Population dynamics, spatial distribution, and sex ratio of Trachelipus rathkei Brandt (Isopoda : Oniscidea) in a wetland forest by the Drava river

Pitfall samples were taken during 1995 from several locations in the Danube -Drava National Park, Hungary, including the forest of Gordisa. Twenty trap s were laid in two different patches, catching a total of 10,338 isopods be longing to three species: Trachelipus rathkei Brandt, Armadillidium vulgare Latreille, and Hyloniscus riparius Koch. Spatial distribution was influenc ed by patch type; all three species were found in varying numbers in the fo rest patch of older poplar, while H. riparius was absent in the young willo w plantation. T. rathkei and A. vulgare populations were low in the spring, and increased gradually until summer. After July they decreased to the lev el of April. In T. rathkei the sex ratio was biased toward females througho ut the sampling period: the percentage of females was approximately 60% unt il July and around 86% thereafter. The reproductive period of T. rathkei wa s long, with 5.38% of the females still gravid in late September. Populatio n structure of T. rathkei was analyzed by head capsule. The results suggest that sexual maturation in this species occurs at an earlier life stage tha n in other populations.