Environmental Engineering Reference
In-Depth Information
Defoliation by domestic herbivores and the occurrence of frequent i res
in association with periodic droughts are also thought to have promoted
plant diversii cation particularly of annual species and initiated adapta-
tions to drought, i re and grazing (Pignatti, 1978; Naveh, 1994).
Many bird species from northern Europe overwinter in the
dehesas
and
are reliant upon the
dehesas
as a food source. Telleria et al.
(1992) provide
evidence that suggests that
dehesas
may support more diverse communi-
ties of passerines than neighbouring stands of high forest.
Dehesa
habitat
supports 64 per cent of the population of common cranes wintering in
Spain (50 000-60 000 birds), which represents 70-85 per cent of the western
European population (Alonso
et al., 1990). The crane population is not
widespread in other habitats as cranes rely on acorns, so holm oak
dehesas
are of great importance for this species, considered vulnerable because
of their decreasing population trend (Tucker and Heath, 1994). It is also
thought that the distribution of white stork is most strongly associated
with open holm oak wood pasture and it has been suggested that conver-
sion of this habitat could lead to a decline in its populations (Carrascal et
al., 1993).
Managed woodland and grazing systems elsewhere in Europe are also
important for maintaining biological diversity. The bird community of
western oak woods in the United Kingdom, particularly the abundance of
the wood warbler (
Phylloscopus sibilatrix
), pied l ycatcher (
Ficedula hypo-
leuca
) and redstart has long been recognized as unique and some grazing
may help to create the open conditions in the understorey and i eld layer
favoured by these species. Some studies that focus on grazing even report
that subspecies of grasses may develop according to the specii c ecological
conditions that occur in a grazed or mowed sward (Reinhammar, 1995).
The development of species-rich raised coastal dune and bog habitats in
north-western Europe, known as machairs, is also thought to be strongly
associated with agriculture and human activity, particularly i re and
grazing (Mate, 1992; Edwards, et al.,
2005). Machairs, which are priority
habitats under the European Habitats Directive, are unique ecosystems
coni ned, in the northern hemisphere, primarily to west and north-west
coasts of Ireland and Scotland. Machairs are priority habitats because
of the high plant species richness which contain elements of calcareous
grassland and sand dune plant communities. We now turn to the issue of
abandonment.
Londo (1990) reports that, in the absence of management, semi-natural
grassland communities revert by processes of natural succession to natural
woodland and forest and the diversity of herbaceous species falls. Many
traditional extensive farming practices have been shown to maintain plant
and animal diversity (González Bernáldez, 1991; Naveh, 1994), and where
