were mostly clear, still and very hot; the conditions under which
this behaviour was previously observed. On several occasions individuals were seen
rising vertically into the air, but on all occasions they were observed alighting on a tree
branch nearby or moving horizontally to descend in an area of adjacent vegetation. It is
possible that, on a bright day, similar local movement events could be mistaken for
movements to a much greater altitude.
Following the 2005 study, it was thought that the absence of extra-site dispersal events
might simply be because the habitat was ideal, maintained and large. Consequently,
there was no stimulus to disperse. It is likely that greater movement than that recorded
is possible, as the early-successional habitat more typically occupied by I. pumilio
populations is prone to dry out or become overgrown, forcing populations to move on. It
was partly for this reason that the study was repeated, in 2006, in an area where the
habitat was more polluted, less maintained and comprised several smaller potential
sites to which individuals could disperse. However, no long range dispersal movements
were recorded.
2.4.3 Factors affecting movement
Distance moved during first mature movements was most affected by the length of time
between captures. This has also been observed in C. mercuriale (Purse et al. 2003;
Rouquette and Thompson 2007a), Calopteryx splendens (Schutte et al. 1997) and
Enallagma cyathigerum (Garrison 1978). Similarly, lifespan (time between first and last
captures) had a highly significant effect on all of the lifetime movement parameters
considered, but the effect sizes were small.
The distance moved by I. pumilio individuals during their first mature movement was
inversely density dependent. Inverse density dependent movement was observed in a
study of C. mercuriale (Rouquette and Thompson 2007a) which attributed this unusual
strategy to individuals aggregating in areas of more suitable habitat. More commonly
observed strategies are a tendency to spread out evenly within the available habitat in
territorial species (Stettmer 1996; Beukema 2002) or classic density dependent
dispersal, where the likelihood of movement increases in areas of high density. The
latter is often said to be mediated by competitive effects on fitness or deteriorating
environment due to crowding (Denno and Peterson 1995; Matthysen 2005). Inverse
density dependent movement may be caused by conspecific attraction (Stamps 1991)
which can be explained as either benefits from aggregation, such as decreased
predation risk, or the use of conspecifics as a cue for resource or mate availability. It
may also be a response away from low density areas, which may suffer Allee effects
where aspects of fitness decline with population size (Allee 1949; Kuussaari et al.
1998; Stephens and Sutherland 1999; Stephens et al. 1999). Alternatively, the “social
36 Science Report – The ecology and conservation of threatened damselflies