Environmental Engineering Reference
In-Depth Information
thousands of kilometres and was highly relevant for
gene flow between continents.
The importance of wind dispersal changes dramat-
ically in more heterogeneous landscapes. Steep slopes
and irregular warming of diverse surface materials
lead to an increasing importance of thermal updrafts
for airflow. Unlike the situation in homogeneous
landscapes, air transport in heterogeneous landscapes
does follow more or less distinct corridors, namely those
parallel to steep slopes or along edges between dif-
ferent surface types. This can occur on a scale of
several hectares but also on much larger scales.
Tackenberg
et al.
(2003) modelled seed dispersal of rare
plant species that grew on small porphyric outcrops
in an otherwise intensively used agricultural landscape,
illustrating that heat-induced air turbulence enables
seed exchange between sites with distances of up to
some hundreds of metres for many species despite a
small size and a low number of seeds. Pennycuick
(1972) studied bird movement by thermal updrafts and
showed that both daily movements of vultures over
distances of several hundreds of kilometres and sea-
sonal eagle migration over thousands of kilometres
were energetically only possible because of this type
of airflow.
drift-line material (Cappers 1994, Knörzer 1996).
Moreover, even seeds with a restricted floating capa-
city can probably travel large distances in running
waters. Nilsson
et al.
(1991) calculated that only
2.5 days should be sufficient to cover a 230-km long
river stretch in northern Sweden during the spring
floods. Studies of riparian vegetation have indeed
shown a large floristic similarity between upstream and
downstream areas, which decreased slowly with dis-
tance (Tabacchi
et al.
1990, Johansson
et al.
1996).
Tabacchi
et al.
(1990) found a sharp decrease in
similarity only when the river merged with major side-
branches, suggesting that at such points 'propagule
flows' of two (or more) areas with different species
pools (see Zobel
et al.
1998; see also Chapter 2 in this
volume) are combined.
Dispersal by animals and humans
With the exception of waterfowl and fruit-eating
birds (Vivian-Smith & Stiles 1994, Galatowitsch & van
der Valk 1996, Handel 1997) animals are considered
poor dispersal agents in the modern landscape. In
contrast to the past when livestock was transported
from winter to summer areas and vice versa over dis-
tances of up to hundreds of kilometres, seed exchange
between areas by moving animals has probably
almost disappeared (Poschlod
et al.
1996, Poschlod
& Bonn 1998). This has been considered the reason
why gene exchange between isolated nature reserves
has become minimal (Bokdam & WallisDeVries 1992,
Grashof-Bokdam 1997, Mouissie 2004).
Humans have always played a role as a 'moving
corridor' (Poschlod
et al.
1996). Plants lack all mor-
phological adaptations to dispersal by human beings,
yet people have probably been the most effective
long-distance dispersers since the Middle Ages
(Sukopp 1972, Sykora
et al.
1990, Poschlod
et al.
1996).
For example, modern traffic facilitates rapid dispersal
of halophytes along motorways and a rapid invasion
of alien weeds along railway lines (Ernst 1998). At
the same time the decreasing degree of agricultural
activities in most nature reserves has probably led to
less exchange with other areas (Poschlod & Bonn 1998).
The significance of humans for the dispersal of rare
plants to such reserves is, therefore, likely to be very
low except when a deliberate management scheme is
set up (Strykstra
et al.
1996a).
Dispersal by water
Several field studies (Johansson
et al.
1996, Bill
et al.
1999, Middleton 2000, Andersson & Nilsson 2002)
showed that running waters contain large numbers of
seeds and this suggests that they are most likely to be
a major dispersal agent in wetlands. In comparison
to wind, water has some typical characteristics. Water
dispersal takes place only via corridors and it is
highly unidirectional, i.e. downstream. It is only omni-
directional in the case of standing water, but seed dis-
persal in these systems is quantitatively probably of
minor interest, as transport rates are only a fraction
of those of running waters. Laboratory measurements
on seed buoyancy of characteristic plant species of bogs
(Poschlod 1990) and riparian habitats (Danvid and
Nilsson 1997, van den Broek
et al.
, personal com-
munication) showed that seeds of most species were
capable of floating for many weeks, including seeds
without clear morphological adaptations. This picture
was confirmed by palaeobotanical studies when dias-
pores of a large number of species were found in buried
