Environmental Engineering Reference
In-Depth Information
the end of the Middle Ages (ca. 1500 CE), for example
in England where most woodlands had been cut for fuel
and people had to switch to peat for heating. Another
example is the western part of the Netherlands and
Belgium where people started to protect themselves
against fl ooding by building dykes and creating polders
from the fi fteenth century onwards.
Notwithstanding the fact that the latter landscapes
have very little overlap with earlier seminatural land-
scapes, both belong to the variegated type. In both
cases, there is a large connectivity between the differ-
ent types of landscape patches. Variegated landscapes
were dominant in Europe until the beginning of the
twentieth century but were much less common in
North America or Australia. In the latter regions there
was often a direct conversion from natural habitats
into large-scale industrial agriculture, almost without
any remnants of untouched habitat.
2002). What is clear is that exchanges of species
between patches, at least in the case of organisms with
little mobility, such as plants, have decreased signifi -
cantly in recent decades, either because of ongoing
fragmentation
or because of the cessation of tradi-
tional management practices such as
transhumance
(Poschlod & Bonn 1998). Probably there is still some
interchange between subpopulations but it is unknown
whether this is suffi cient to sustain a functioning
metapopulation
(Opdam 1991 ; Hanski 1999 ; see
Chapter 7). Nevertheless, it is feared that present-day
distribution patterns of many organisms are in fact
spatially isolated relics of a former continuous popula-
tion that has become fragmented. The danger is very
real that many of these small populations will die out
and that species will become locally extinct due to
chance processes alone.
5.4.4
Relictual landscapes
5.4.3
Fragmented landscapes
When land use pressure increases further, most of the
remaining patches with high ecological quality disap-
pear and the ecological value of the landscape decreases
further. The few remaining high-value patches are
under high pressure. Most species are ubiquists, occur-
ring everywhere, not only in intensively used patches
but even in more natural sites. Land use in relictual
landscapes is by defi nition not sustainable and - apart
from threats to biodiversity - there are generally a
range of environmental problems associated with it as
well, such as groundwater pollution, salinization of the
topsoil, peat subsidence and associated CO
2
emissions
and soil erosion. Typical examples are found in densily
populated urban areas (e.g. in north-western Europe),
but also in areas that are optimal for intensive agricul-
tural production such as the midwestern United States,
parts of south-western Australia and collective farming
areas in former communistic countries. Intensive land
transformation in these areas leads to only small rem-
nants of pre-existing ecosystems being retained in an
otherwise completely modifi ed matrix.
Developments in technology stood at the basis of the
phase with industrial agriculture and led to landscape
changes in rural areas at an ever-increasing speed and
scale. Chemistry enabled the development of artifi cial
fertilizer, and engineering sciences made large-scale
mechanization possible. These developments became
especially apparent after the Second World War, par-
ticularly in Europe where politicians feared famine and
stimulated large-scale land reallotment programmes
and other rationalization of agriculture. This process
was accelerated further through the implementation
of the Common Agricultural Policy in the European
Community from the 1960s onwards.
Compared to variegated landscapes there is much
less connectivity between patches, borders are much
sharper and the ecological quality is much lower.
Patches with a high ecological quality have become
islands surrounded by a matrix of unsuitable habitat.
Island theory (MacArthur & Wilson 1967) predicts
that the probability of local extinction increases with a
decreasing patch size and that the probability of re-
colonization decreases with increasing distance.
Among conservation scientists today, the prevailing
view is that this is also true for small patches of suita-
ble habitat in otherwise unsuitable surroundings
(Soul é & Simberloff 1986 ; Opdam
et al
. 2003 ). Though
this hypothesis has never been proven, it is supported
by the fact that small patches are generally poorer in
species than larger ones (Rosenzweig 1995; Cook
et al
.
5.4.5 Effects of land use changes on
abiotic conditions
Human-induced changes in landscape structure affect
not only species exchange patterns but also abiotic
fl ows. The building of dams and canals, as well as the
straightening of river tracts, affect water fl ow rates and
fl uctuation patterns to a high degree. Sometimes this
