Environmental Engineering Reference
In-Depth Information
2002). Not only rare habitat specialists, but also some
common, widespread butterfl y species declined during
the past 20 years in the Netherlands and Belgium.
They maintained their populations in nature reserves
such as dunes, heathlands and seminatural grasslands
as a result of management practices, but they strongly
declined in farmland and urban areas (van Dijck
et al
.
2009). In the Netherlands, 10 and 5 bird species,
respectively, breeding in open grasslands and in dunes,
have disappeared from these ecosystems between
the periods 1973-1978 and 1998-2000 (Saris
et al
.
2002). At the same time, six bird species of dense
shrubland on sand dunes showed an increase. This
implies that shrub encroachment has strongly infl u-
enced the breeding bird population in dunes (Sierd-
sema & Bonte 2002 ).
increased nutrient levels. Hence, high nutrient levels
are a constraint for re-establishment during restora-
tion. In western Europe, total atmospheric input of
inorganic nitrogen ranges from 2-40 kg N ha
− 1
yr
− 1
,
which is related to mean species numbers of 22-10,
respectively, in plots of 2 m × 2 m in acidic grasslands
(Stevens
et al
. 2010). Moreover, a synthesis of experi-
ments all over the world with addition of nitrogen
shows a decline in species numbers across a latitudi-
nal range of ecosystems from arctic and boreal
systems to tropical forests (Bobbink
et al
. 2010 ). The
magnitude of the potential diffi culty can be assessed
by comparing these fi gures with the calculated
critical
nitrogen loads
, that is, effect thresholds for atmospheric
deposition, which are below 20 kg N ha
− 1
yr
− 1
for
heathland and grassland (Bobbink
et al
. 1998 ) and
coastal sand dunes (Jones
et al
. 2004 ). A 23 - year fi eld
experiment with addition of 10 kg of N ha
− 1
yr
− 1
on
top of the local atmospheric wet deposition of 6 kg of
N ha
− 1
yr
− 1
revealed a plant species reduction of 17%
in prairie grasslands in the United States (Clark &
Tilman 2008). The decline of a number of butterfl y
species in Sweden is related to the relatively high
atmospheric deposition, which is 12 kg of N ha
− 1
yr
− 1
( Ö ckinger
et al
. 2006 ).
It is also important to consider interactions among
nutrients, especially the balance between the main
macronutrients (i.e. phosphorus, potassium and nitro-
gen). Moreover, when two or more nutrients are limit-
ing plant productivity, the system is less sensitive to
temporary rises in the availability of a single nutrient.
Sod cutting and topsoil removal can be effective in
removing nitrogen but have much less effect for phos-
phorus (e.g. Bekker 2009; Figure 14.7 in Box 14.2).
In the great majority of dry grasslands and heath-
lands, productivity is only limited by nitrogen avail-
ability. Hence, these systems are sensitive to small
fl uctuations in N inputs. This is especially true where
atmospheric deposition can be more than double the
critical loads for oligotrophic target communities
(Bobbink
et al
. 1998 ).
14.3 CONSTRAINTS FOR
RESTORATION
The aforementioned shows that species have disap-
peared after changes of the ecosystem by exploitation,
enrichment of soil nutrient status or changes in land
use intensity. What is needed for restoration of these
modifi ed ecosystems is reduction in nutrient avail-
ability and re-establishment of now-absent species,
through either re-emergence from the soilborne seed
bank or else dispersal of propagules (seeds, fruits or
vegetative parts) from the surrounding area into the
site to be restored. Hence, we will discuss the issues of
nutrient status of the soil, dispersal capacity of target
species, longevity of seeds in the soil and interactions
between organisms as preconditions for succesful res-
toration. The fi rst step for re-establishment of species is
their dispersal to the target site. Once arrived, they can
either survive in the soil seed bank, or germinate and
establish as an adult plant. During this process of
establishment, of course, seeds encounter many limita-
tions resulting in few adults compared to the large
amount of arrived seeds (Figure 14.1; Baeten
et al
.
2009). Hence, a large amount of seeds should arrive
at the site to be restored for the successful establish-
ment of now - absent species.
14.3.2
Dispersal in space
Plant species that have disappeared and do not have a
long-term persistent seed bank must have adequate
long-distance dispersal mechansisms in order for them
to be able to re-establish themselves in a fragmented
landscape. Hence, it is important to know which dis-
14.3.1
Nutrient status of the soil
Plant and animal species have disappeared as a result
of agricultural intensifi cation, often as a result of
