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

138 Karel Prach et al.
and preferably controlled or eradicated (Myers and Bazely 2003). However,
species removal in isolation can result in unexpected changes to other ecosystem
components, such as trophic interactions (Zavaleta et al. 2001), and open
the way to reinvasion by the problem weed species or other aliens. To avoid
this reinvasion, appropriate restoration measures need to be taken, and this is
analogous to removal of late-successional species when succession is being
reversed, e.g., on heathlands (see Sections 6.3.1. and 6.4.2b).
With increasing dominance of an invasive species, and increasing difficulty
to control it, possible action moves along the sequence from unassisted succession
to biological manipulation to technical reclamation. Planting or sowing
indigenous species should accelerate recovery of resident vegetation in highly
degraded sites after the clearance of dense and extensive stands of alien plants,
and where the likelihood of recovery from seed banks is low (Richardson et al.
in press). Appropriate management, aimed at manipulating the site characteristics
is difficult. Moreover, some highly invasive species (transformers sensu
Richardson et al. 2000) alter ecosystem functioning and change the site conditions.
Conventional wisdom suggests that disturbance, and both site nutrient and
moisture status as reflected in site productivity, all play a crucial role in plant
invasions (Rejmánek 1989, Hobbs and Huenneke 1992, Davis et al. 2000).
Manipulation of both the disturbance regime and productivity are, therefore,
two major options for the subsequent control of alien species. Manipulation
of disturbance is often aimed at restoring a management regime that was typically
applied to the habitat before invasion. The effect of such management
depends on the site productivity. In productive environments a reduction in disturbance
is likely to allow native species to outcompete aliens (Huston 2004).
However, manipulating productivity is much more difficult to manage than
disturbance.
Because invasions are context-dependent and individual habitats differ
largely in the level of invasion and invasibility (Lonsdale 1999, Chytr´y et al.
2005), it is essential that the character of the restored site is taken into account.
Spontaneous succession either assisted or not, may be less efficient in riparian
zones because of their high water-flow dynamics and continuous addition of
propagules of alien species. The nature and effects of the fluctuating conditions,
such as timing of floods, make the role of aliens in riparian succession
difficult to predict, especially on recolonization processes after removal. The
restoration strategy must, therefore, take into account the pattern of arrival of
alien species propagules into a site subjected to restoration (Richardson et al.
in press). Clearly, the source of reinvasion is likely to be from upstream, so if
aliens are to be controlled then upstream source populations must be identified
and controlled before restoration is started. The pattern of propagule arrival
is much more difficult to predict in habitats outside river corridors where the
arrival of propagules is nondirectional, from various sources and highly stochastic.
Where the pool of alien species is large and diverse (e.g., urban wasteland)
sowing or planting of native species may prevent invasion.
Under certain circumstances, aliens are used intentionally in restoration programs,
especially where economic priorities prevail (see Chapter 5). For example,
in dry areas of Africa, alien woody species (Albizia spp., Acacia spp.,
Eucalyptus spp., or Prosopis spp.) are largely planted into different successional
stages to increase productivity and help prevent desertification (Adams