Environmental Engineering Reference
In-Depth Information
species pool from a more general species assemblage
through environmental 'filters'. Species can colonize
new or existing plant communities either from a seed
bank or by spatial dispersal. Seed banks do not, by
definition, occur in the early stage of a primary suc-
cession. Here, the species composition during the colon-
ization phase depends on the arrival of propagules
from elsewhere. This is in concert with the classic
approach of 'relay floristics' as proposed by Egler
(1954), which does not exclude that one species may
facilitate the establishment of another species, and could
give rise to 'obligate succession' (Horn 1976). The reap-
pearance of plant species in restoration projects may
depend on their persistence in the soil seed bank as
a 'memory' of the original plant community (Bakker
et al.
1996b). Many species of the north-west Euro-
pean flora have only a short-term persistent seed bank
(Thompson
et al.
1998), which implies that dispersal
is the bottleneck in regeneration. If a species has been
lost from the persistent soil seed bank, it has to be
transported to the site of reappearance by some
vector, for example wind, water, animals or humans,
and incorporated into the fresh seed bank, waiting for
safe sites for germination and establishment. Seed
dispersal is an important key for the establishment of
target communities or species (Poschlod & Bonn
1998). Also for aquatic systems, Palmer
et al.
(1996)
argued that dispersal needs to be viewed as a regional
process that may routinely influence local benthic
dynamics, because fauna can move to and from water-
column dispersal 'pools' and may do so at frequent
intervals; benthic invertebrate assemblages may be
influenced in an ongoing fashion by dispersal.
communities. Invasions of single-species populations
being a natural phenomenon in terms of island
biogeography, they may remain ineffective in case of
dispersal constraints and they may come to play a par-
ticularly negative role in affecting communities when
the species are alien to the local community. The vigour
and success of aliens in areas where they have been
introduced has been attributed not only to more
favourable environments, but also to the release from
natural phytophagous enemies (Crawley 1987). For
Lythrum salicaria
, introduced to North America some
200 years ago, both increased competitive ability and
release from herbivores have been suggested as an
explanation of increased alien vigour (Blossey &
Nötzold 1995). The invasion of
Senecio inaequidens
,
a perennial and highly self-fertilizing pioneer plant
transported in the early 20th century with sheep's wool
from South Africa to Belgium and the Netherlands,
coincided with its adaptation to the Atlantic climate
by early flowering, with flowering time moving from
August to May, and prolongation of the flowering
period to the end of December (Ernst 1998).
A survey of plant traits in the Czech flora (Py
s
ek
1995, Py
s
ek
et al.
2002) revealed that the alien spe-
cies differed significantly from the native flora in the
frequency distribution of life forms and life strategies.
Therophytes and phanerophytes showed remarkably
higher representation among aliens, whereas hemi-
cryptophytes and hydrophytes were under-represented.
Across 302 nature reserves, the mean proportion of
aliens was 6.1% (range 0 -25%). Vegetation type
alone explained 14.2 and 55.5% of the variation in
the proportion of aliens in regions of mesophilous
and mountain flora, respectively. Taken together, the
environmental variables used (altitude, climate and
human impact) explained 44% of the variation in the
representation of all alien plants. The positive re-
lationship between the occurrences of neophytes and
native species could indicate that the two groups do
not directly compete. To the contrary, Levine and
D'Antonio (1999) and Alpert
et al.
(2000) concluded
from their survey of spatial pattern and invader addi-
tion studies that exotic and native species respond to
the same factors in a similar way; it has proved diffi-
cult to identify particular traits that are consistently
associated with the tendency of plant species to
invade. The traits that seem to best explain invasive-
ness are probably a broad native range and rapid
5.2.2 Alien invasive species
In Chapter 7, risks of re-introductions will be discussed,
meant to restore or to complete a community species
pool. Here, I will deal with the problem of the
unintended invasion of alien species, not belonging
to the community species pool (see also Chapter 2).
According to McCann (2000), the current evidence indic-
ates that, although most species invasions have a weak
impact on ecosystems, the occasional invasive exotic
species alters an ecosystem profoundly. It is, there-
fore, necessary to carefully distinguish between native
and alien (exotic, non-indigenous) immigrants into
