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Effects of Invasive Non-Native Species on the Native Biodiversity in the River Rhine 269
invaders than would introductions of unacquainted species (Ricciardi 2005).
This could be an alternative to the enemy release hypothesis, which relates the
success of an invader to the absence of its natural predators and parasites in
the invaded region (Chap. 6). Each of the examples presented in Sect. 15.4
could be explained by at least one of these six hypotheses. However, experimental
tests of these hypotheses are lacking for the Rhine.
15.6 Conclusions
The river Rhine is a good example for how a combination of different factors
structure benthic communities. River modification deteriorated certain habitats
but also created new habitats. Prolonged pollution changed the original
communities and caused the loss of certain species, creating open niches for
pollution-tolerant invaders. Major disturbances, such as the Sandoz accident
in 1986, subsequently enabled the invasion of many new species which
reached unprecedented densities. The Rhine-Main-Danube Canal, opened in
1992–1993, provided additional opportunities for the immigration of nonnative
species from the Ponto-Caspian region, some of them being coadapted.
After reduction of the pollution in the Rhine, recolonisation seemed
to favour invaders, rather than native species. These invaders suppressed the
development of populations of native species. At the present day, the number
of invaders is still increasing.
For the development of appropriate conservation strategies for the river
Rhine, detailed knowledge of the ecological consequences of invasive nonnative
species for the native biota is required. The present review shows that,
in most cases, negative impacts of invasive species on native species have been
deduced from correlative evidence. Evidently, there is an urgent need for
experimental studies on interactions between invasive and native species.
Numerous rare native species in the Rhine are threatened with extinction by
the combined impacts of environmental degradation and species invasions
(e.g. by D. polymorpha). From a conservation perspective, the habitat requirements,
population dynamics and persistence of rare native species deserve
increased attention. Restoration to pristine conditions is not feasible in the
Rhine. However, several promising ecological restoration projects are of vital
importance to preserve those facets of the originally unique biodiversity of
the river Rhine and its floodplain still present today.
Acknowledgements.
manuscript.
We thank A. Baur and P. Stoll for constructive comments on the