Environmental Engineering Reference
In-Depth Information
wilderness still exist in North America, whereas such
areas are no longer present in western Europe. It is
therefore not surprising that wilderness is the major
reference for restoration in North America.
probability of recolonization decreases with increas-
ing distance. Present ideas in conservation theory
are that this is also true for small patches of suitable
habitat in otherwise unsuitable surroundings such as
nature reserves surrounded by agricultural fields
(Soulé & Simberloff 1986). Though this hypothesis has
never been proven unambiguously, it is supported by
the fact that small habitat fragments are generally
poorer in species than larger ones (Rosenzweig 1995,
Cook
et al.
2002). What is certainly clear is that
exchange of species between patches, at least in the
case of sessile organisms like plants, has decreased
significantly (Poschlod & Bonn 1998). Probably there
is still some interchange between sub-populations
but it is unknown whether this is sufficient to sustain
a functioning metapopulation (Opdam 1991, Hanski
1999; see Chapter 6 in this volume). Nevertheless, it
is feared that present-day distribution patterns of
many organisms are in fact spatially isolated relics from
a former continuous population that has become
fragmented. The danger is very real that many of these
small populations will die out and species will
become locally extinct due to chance processes alone.
Changes in land use did not only change species-
exchange patterns: water flows have also been
modified by human activities. The building of dams
and canals, but also the straightening of river tracts,
affected water flow rates and fluctuation patterns to
a high degree. This sometimes even resulted in a
complete reversal of water flow direction (see Chapter
11). Such activities have a large impact on wetlands
downstream. The straightening of the Upper Rhine
along the French-German border resulted in the loss
of 87% of characteristic floodplain woodland, on both
the German and French sides (ICPR 1998).
Changes in groundwater flows are less obvious but
can potentially have equally large effects. The most
obvious effects occur when the water regime is
changed. Lowered water tables during the dry season
lead to increased mineralization of organic matter
and hence increased nutrient availability. In low-
productivity wetlands this results in a substantial
increase of non-characteristic ruderal plants at the cost
of typical species of low competitiveness (Grootjans
et al.
1985). Changes in water chemistry are more insidi-
ous but can have similarly large impacts. A ground-
water-abstraction facility does not necessarily lead to
lowered water levels in a nearby groundwater-fed mire
3.5.3 Fragmentation
Gradients in a natural landscape are closely linked to
geomorphologic structures and soil types. As long as
these features are relatively uniform - for example,
in flat areas - the landscape is also rather mono-
tonous and gradients are large. Human activities often
split such larger gradients partly into smaller ones and
the landscape becomes fragmented to some degree.
This is probably not a big problem for most organ-
isms as long as the patches are connected, though large
mammals might experience problems even at a low
degree of fragmentation (Soulé & Terborgh 1999). Under
little-to-moderate human impact this process often leads
to a local increase in biodiversity, creating additional
suitable patches for species that had a very limited
distribution in pristine landscapes (Sukopp & Trepl
1987; Fig. 3.9).
The situation changes dramatically when land use
becomes so intense that corridors between patches
disappear. Island theory (MacArthur & Wilson 1967)
predicts that the probability of local extinction
increases with a decreasing patch size and that the
1975
1950-1960
1750-1850
800-120
A
BC
5000-3500
10,000
Species number
Fig. 3.9
Species richness of higher plants in Europe in
time (after Sukopp & Trepl 1987).
