Climate Change and the Hydrology and Morphology of Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 77

Figure 4.6 (a) Single-channel and (b) restored multiple-channel sections along the river
Lahn. (Photographs by S. Jähnig and A. Lorenz.)
However, this is by no means a guaranteed outcome (Palmer et al . 1997). Whether
the original species are able to re-colonize the restored stream depends not only
on the quality of the restored habitat but also on a number of factors such as the
dispersal capacities of the species and the presence or absence of migration
barriers between the source populations and the restored areas (Hughes 2007).
Establishment of an invasive or non-native species may also hinder re- colonization,
and biodiversity may, in general, be threatened by invasive species replacing
native ones. Colonization may be less successful than hoped for, as illustrated by
a study from the German Central Highlands, where the benthic macroinvertebrate
biodiversity of multiple channels re-created in seven rivers was compared with
that of seven engineered straight channels (Fig. 4.6).
The hydromorphological diversity of the multiple-channel sections
approached the intended reference condition. Habitat diversity increased, and
increased sediment dynamics were observed. However, substrata in single- and
multiple-channel sections still had similar macroinvertebrate community
composition, and the alpha diversity of substratum-specific communities did
not change. Different substrata, however, did host distinct macroinvertebrate
communities, so beta diversity was greater in the restored channels as they
tended to have a greater range of substrata. A comparison of representative
communities in single- and multiple-channel sections showed very high Bray-
Curtis similarities (69%-77%). Mean similarity analyses (using MEANSIM)
revealed that the macroinvertebrate community composition within a channel
type was less similar (single-channel within-group similarity: 0.61, multiple-
channel within-group similarity: 0.63) than the composition between channel
types (between-group similarity: 0.66).
Differences between paired stream sections could mainly be attributed to single
taxa that occurred solely in either the single- or multiple-channel sections. These
exclusive taxa were mostly found on organic substrates such as living parts of
terrestrial plants, large wood, coarse particulate organic matter and mud. Whether
this occurrence is related to the specific substrata or to chance is still to be
resolved. The overall high similarity of macroinvertebrate communities from
single- or multiple-channel sections could be due to prevailing large-scale