Environmental Engineering Reference
In-Depth Information
especially by nitrogen. It coincides with the dominance
of indigenous plant species such as
Deschampsia
flexuosa
in dry heathland,
Molinia caerulea
in wet
heathland,
Calamagrostis epigejos
in dunes,
Brachy-
podium pinnatum
in calcareous grasslands and
Elymus athericus
in salt marshes on the continent. In
the UK an increase of the indigenous bracken
(
Pteridium aquilinum
) and the alien
Rhododendron
ponticum
, planted in 1763, is found in heathlands
(Cross 1975). In North America invasive species can
take over. They pose a serious threat in at least 194
of the 368 National Park Units in the USA (D'Antonio
& Meyerson 2002). Many invasive species are peren-
nial plants or persistent annuals that set up positive-
feedback mechanisms that perpetuate their own
existence. The list of 'One hundred of the world's
worst invasive alien species', including all groups of
organisms, published in the Global Invasive Species
database (www.issg.org/database) reveals that plant
species make up 60% of invasive aliens in grasslands
and shrublands. One of the most troublesome species
is the perennial shrub
Chromolaena odorata
which is
invasive in Australia, Africa and Oceania (McFadyen
& Skarratt 1996).
Apart from the aforementioned changes in plant
diversity, dramatic shifts in birdlife have been mon-
itored as a result of changes in land use. A compar-
ison between data in the new breeding bird atlas of
the Netherlands (1998-2000; SOVON Vogelonderzoek
Nederland 2002) and the previous atlas (1973-7)
revealed a decline of species groups breeding in open
grassland, and in dunes and heathland, with ten and
five species, respectively (Saris
et al
. 2002). At the
same time six species of dense scrub showed an
increase. The encroachment of scrub has strongly
affected the breeding bird population in dunes. Typ-
ically scrub-dependent species such as lesser white-
throat (
Sylvia curruca
) and nightingale (
Luscinia
luscinia
) have increased. However, birds character-
istic of more-open dunes have declined dramatically,
such as Montagu's harrier (
Circus pygargus
), wryneck
(
Jynx torquilla
), nightjar (
Caprimulgus europaeus
)
and red-backed shrike (
Lanius collurio
) (Sierdsema &
Bonte 2002).
Many insect species depend on grasslands with a
typical structure of the sward or a mosaic of short
canopy and scrub. Up to 20% of European butterflies
are threatened or near threatened (Bourn & Thomas
2002). Calcareous grasslands are extremely important
for butterflies in Europe: 48% (274) of all native
species (576) occur in calcareous grasslands (van
Swaay 2002). Only for the UK and the Netherlands
are historical data available on changes over the past
century. In Suffolk, UK, 42% of the butterfly species
became extinct (Bourn & Thomas 2002). The Nether-
lands harbour a total of 71 native species of butterflies,
of which 17 (24%) have become extinct and 30 (43%)
are endangered (van Ommering
et al
. 1995).
Data on the decline of biodiversity as a result of
changes in land use should be treated with caution.
We will elucidate some plant data collected before
1950 and after 1970 in the well-monitored province
of Drenthe, The Netherlands. This province showed a
dramatic decline in the area covered by heathlands
and bogs between 1850 and 1990 (Plate 8.1). For the
whole of the Netherlands the area decreased from
about 600,000 ha in 1833 to 36,000 ha in 1990.
Despite a strong intensification of agricultural prac-
tices, expansion of urban areas and other significant
land-use changes, the biodiversity expressed as the
average frequency of 126 indicator species of nine plant
communities hardly declined (van Diggelen
et al.
2005).
The first reason for this phenomenon is that the
data on the occurrence of plant species are presence/
absence data, and do not take into account the abund-
ance of species, as shown by the following example.
The number of marked grid cells (cell size 1 km
2
) after
1970 shows a nearly complete coincidence with the
former heathland area of the dominant species of
dry heathland,
Calluna vulgaris
(Fig. 8.1a). At the
national level the number of marked grid cells
decreased from more than 10,000 in 1930 to 3000 -
10,000 in 1995 (Tamis & van 't Zelfde 2003). Small
fragments of heathland harbouring
C. vulgaris
still
exist in most grid cells. Moreover, after reclamation
of heathland, the seeds of
C. vulgaris
may survive for
many decades in the soil (Thompson
et al
. 1997).
Intensively exploited arable fields still harbour viable
seeds 70 years after reclamation from heathland (ter
Heerdt
et al
. 1997). The intensification of infrastruc-
tures such as roads and ditches often brings viable seeds
to the surface and hence allows the re-establishment
of
C. vulgaris
in road verges. In contrast,
Arnica
