Ábrahám Levente: Vegyes tanulmányok - Natura Somogyiensis 3. (Kaposvár, 2002)

Vadkerti Edit - Farkas Sándor: The terrestrial isopod fauna of the Rinya region IV. Szilonics-puszta. (Isopoda Oniscidea) - A Rinya régió szárazföldi isopoda faunája IV. Szilonics-puszta

30 NATURA SOMOGYIENSIS (2) By the results of the cluster analysis of the isopod assemblages cought by the traps there is no habitat preference; the occurrance of the species does not reflect the difference between the two associations (Fig. 2). It is not surprising because H. riparius occurs everywhere independently from the type of association where there are moist microhabitats (GRÜNER 1966). The other four species occur in all of the habitats in this region (FARKAS 1999). (3) The total of 21,664 individuals of the 5 species were collected. Comparing these results to other similar long term experiments the number of individuals were 5 to 20 times more. The low number of Hyloniscus riparius (less then 1%) did not enable a reliable analysis so they were omitted from the statistics. The number of invidiuals was the highest at Szilonicspuszta among the habitats examined in the regions of DDNP. The surface activity of the four species had three peaks (Fig. 3). It may be concluded that the surface activity was the highest at the beginning of the summer. After a decrease at the end of June the activity showed an increasing tendency again. The decrease in the number of trapped animals on 19 August was probably due to a sampling error because the activity of the species was the highest in other sampled areas in this period (FARKAS, 1998a, 1998b). (4) The temporal pattern of the composition of isopod assemblages did not change during the sampled period (Fig. 4) apart from A. vulgare whose participation decreased continuously. T. ratzeburgi was the dominant species in both vegetations; its participation did not drop below 30 %. (5) The seasonal changes in sex ratio of the species (Fig. 5, 6, 7, 8) were different. The amount of gravid females of three species (T. rathkei, T. ratzeburgi, P. collicola) showed a similar tendency. The number of gravid females increased rapidly in June, peaked in July and decreased gradually from August. A. vulgare showed a different pattern: it peaked in June with a value at 30 %. MCQUEEEN (1976) reported that temperature is the major factor regulating growth, mortality and reproductive timing of Trachelipus rathkei. He found that 83% of mated adults produced one brood, and 42% a second, with some indication that the reproductive rate might be temperature-dependent. High temperature affects the reproductive pattern of isopods (HORNUNG and WARBURG 1993). The mean duration of gravidity were 51.4, 30.7, and 17.6 days at temperatures of 15.6,21.0 and 26.7 ° С (SNIDER and SHADDY 1980). They found that the most favorable temperature was 21 ° C, for both breeding and non-breeding females. The number of consecutive broods produced was dependent on initial female weight, the smaller females tending to breed only once (it was also influenced by temperature, since the highest temperature increased the incidence of second and third broods. From the end of May to middle of August 60-93 percent of the females were gravid. In our study the proportion of gravid T. rathkei females was 30-60 % in June and July, and 10-20 % during August and September. The gravid T. ratzeburgi females had a higher peak in July (60 %) and a smaller one in September (20-30 %). The number of nongravid females exceeded the gravids of both species from August. PorceUium collicola has one big, longtenn breeding period. The amount of gravid females ranged between 40-90 percent. There were no available data on the population biology of T. ratzeburgi and P. collicola to compare these results. For Armadillidium vulgare growth was more sensitive to changes in food quality than to temperature (HELDEN and HASSALL 1998). At low food quality 30-50 % of the females were gravid while at high food quality this amount was 70-90 %. There is a strong selection pressure for juveniles to respond strongly to temperature with respect to growth rates. In the spring and early summer, when they are larger, food availability is more limiting. At Szilonicspuszta the proportion of gravid females was low, 10-30 %. DANGERFIELD and TELFORD (1994) studied the sex ratio of 5 isopod species. The per-