Environmental Engineering Reference
In-Depth Information
Box 13.1 Salt-marsh succession and plant-animal interactions
The back-barrier island of Schiermonnikoog, the Nether-
lands, extends eastward, thus featuring a chronosequence
from east to west. With increasing age, the layer of
deposited silt gets thicker. A positive correlation is found
between the thickness of the silt layer and both the nitro-
gen pool (Olff
et al.
1997) and the availability of nitro-
gen for plants. Hence, the chronosequence represents a
productivity gradient featuring low-statured plants in
early stages and tall grass in later successional stages,
and a decrease in the number of plant species (Bakker
et al.
2002b). As a result, the forage quantity for
natural herbivores such as spring-staging geese and
resident hares increases, but the quality, expressed as
leaf/stem ratio, decreases (van de Koppel
et al.
1996).
The distribution of the palatable species
Triglochin
maritima
is sandwiched between intensive grazing by
geese and hares in the younger marsh and increas-
ing competition for light in the older marsh (van der
Wal
et al.
2000a, 2000b). Hence, geese and hares are
evicted by vegetation succession. A resetting of the suc-
cessional clock is only possible when large herbivores
such as livestock are introduced.
The North American mid-continent population of
lesser snow geese (
Chen caerulescens caerulescens
)
increased strongly in recent decades. The sustained
increase in population size is thought to be the result
of increased dependence of the birds on agricultural food
sources and food provided in refugia on wintering
grounds and along flyways towards their breeding
sites along the coast of Hudson Bay, Canada. The
increase in snow geese numbers has resulted in a
widespread consumption of the coastal marsh vegeta-
tion in breeding colonies. High consumption rates have
resulted in loss of vegetation cover, subsequent expos-
ure of surface sediments and development of hypersaline
soil. Several thousands of hectares of natural coastal
marshes have already transformed into 'arctic desert'
(Jefferies & Rockwell 2002).
In an artificial salt marsh in the Dollard, the Neth-
erlands, drainage works were abandoned in the early
1980s. This resulted in filling the ditches with sediment
and subsequent waterlogging of the cattle-grazed salt
marsh. Wintering greylag geese (
Anser anser
) main-
tained bare soil in inundated depressions by grubbing
below-ground parts of
Spartina anglica
. During recent
decades the wintering population of barnacle geese
(
Branta leucopsis
) increased strongly, but seemed to reach
carrying capacity during the past few years. Barnacle
geese grazed the
P. maritima
sward that degraded into
secondary pioneer vegetation. Experimental exclosure
of cattle revealed that the geese can only affect the
vegetation when facilitated by the large herbivores, in
contrast to the above-mentioned lesser snow geese
(Esselink
et al.
2002).
(Mendelssohn 1979, van Wijnen & Bakker 1999).
Nitrogen supply can dictate the competitive relations
of marsh plants and hence has important consequences
for the abundance and distribution across marsh land-
scapes (Levine
et al.
1998). The lower tidal boundaries
of marsh-plant distributions are generally set by phys-
ical stress, whereas the upper boundaries of plants are
set by competitive exclusion. Hence nitrogen supply
may affect the elevation of zonal borders in marshes.
The influence of fresh water discharged by rivers
from the hinterland creates a gradient of decreasing
salinity away from the sea. The vegetation features
Scirpus maritimus
and especially
Phragmites aus-
tralis
, as in estuarine marshes (Esselink 2000), the Baltic
Sea in Europe (Jeschke 1987, Dijkema 1990, Puurman
& Ratas 1995) and the Atlantic coast in the USA
(Bertness 1999).
13.3 Changes in intertidal flats
13.3.1 Human exploitation and roads
towards extinction of benthic
organisms
Summarizing the extensive and detailed faunistical
information from the Sylt area in the northern part
of the Schleswig-Holstein Wadden Sea dating back
to 1869, Reise (1982) came to the conclusion that
whereas bivalves and some other groups of inver-
tebrate animals show long-term decreases in species
diversity, the smaller polychaete species with short life-
spans are doing well (Table 13.2). Small polychaetes
can take rapid advantage of environmental disturbances
leading to faunal depletions. Reise (1982) attributed
the disappearance of 28 common macro-invertebrate
