Dr. Murai Éva - Gubányi András szerk.: Parasitologia Hungarica 28. (Budapest, 1995)

operating at the host species level. Along this axis the digenean assemblages of the two Ardea species were distinguished. On the other hand, the second DCA axis seemed also to reflect a seasonal gradient in the structure of the assemblages in these hosts. Comparison of digenean alpha diversity was not possible for all host samples due to small sample size. Nevertheless, the analysis, where possible, confirmed the trends established above. Species richness and diversity were found to vary seasonally (Table 1). This effect was best demonstrated for trematode assemblages in A. purpurea, where more data were available. Having the same number of species, the trematode assemblage in this host in April had higher diversity and lower concentration for dominance than in October. Similar variation was observed in A. ralloides samples. Assemblages in A. cinerea seemed to present an exception to this pattern, further indicating the effect of local ecological events. The dominant structure (based on relative abundances) of digenean assemblages also showed seasonal changes. In general, a polydominant structure of the digenean assemblages seemed to be characteristic for the spring host samples, while in autumn the assemblages were dominated by fewer trematode species with higher relative abundances (Table 1). The species composition of dominants differed among the hosts and also seemed to vary throughout the year. Hostspecificity was identified as the main factor responsible for structuring of the trematode assemblages of ardeids in our study. Specificity was found to operate both at a larger scale (host generic - Aei Apr • Aci Oct - Apu Apr - Apd Jun - Apu Oct - Ar» Jun - Ar» Oct - B»t Apr - E<3» Oct Fig. 1. Group average clustering dendrogram of the trematode assemblages in host samples based on Czekanowski Sorensen similarity index for presence/absence data. (Aci = Ardea cinerea, Apu = A. purpurea, Ara = Ardeola ralloides, Bst = Botaurus stellaris, Ega = Egretta garzetta, Apr = April, Oct = October, Jun = June). level) and at a finer scale (host species level). However, within the framework set by specificity, local ecological factors (shifts in host diet and/or changes in habitat) seemed to be responsible for the finetuning of the trematode assemblages and produced the observed seasonal variations in their structure and diversity. These findings support recent theoretical considerations on parasite communities in avian hosts (e.g. see Bush 1990). ACKNOWLEDGEMENT This study was completed with financial support granted by the National Scientific Research Foundation of Bulgaria, Grant B-37/1991.