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B. Baur and S. Schmidlin
and C. fluminalis invaded the Rhine (Sect. 15.4).Already in 1989, the population
density of C. curvispinum in the Middle and Lower Rhine was so
high that the hard substrate of the channel bottom had been essentially
completely overgrown due to the species’ engineering activity. The D. polymorpha
population collapsed because adult shells were rapidly overgrown
by C. curvispinum, and their muddy tubes inhibited the development of
new D. polymorpha patches – the planktonic larvae can settle only on hard
surfaces (Van der Velde et al. 1994; Tittizer and Krebs 1996; Haas et al.
2002).
3. A new phase started with the invasion of the amphipod Dikerogammarus
villosus in 1995 (Sect. 15.4). In 2000, maximum densities of 3,000 individuals
m –2 were recorded. Since 1996, the population densities of C.
curvispinum have decreased whereas D. polymorpha has recovered and
again reached high densities. Subsequent to the appearance of D. villosus,
two other amphipods, Gammarus tigrinus and Echinogammarus ischnus,
have declined in the Upper Rhine; G. tigrinus finally disappeared in 1999.
In 1997 and 1998, three new non-native invertebrates reached the river
Rhine, originating from the Danube and the Ponto-Caspic region: the isopod
Jaera istri, the turbellarian worm Dendrocoelum romanodanubiale
and the polychaete Hypania invalida (Haas et al. 2002).
There is an accelerating colonisation rate of non-native macroinvertebrate
species in the Rhine (Fig. 15.2). The shape of the cumulative colonisation
curve shows that 55 % of the total number of colonisations were recorded
after 1970. Thus, more than half of all colonisations in the 175-year record
have been reported these last 35 years. The average rate of colonisation has
increased from 0.15 new species established per year in the period 1831–1970
to 0.74 new species per year for the period 1971–2005. Considering exclusively
the period 1991–2005, the current rate of colonisation averages 1.27 new
species per year.
Similarly to macroinvertebrates, fish species composition in the river
Rhine has altered in the past century. There is ample evidence that the river
engineering works have had deleterious effects on the species number and
abundance of fish (Lelek and Köhler 1989). Associated river modifications
have led to the disappearance of specific spawning grounds, feeding biotopes
and nursery areas, and to the obstruction of migration routes. The construction
of fish passes at almost every weir along the main stream section seems
to have been insufficient to prevent the decline of the migrating fish populations.
Low oxygen concentration and the massive discharge of toxic materials
contributed substantially to this decline. Since the water quality of the Rhine
began to improve in the 1970s, however, the fish community has been recovering
(Cazemier 1988; Lelek and Köhler 1989).
Lelek (1996) presented a list of 27 non-native fish species for the German
part of the Rhine. Eighteen of the 27 species (67 %) were intentionally intro-