Environmental Engineering Reference
In-Depth Information
system with mainly cattle and sheep. Often dung was
collected in overnight stables to fertilize the arable fields
in spring (Bonn & Poschlod 1998). In north-west
Europe, sods were also cut to enhance the production
of arable fields. Cutting for hay only developed
during the last 2000 years, when animals needed for
traction power (oxen, horses) were used intensively;
they needed hay for winter forage. The removal of
biomass resulted in oligotrophication of the soil; that
is, lowering of the nutrient availability. This is a
characteristic phenomenon of low-intensity farming
systems.
North American tallgrass prairies expanded greatly
beginning about 8000
BP
. From that time until about
3000
BP
the region's climate was hotter and drier
than at present, favouring grassland development as
woodlands retreated. The origins of tallgrass prairies
and the roles of climate, grazing animals and human
activity are still not completely understood. Humans
have been part of the prairie environment since
before the last glaciers retreated from the Midwest.
Prairies were home to Native Americans for thousands
of years prior to European settlement. They managed
them according to their needs and abilities. Deliberate
fires, along with occasional lightning, were major forces
in shaping prairie vegetation. Landscape fires were used
to facilitate travelling and hunting, to stimulate new
growth for game, to reduce fuel loads near habitations.
Large areas of former prairie that have not been
destroyed through conversion to agriculture, intensive
grazing or development, have been degraded through
fire suppression, drainage or other alterations (Knapp
et al
. 1998).
Grazing by domestic livestock has most likely dis-
persed many plant species from the Middle East into
Europe (Bonn & Poschlod 1998). Transhumance must
have played an important role in the long-distance
dispersal of diaspores (Poschlod & Bonn 1998).
For example, the species composition of calcareous
grasslands has changed during the past millennia.
Archeological finds of macro-fossils (seeds, leaves,
stems) indicated that the current dominant grass
Bromus erectus
was not found in sites occupied 500 -
1000 years ago (Poschlod & WallisDeVries 2002). The
invasion of species from the Middle East during the
past millennia means that in Europe only a few invas-
ive species are currently considered a problem in
conservation. In contrast, in North America the inva-
sion of plant species from other continents over the
past few centuries is considered one of the problems
for biodiversity (D'Antonio & Meyerson 2002).
Both intermediate disturbance and intermediate
nutrient supply are believed to support species rich-
ness in grasslands. Stability of vegetation dynamics
in dry acidic grasslands is preserved and promoted by
sufficient small-scale disturbance under conditions of
low resource availability (Jentsch & Beyschlag 2003).
Disturbances in grasslands are often caused by
herbivores. Both small and large herbivores have a
major impact on dispersal and colonization of plant
species. Cattle are identified as most important for
seed dispersal, whereas rabbits have a main effect as
creators of disturbances (Bakker & Olff 2003).
8.2 Threats for biodiversity due to
changes in land use
Changes in land use have resulted in reclamation of
grassland and heathland. Intensification of agricultural
practices has led to a strong decline of these areas.
On the other hand, many marginal agricultural areas
have nowadays been abandoned, with subsequent
bush encroachment and hence a decline in the area
of grassland and heathland. Examples of the decrease
of habitats are heathland (Webb 1997), calcareous grass-
lands in north-west Europe (WallisDeVries
et al
.
2002), wooded meadows in Sweden (Mitlacher
et al
.
2002), tallgrass prairies in the USA (Samson & Knopf
1994) and changes within habitats, such as bush
encroachment in alvars in Sweden (Rejmánek &
Rosén 1992). Abandonment coincides with bush
encroachment accompanied by a decrease in plant
species richness. For instance, on the alvar grasslands
in Öland, Sweden, the number of characteristic alvar
species has declined continuously with an increasing
cover percentage of
Juniperus communis
scrub and a
dramatic drop in species number between 75 and 100%
of shrub-cover species (Rejmánek & Rosén 1992). As
a result of previous agricultural practices, dry grass-
lands and heathlands occur on nutrient-poor soils.
This makes them vulnerable to impacts from adjacent
fertilized agricultural areas, rivers and coastal seas, and
atmospheric deposition resulting in eutrophication,
