Linking Restoration and Ecological Succession (Springer ... - Inecol

phases and a domination by late-successional species; e.g., Elymus athericus
(Bos et al. 2002) or Phragmites australis (Esselink et al. 2000). The overall
result is a loss in biodiversity. Succession in salt marshes can be manipulated
back to earlier stages by reopening existing embankments and flooding.
In a survey of 70 restored flooded sites in northwestern Europe, the percentage
of target species was as high as 70% in the best examples (Wolters et al.
2005).
6.5 The Role of Aliens
6.5.1 Successional Pattern: Repairing Function of Native Vegetation
Recent attempts to bring the science of invasive species and succession together
have proved profitable (Davis et al. 2000, 2005; see Chapter 3). Alien
species (defined as those whose presence in a given area is due to intentional
or unintentional human involvement, or which have arrived without the help
of people from an area in which they are alien; see Pyˇsek et al. 2004b) are
increasingly common in successional seres. For example, in 55 successional
studies in central Europe and North America, 25% of the species (range 2–
81%) were aliens. Aliens were most prevalent in ruderal habitats and old fields,
and their representation declined during the successional process. The rate of
this decrease was context-dependent: industrial habitats had a greater proportion
of aliens at the start, but a faster decline with time than habitats associated
with agricultural landscapes. Alien species contributed more in terms of species
number than cover reflecting that many of them are rare casuals (Richardson
et al. 2000, Pyˇsek et al. 2004b). Those aliens classified as neophytes (species
introduced after 1500AD) were most likely to become dominants (Pyˇsek et al.
2004a).
It is not known how much the pattern of decrease of alien species during
succession is determined by the exposure of various successional stages to
different propagule pressures of alien species. It is predicted that colonization
by diaspores will be greater at the beginning of succession (Rejmánek 1989).
Experimental studies on the invasibility of successional stages are rare, but
Bastl et al. (1997) found that early, but not initial, successional stages were
most prone to plant invasions. The establishment of aliens in the initial stages
of succession was probably restricted by adverse abiotic conditions, whereas in
later successional stages, intensive competition from resident species appeared
more important. This seems to be a common pattern (Rejmánek 1989) and
should be considered in restoration practice. It has been suggested that the
maximum cover and proportion of aliens are found in the initial stages of mesic
succession (Rejmánek 1989). These results suggest support for the successional
repairing function of native vegetation (Rejmánek 1989) indicating that during
spontaneous succession alien species should disappear in time (Pyˇsek et al.
2004a).
6.5.2 Manipulating Succession in Invaded Sites
The unassisted recovery of native vegetation during succession is of little practical
use when dealing with those large-scale invasions where immediate action
is needed. In such cases, the dominant alien species must be at least contained
Chapter 6 Manipulation of Succession 137