Chironomids (Diptera, Nematocera) of Temporary Pools - an Ecological Case Study

摘要

The main aim of the present study was to determine how Chironomidae cope with the environmental changes to which temporary pools are exposed. Are the species specifically adapted to the habitat or opportunistic? The problem was approached by: (a) an emergence study done in the Lahnberge mountain range (Marburg, Hesse, Germany) on three pools that were subjected to different lengths of drought (two of which were really temporary and one semi permanent); (b) an emergence study done in order to investigate the dispersal ability of Chironomus dorsalis (colonizing experiment) in ten experimental pools that had been exposed in the field in 1998 and that mimicked spatially unstable pools; (c) laboratory investigations of some fundamental biological characteristics (role of temperature, photoperiod and density in growth and development, drought tolerance and parthenogenesis) of the four principal temporary pool dwellers Limnophyes asquamatus, Paralimnophyes hydrophilus, C. dorsalis and Polypedilum tritum. Contrasting with what is known for many other inhabitants of temporary pools (e.g. mosquitoes or water beetles), the temporary pool chironomids presently investigated showed only one programmed life history trait - the way of the annual timing - which is widespread amongst Chironomidae. All other life history traits were highly flexible and consecutively followed the actual situation within the habitat. The life histories of Limnophyes asquamatus, Paralimnophyes hydrophilus, Chironomus dorsalis and Polypedilum tritum are therefore rather opportunistic. A mixture of r- and A-selected traits achieves this enormous flexibility, which also seems to be widespread amongst Chironomidae. The ability to enter dormancy when environmental factors go below/beyond a given limit is the central element of the species? life histories. I call this capacity the quiescence strategy of Chironomidae, the knowledge of which is still fragmentary. Facultative dormancy, high levels of physiological tolerance of the larvae and many r-selected traits, lead to a high plasticity of life histories. The fine-tuning to the temporary habitat has been mainly achieved by an adaptive improvement of a few preadaptive properties present in Chironomidae: (a) The effective colonization of spatially unstable temporary pools was mainly achieved by the improvement of the dispersal abilities in Chironomus dorsalis; (b) The improvement of larval drought tolerance and its related features (such as acceleration of development at high larval densities and the capacity for terrestrial eclosion) enabled Limnophyes asquamatus, Paralimnophyes hydrophilus and Polypedilum tritum to effectively colonize spatially stable temporary pools. The evolution of an expert invader as well as of drought tolerance can be regarded as a strategy of being the first: the first species present after pool formation has the decisive advantages of (a) larger larval size in relation to other potential competitors and; (b) low numbers of predators. Many other insects of temporary waters were forced to evolve life cycles specifically linked to drought because they are able to survive drought only in a species-specific development stage (e.g. the egg stage in mosquitoes). This was quite different in the drought tolerant species investigated in the present study: all instars proved to be drought tolerant and resumed development without any risk whenever water was present. Whether the high thermal coefficients (4-6) over a wide range of favourable temperatures (5-15 °C) for growth and development that were observed in Paralimnophyes hydrophilus are really an adaptive feature still remains questionable.