• Bray–Curtis;
  • freshwater and impoundment;
  • historical fish data;
  • metapopulation and metacommunity;
  • biological invasions;
  • true diversity



Determination of taxonomic diversity changes, and patterns and drivers of homogenization/differentiation processes in fish (meta)communities at spatial watershed and temporal decadal scales.


The 333-km-long lowland Pilica River, with a solitary large dam reservoir constructed in the early 1970s, central Poland.


Fish data sets obtained along the river upstream and downstream of the reservoir in the 1960s and 2000s are compared in terms of several community properties. The true gamma diversity of order one (1Dγ), decomposed into true alpha (1Dα) and true beta (1Dβ) components, is calculated for the taxonomic classification of the sets. Pairwise community distances against the fluvial distances of each course and period are also assessed using the Bray–Curtis measure (BC) applied to occurrence and by 1Dβ, applied to abundance.


Based on 1Dβ, the fish fauna became homogenized taxonomically by 33.0% upstream, while it differentiated by 8.6% downstream. 1Dα and 1Dγ decreased considerably in both courses. The changes in diversity were due to the range expansions of the native species. Despite the exposure to alien invasive species, the Pilica was not invaded by any of them between the surveys, although one species probably established itself in the lower course.

Main conclusions

Decreases in 1Dα and 1Dγ in both Pilica courses and in 1Dβ upstream were not sharp enough to allow the invasions of alien species to which the Pilica was exposed. Consequently, either other drivers than river fragmentation, introductions, climate warming and improvement in water quality may be essential for such invasions, or longer time periods than those presently considered are necessary for them to occur. An individual species' occurrence and abundance analysis suggests that the homogenization and differentiation resulted from variously combined impacts of several natural and human-mediated drivers. The river fragmentation exerted both course-specific and species-specific impacts.