The structure and dynamics of a water beetle community in a ...

were focused on daily flight periodicity, thus the upper and lower thresholds may arise from
the fact that the flight was observed to be most intense around sunset and sunrise. According
to Csabai et al (2006), this daily flight pattern is caused by changes in polarotactical
detectability of water bodies and not by temperature. The existence of lower temperature
threshold is plausible, but the upper threshold may simply arise from the fact that none or
very low flight activity was observed around noon. However, in several species of insects,
Taylor (1963) found an upper threshold even when the data were restricted to those parts of
the day when flight occurs. In water beetles, the upper temperature threshold may occur at
higher temperatures than were available in my case.
The Scirtidae have terrestrial adults but aquatic larvae and thus desiccation means loss
of habitat for oviposition and larval development. I therefore expected to find some effect of
water depth on their flight activity. Although they show a very similar pattern of temperature
dependence in flight activity as the Dytiscidae and Hydrophilidae, I could not detect any
influence by water depth.
The seasonal differences in the relative species composition in light trap samples are
likely to come from differences in life-histories (e.g. the dominance of the Scirtidae in spring,
which may represent the pre-breeding dispersal) and different responses to desiccation (clear
dominance of the Hydrophiloidea in mid-summer). The responses to temperature and water
depth seem to be species specific and may also be a result of differences in life histories.
Moreover, as mentioned above, different species may have different temperature thresholds
because of physiological constraints. The lack of a significant result in CCA in the Scirtidae
may simply come from a low taxonomical and ecological diversity – the family is represented
in my data only by six species of Cyphon and Microcara testacea. The Dytiscidae and
Hydrophiloidea in the light trap samples are much more diverse taxonomically as well as
ecologically.
Females of the diving seem to disperse more than males, although I also cannot
completely rule out methodological artefacts (e.g. sex-specific attractivity of the light trap). A
possible explanation of this phenomenon is that females are more sensitive to habitat
degradation because they need to oviposit in a safe habitat. The female-biased flight may also
be independent of local conditions. Sex-biased dispersal has been observed in birds and
mammals, where it is assumed to decrease intrasexual competition and avoid inbreeding (e.g.
Greenwood 1980). In insect, sex-biased dispersal is well known e.g. in social Hymenoptera
(e.g. Kukuk et al. 2005). No thorough examination of this topic has been carried out in water
beetles and most other aquatic insects so far.
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