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B. Baur and S. Schmidlin
Strayer 1999). The clam severely affected native mollusc assemblages in some
North American rivers but can coexist with other bivalves at other sites. Similar
information on the impact of Corbicula on native macroinvertebrates in
the river Rhine is not yet available.
15.5 Why Are There so many Non-Native Species
in the Rhine?
The number of non-native animal species colonising the river Rhine is still
increasing (Fig. 15.2). Furthermore, non-native plant species constitute a significant
proportion of the vegetation of the river bank and floodplain
(Schwabe and Kratochwil 1991).A variety of mutually non-exclusive hypotheses
have been suggested to explain the success of invaders in the river Rhine:
(1) vacant niches, (2) disturbances preventing strong interspecific competition,
(3) the creation of new niches by invasive species, (4) ecosystem instability
(invasional meltdown), (5) groups of co-adapted invaders, and (6) enemyfree
space.
It has been argued that human alterations of habitat make a community
vulnerable to invasions and that extreme natural disturbances facilitate the
establishment of non-native species (Mack et al. 2000). Community vulnerability
to invasions has been ascribed to a combination of several factors, such
as the presence of vacant niches, habitat modification, and disturbance before
and after invasion. Recent findings indicate that species-rich communities are
less vulnerable to invasions (at least, in terrestrial habitats; Cox 2004). Moreover,
invasibility is known to increase if a community lacks certain species
present under normal conditions (Chap. 11).
The invasional meltdown model (Chap. 6) predicts that ecosystems subjected
to a chronically high frequency of species introduction will become
progressively unstable and easier to invade, as each introduced species has the
potential to facilitate subsequent invaders (Simberloff and Von Holle 1999).
Invasional meltdown may occur through one of two processes: frequent disturbance
through species introductions progressively lowers community
resistance to invasion, and increased introductions lead to a higher frequency
of potential facilitations and synergies between invaders (Ricciardi 2005).
Highly active invasion corridors (in the present case, canals) may introduce
numerous species from one and the same region (e.g. the Ponto-Caspic
region), and thus may reunite groups of co-adapted species, either in simultaneous
introductions (e.g. a host arriving with its parasites) or in successive
introductions, thereby assembling contiguous links of a non-native food web.
If co-adaptation reduces the intensity of predation and parasitism, then
positive interactions probably dominate invasion ‘groups’, and successive
introductions of co-adapted species might result in a higher success of