Arch. Hydrobiol. 166 1 23–39 Stuttgart, May 2006 To conquer and persist: colonization and population development of the Ponto-Caspian amphipods Dikerogammarus villosus and Chelicorophium curvispinum on bare stone substrate in the main channel of the River Rhine M. C. van Riel 1, 3 , G. van der Velde 1, 3 and A. bij de Vaate 2, 3 With 7 figures and 2 tables Abstract: Macroinvertebrate communities on the stones in the Rhine are dominated by the Ponto-Caspian amphipods Chelicorophium curvispinum (since 1987) and Dikero- gammarus villosus (since 1995), which have invaded the Rhine through canals con- necting the large rivers of Europe. Colonization of bare stones suspended in the water of the Rhine main channel was studied. At the same time the macroinvertebrates drift- ing in the water layer were sampled. Macroinvertebrate populations on the newly colo- nized stones were followed for two months (June – August 2002). Bare stones were colonized from the water layer, with D. villosus and C. curvispinum most numerous from the start. Species richness was highest after one month. D. villosus and C. curvi- spinum continued to dominate the macroinvertebrate community on the stones throughout the experiment, representing 70 – 95 % of the total number of macroinverte- brates. In the first period week of colonization, especially juveniles of both amphipod species settled on the bare stones. After one week, the number of adults of D. villosus increased. After one month, ovigerous females of D. villosus became abundant on the newly colonized substrate. The numbers of adult C. curvispinum increased after one month and ovigerous females were present after two months. Newly settled popula- tions resembled the amphipod populations present in the water layer, but started to de- viate as colonization time increased, indicating that development of populations on stones became increasingly autonomous and less dependent on new colonization by 1 Authors’ addresses: Department of Animal Ecology and Ecophysiology, Section Aquatic Animal Ecology, Institute for Wetland and Water Research, Radboud Univer- sity Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands; E-mail: gvandervelde@science.ru.nl 2 Ministry of Transport and Public Works, Institute for Inland Water Management and Waste Water Treatment, P.O.Box 17, NL-8200 AA Lelystad, The Netherlands 3 Member of Netherlands Centre for River Studies, P.O.Box 177, 2600 MH Delft, The Netherlands. DOI: 10.1127/0003-9136/2006/0166-0023 0003-9136/06/0166-0023 $ 4.25  2006 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart