Environmental Engineering Reference
In-Depth Information
Table 5.3
Four states of landscape alteration defi ned by degree of habitat destruction (after McIntyre & Hobbs 1999).
Level of
alteration
Degree of
destruction
of habitat
(% remaining)
Connectivity of
remaining habitat
Degree of
modifi cation of
remaining habitat
Pattern of
modifi cation of
remaining habitat
Intact
Little or none
(
>
90)
High
Generally low
Mosaic with gradients
Variegated
Moderate
(60-90)
Generally high, but lower for
species sensitive to
habitat modifi cation
Low to high
Mosaic that may have
both gradients and
abrupt boundaries
Fragmented
High (10-60)
Generally low, but varies
with mobility of species
and arrangement on
landscape
Low to high
Gradients with fragments
less evident
Relictual
Extreme (
<
10)
None
Generally highly
modifi ed
Generally uniform
dominant landforms and the climatic conditions were
so harsh that hardly any organism could persist.
Almost all species had to (re)colonize the released area
during the following interglacial period. Delcourt and
Delcourt (1983) have presented a detailed reconstruc-
tion of this process and showed that colonization rates
differed signifi cantly between species. It is assumed
that this process is not yet complete, especially in the
case of species with low colonization rates such as
plants or low-mobility insects. The inhibition model of
Connell and Slatyer (1977) predicts that under such
conditions, 'priority effects' will lead to different species
assemblages in otherwise similar ecosystems and such
results are indeed found in the fi eld (e.g. Urban & De
Meester 2009 ; Klimkowska
et al
. 2010a ). This implies,
amongst other things, that the 'natural' or ideal
refer-
ence system
for a restoration project in a given type
of landscape may differ from place to place.
Contrary to what is sometimes assumed, most intact
landscapes have been subject to human impact, often
for a considerable period; at least 5000-7000 years in
north-western Europe and North America, 8000-
13 000 years in the Mediterranean region, and at least
40 000 yr in Australia. Human densities were very
low (<0.5 inhabitants per km
2
) in the distant past,
but in some cases human impacts still affected
the landscape structure. Most prehistoric peoples
lived as nomads and those who practised agriculture
generally employed the slash - and - burn system whereby
a certain area of woodland was burned down, used for
a few years until the nutrients were depleted, and then
left again in fallows for a long time. This human-
initiated process led to a structured landscape of
patches of different age since disturbance. Intact land-
scapes or landscapes structured by indigenous human
communities dominated the scene in Europe until the
fi rst centuries
CE
and in North America and Australia
until the arrival of European settlers. Nowadays they
persist only in sparsely populated parts of tropical rain
forests, in boreal woodlands in the cold parts of the
northern hemisphere, or in hot and cold desert regions.
5.4.2
Variegated landscapes
The next, seminatural stage is associated with the devel-
opment of a more sedentary form of agriculture. The set-
tlements became fi xed and human impact on landscapes
increased. Uniform landscapes became more heterogene-
ous, and large-scale abiotic gradients were replaced by
smaller gradients of human impact. This resulted in a
local increase in biodiversity, through the creation of suit-
able patches for species that had a very limited distribution
in intact landscapes (Sukopp & Trepl 1987). The land-
scape did become fragmented to some degree, but the
connectivity between more or less affected patches was
still very high. It is unlikely that these developments lead
to problems for most organisms, except for large mammals.
These might experience problems even at a low degree of
fragmentation (Soul é & Terborgh 1999 ). Not surprisingly,
large grazers such as auroch (
Bos primigenius
) or wisent
(European bison,
Bison bonasus
) disappeared from Europe,
although bison populations have recovered in some areas.
This process continued with time and human impact
became ever more dominant. In certain areas humans
had altered the structure of landscapes completely by
