Geoscience Reference
In-Depth Information
ecosystems and ecological flows. Yet Karr (1991)
in commenting on the perception of biological
degradation of rivers as the stimulus for (US) state
and federal legislation on water quality exposed
the reality that 'any biological focus was lost in the
search for easily measured physical and chemical
surrogates'.
Two centuries of taming rivers has simply
provided more opportunity for the demonstration
of human frailty as floods increase in frequency
and volume, often in countries where they have
not previously been seen. And, at the same time,
other (typically endorheic) systems are simply
evaporating away, merely leaving their impression
on the landscape. Ellis
et al.
(2010) noted that much
of the earth's landscapes have become fragmented,
and anthropogenic. They emphasized that this
has been an accelerating trend since 1700, and
that in 2010 'less than 20% of the biosphere
remains semi-natural and only a quarter is left
wild.'
Despite being omnipresent landscape features,
rivers are often seemingly ignored or forgotten, or
they are seen as dividers rather than integrators
of the landscape. Informed by the collective body
of river science over recent decades, the following
sections address the role of rivers as landscape
integrators and connectors, as ecosystem/landscape
engineers, illustrating the themes with case studies
from China.
and other global changes. Equally important is
an understanding of connectivity through time,
so integrating landscape change is a vital area of
river ecology, and subsequent management and
conservation action.
Lateral connections between rivers and
their surrounding landscapes are essential for
maintaining biodiversity in both the riverine
and in the associated floodplain and riparian
ecosystems. These connections are driven in large
part by hydrological processes; with the interaction
between river flows and floodplains creating the
dynamic conditions that are the basis for associated
wetlands. Riparian and floodplain ecosystems and
their connectivity with river systems are important
for conservation of all the component ecosystems
(Naiman
et al.
, 1993; Hughes
et al
., 2005).
Conservation of species and habitats usually
requires looking beyond surface hydrology.
Groundwater-fed systems are linked to river
systems in often complex space-time dimensions,
so conservation requires maintaining groundwater
flows as well as flowing surface waters.
Groundwaters, such as in karstic areas, provide
habitat for highly specialized species, as well as
water for millions of people (Leibundgut
et al.
,
1998). Groundwater catchments and surface water
catchments may not spatially or geopolitically
coincide, adding an additional layer of complexity
to conservation. While rivers are ecosystems
themselves, they influence adjacent ecosystems
from origin to ocean. In many cases this has
created ecosystems where flooding effects are
mollified rapidly through creating and maintaining
wetlands, including flooded forests.
During the last century human populations have
attempted to tame this feature of river life, by
canalizing or channelling rivers, creating dams and
by using increasing amounts of water from source
to sea for domestic human use but especially for
agricultural irrigation. The scientific challenge in
understanding the provision of ecosystem services
remains the ability to predict how any river system
might respond over time to new water withdrawal,
or to the restoration of previous flow regimes.
These problems will become even more difficult as
human demands for fresh water and pressures on
Rivers as landscape integrators
As highly linear ecosystems, rivers vary
considerably along their length. Rivers and their
networks, from origin as spring or glacier until
they reach the sea, have also lateral, vertical and
temporal dimensions varying along their length.
Ensuring longitudinal connectivity - the linkages
of habitats, species, communities and ecological
processes between upstream and downstream
portions of a river corridor or network - must be
a key goal of river management and conservation
(Boon, this volume). Ensuring longitudinal
connectivity is also critical for the maintenance
of system resilience under pressure from climate
