View - ResearchGate

264
B. Baur and S. Schmidlin
others seem to be restricted to flowing water (Van der Velde et al. 2002). Cargo
shipping appears to influence the velocity of spread in invasive species. For
example, the clam C. fluminea spread approximately 150 km per year in the
navigable part of the Rhine but only 2.4 km per year upstream of Basel, where
cargo shipping is largely reduced (Schmidlin and Baur 2006). Corbicula fluminea
may also be displaced by waterfowl, because juvenile clams use their
mucous secretions to stick to ducks’ feet.
Interestingly, the number of non-native species decreases significantly
upstream of Rheinfelden where cargo shipping ends (Rey et al. 2004). However,
the weir in Rheinfelden is not an absolute barrier for the spread of invading
species. In fact, several non-native species have crossed the weir and are
now spreading upstream (e.g. D. polymorpha, C. fluminea, and the annelids
Branchiura sowerbyi and Caspiobdella fadejewi), some having even entered
the tributary Aare (e.g. the gastropod Potamopyrgus antipodarum and the
flatworm Dugesia tigrina; Rey et al. 2004).
15.4 Species Interactions and Mechanisms of Replacement
15.4.1 Amphipods
The amphipod Corophium curvispinum, originating from the Ponto-Caspic
region, was first observed in the Middle and Lower Rhine in 1987 (Schöll
1990). A few years later, C. curvispinum was found to be by far the most
numerous macroinvertebrate species in the Lower Rhine (Van den Brink et al.
1991). Its density increased up to 200,000 specimens m –2 on groynes (Van den
Brink et al. 1993). It has been claimed that C. curvispinum had filled an ‘empty
niche’ because it was the first tubiculous amphipod to colonise the Rhine
(Den Hartog et al. 1992). The animals produced extensive mats of dense silty
tubes which covered all available hard surface. As a consequence, other
epilithic invertebrates were negatively affected by this muddy layer. Significant
declines in population densities were recorded for the amphipod Gammarus
tigrinus, the zebra mussel Dreissena polymorpha, the gastropod Potamopyrgus
antipodarum, the caddis fly Hydropsyche contubernalis, and several
species of Chironomidae (Van den Brink et al. 1993). The former three are
non-native species whereas H. contubernalis is native. It has been suggested
that these changes in abundance were at least partly the result of competition
for food – C. curvispinum, D. polymorpha and H. contubernalis are all filterfeeders
(Rajagopal et al. 1999). In fact, the exponential increase in the density
of C. curvispinum during 1989–1991 coincided with a decrease in the concentrations
of total organic carbon and total suspended matter in the Lower
Rhine, which may be related to an increase in filtration capacity in the river.