Geoscience Reference
In-Depth Information
water resources increase with global environmental
changes.
capable of supporting herbaceous plant species
not found elsewhere in the riparian zone'. They
are specific in talking about 'beaver modified
patches', and ecosystem engineering resulting in
increased landscape patchiness or heterogeneity
is also a feature noted by Jones
et al.
(1994).
In Scotland, the European beaver (
Castor fiber
)
has been reintroduced after an absence of 400
years (Halley, 2011), and has set about re-
engineering its habitat in a manner consistent with
its previous role. Interestingly, in this case public
reaction has not all been positive, suggesting that
if ecosystem re-engineering can occur, it needs
to be accompanied by social engineering, or least
increased public awareness.
Like beavers, Milu (
Elaphurus davidinus -
a
species of deer) in China are ecosystem engineers.
Archaeological remains from 7000 BP shows their
semi-domestic use in trampling swamp/marsh
into rice paddy (You, 2005). The trampling tillage
culture for rice paddy was subsequently spread
to South-east Asia by buffalo. In China the
Milu became extinct about 150 years ago, but
in 1985 they were re-introduced from England.
Unfortunately, monitoring shows that these
translocated Milu populations are compacting the
riverine soil, and feeding selectively on only a
few species, thus forcing a simplified vegetation
pattern, and threatening their own habitat
(Zhang
et al.
, 2007). However, in a twist, and
linking to invasive alien species, this ecosystem
engineer is controlling
Spartina alterniflora
,a
wide spread(ing) species, causing ecosystem
damage along the eastern Chinese coast (Ding,
2009).
While the Milu and buffalo ecosystem engineers
were working slowly on producing rice paddies,
human agro-engineers decided to accelerate the
process of wetland reclamation. This reclamation
in the middle and lower Yangtze has reduced the
area of floodplain lakes from 25 828 km
2
in 1949
to 14 073 km
2
. Dongting Lake, which used to
be the largest freshwater lake in the floodplain,
shrank from 4350 km
2
in 1949 to 2625 km
2
in
1995 and lost water capacity of 12.6 billion m
3
.
This reclamation and creation of paddy in too
rapid a fashion changed flooding frequency and
Rivers and their biota as
ecosystem engineers
'Ecosystem engineers' is a term coined by Jones
et al.
(1994) for 'any organism that creates or
modifies ecosystems'. They identified two different
types of ecosystem engineers:
Allogenic engineers
that modify the environment
by changing materials mechanically from one form
to another. Humans are perhaps the best-known
allogenic engineers.
Autogenic engineers
that modify the environment
by modifying themselves, for example the growth
of trees.
Invasive alien species are often ecosystem
engineers, both allogenic and autogenic in nature.
Rivers can be affected by alien species in various
parts (or all) of their lengths, as well as providing
easy means for movement of species and their
propagules
along
the
length
of
the
river,
or
between and within riparian ecosystems.
In river systems, beavers (
Castor
spp.) are
archetypal ecosystem engineers since in their
process of tree cutting and damming they alter
their ecosystem extensively. Beaver dams change
hydrological processes significantly and promote
different species and populations from those in
free-flowing rivers. Aznar and Derochers (2008)
examined beaver (
Castor canadensis
) pond dynamics
and associated riparian bird communities. They
noted that 'following water disappearance,
abandoned beaver ponds were invaded by grassy
areas and alder shrubs and supported higher bird
species numbers than nearby riparian areas.' They
postulated that 'Beaver population control or
geographic confinement may suppress both spatial
and temporal dynamics of beaver pond creation
and
abandonment,
and
could
impact
riparian
vegetation and birds.'
Wright
et al
. (2002) for the Appalachians
found that 'in the central Adirondacks, New
York, USA, ecosystem engineering by beaver leads
to
the
formation
of
extensive
wetland
habitat
