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Effects of Invasive Non-Native Species on the Native Biodiversity in the River Rhine 267
polymorpha can also harm other suspension-feeding bivalves by depleting
food resources (phytoplankton) through massive filtration (Caraco et al.
1997).
Dreissena polymorpha has virtually eliminated the native unionid fauna in
many parts of the lower Great Lakes in North America (Ricciardi et al. 1998;
Strayer 1999). In the Rhine, the decline of the highly specialized and endangered
unionid mussels and other filter-feeding macroinvertebrates could also
partly be due to competition with D. polymorpha. However, D. polymorpha is
not harmful to all riverine species. In North America, the clam provides other
benthic invertebrates with nourishment (in the form of faecal deposits) and
shelter (interstitial spaces between clumped mussel shells), resulting in a local
enhancement of abundance and diversity for these other species (Ricciardi
2005). Non-native deposit feeders may increase in abundance whereas native
filter-feeders are out-competed by D. polymorpha. Among the invertebrates
responding positively to zebra mussel colonisation are non-native oligochaetes,
leeches, amphipods, gastropods, larval chironomids and aquatic
weeds (Ricciardi et al. 1997; Karatayev et al. 2002). Thus, invading species may
also have synergistic impacts which facilitate the establishment of other
invaders.
The clams Corbicula fluminea and C. fluminalis, originating from Southeast
Asia, were first recorded in the Lower Rhine in The Netherlands in 1985
(Bij de Vaate and Greijdanus-Klaas 1990). Six years later, the clams were found
near Karlsruhe in the Upper Rhine and, in 1995, C. fluminea was reported near
Basel in Switzerland (Rey et al. 2004). C. fluminea is restricted to the gravely–
sandy river bottom because sticking structures are lacking. The clam reached
densities of 1,800 individuals m –2 in the Rhine (Haas et al. 2002). Den Hartog
et al. (1992) suspected that the spill of toxic waste from the Sandoz accident in
1986, affecting the Rhine over hundreds of kilometres, contributed to the
clams’ success because most macroinvertebrates were killed and, as a consequence,
their niches were unoccupied.
Several mechanisms by which Corbicula may affect native bivalves have
been proposed (Strayer 1999). Dense populations of Corbicula may deplete
concentrations of phytoplankton and other edible suspended particles,
thereby ‘starving out’ native bivalves. Modest to dramatic declines in phytoplankton
or seston have been recorded in habitats with high Corbicula density
in North America (Leff et al. 1990; Phelps 1994). Dense populations of Corbicula
may ingest large numbers of unionid sperm, glochidia and newly metamorphosed
juveniles (Strayer 1999). Because Corbicula pedal feeds on edible
particles in the sediments, it may deplete also this food resource, affecting
some sphaeriids and juvenile unionids which use benthic organic matter as
food. Corbicula actively disturbs the sediments, so dense populations may
reduce habitat quality and space for native bivalves.
Several studies show that the impact of C. fluminea on native benthic
species depends on both site and community characteristics (Leff et al. 1990;