Environmental Engineering Reference
In-Depth Information
Extreme high water
Average high water
CLAY
Average summer water
SAND
Low water
PEBBLES
annual
weeds
reed
bed
Salix
shrubs
Salix
woodland
Alnus
woodland
Ulmus-Quercus
woodland
deciduous
forest
flood
plain
grass-
land
WATER
FLOODPLAIN
WITHOUT TREES
FLOODPLAIN
WITH WOODLAND
OUTSIDE
FLOODPLAIN
Figure 5.1
Vegetation zonation along rivers in relation to fl ooding frequency. (From Ellenberg 1986. Reproduced by
permission of Verlag Eugen Ulmer.)
Connectivity
and its effects on fl ows of matter and
organisms have become a major research theme in land-
scape ecology. The relation with landscape pattern is
often very complex and may range from broad-scale
fl ows that are hardly related to any landscape features
(e.g. bird migration at higher altitudes) to very localized
fl ows that are completely determined by landscape
structure (e.g. seed transport by fl owing water). Model
simulations show that in purely 'random' landscapes,
circa 60% of the space must be covered by suitable
habitat patches in order to ensure that organisms can
disperse from one side of the area to the other without
having to leave the habitat; that is, be part of a function-
ing metapopulation. In real landscapes, this percentage
can be much lower, due to the presence of
corridors
, rela-
tively narrow strips of suitable habitat that connect
patches. Simulations (e.g. Gardner
et al
. 2007 ) show
that presence or absence of corridors makes a dramatic
difference for the 'percolation' of organisms within a
landscape. One could say that corridors function as
communication channels within the landscape.
However elegant this concept may be, the recognition of
corridors in reality is not easy and may differ from one
species to another. Hedgerows may be corridors for
beetles, but for ospreys the river that fl ows through a
hedgerow landscape may be an appropriate corridor.
5.3 FLOWS BETWEEN LANDSCAPE
ELEMENTS
Even in the second half of the twentieth century, most
landscape ecologists had a rather static view of their
subject. Landscapes were described and classifi ed on
the basis of certain parameters which might refer to
exchange processes, but explicit incorporation of fl ows
of matter and movement of organisms was not nor-
mally done. This changed in the last few decades of the
twentieth century, when it became increasingly clear
that spatial arrangement is a fundamental feature of
terrestrial landscapes. Ecosystems in sites with similar
abiotic conditions, but differing in spatial arrange-
ment, may develop in entirely different ways because
of differences in the infl ow or outfl ow of resources,
and/or different immigration rates of preys and com-
petitors linked to spatial organization.
